ATmega16U4/ATmega32U4 8-bit Microcontroller with 16/32K bytes of ISP Flash and USB Controller DATASHEET Features • High Performance, Low Power AVR® 8-Bit Microcontroller • Advanced RISC Architecture – 135 Powerful Instructions – Most Single Clock Cycle Execution – 32 x 8 General Purpose Working Registers – Fully Static Operation – Up to 16 MIPS Throughput at 16MHz – On-Chip 2-cycle Multiplier • Non-volatile Program and Data Memories – 16/32KB of In-System Self-Programmable Flash – 1.25/2.5KB Internal SRAM – 512Bytes/1KB Internal EEPROM – Write/Erase Cycles: 10,000 Flash/100,000 EEPROM – Data retention: 20 years at 85C/ 100 years at 25C(1) – Optional Boot Code Section with Independent Lock Bits In-System Programming by On-chip Boot Program True Read-While-Write Operation Parts using external XTAL clock are pre-programed with a default USB bootloader – Programming Lock for Software Security • JTAG (IEEE® std. 1149.1 compliant) Interface – Boundary-scan Capabilities According to the JTAG Standard – Extensive On-chip Debug Support – Programming of Flash, EEPROM, Fuses, and Lock Bits through the JTAG Interface • USB 2.0 Full-speed/Low Speed Device Module with Interrupt on Transfer Completion – Complies fully with Universal Serial Bus Specification Rev 2.0 – Supports data transfer rates up to 12Mbit/s and 1.5Mbit/s – Endpoint 0 for Control Transfers: up to 64-bytes – Six Programmable Endpoints with IN or Out Directions and with Bulk, Interrupt or Isochronous Transfers – Configurable Endpoints size up to 256 bytes in double bank mode – Fully independent 832 bytes USB DPRAM for endpoint memory allocation – Suspend/Resume Interrupts – CPU Reset possible on USB Bus Reset detection – 48MHz from PLL for Full-speed Bus Operation – USB Bus Connection/Disconnection on Microcontroller Request – Crystal-less operation for Low Speed mode • Peripheral Features – On-chip PLL for USB and High Speed Timer: 32 up to 96MHz operation – One 8-bit Timer/Counter with Separate Prescaler and Compare Mode Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

– Two 16-bit Timer/Counter with Separate Prescaler, Compare- and Capture Mode – One 10-bit High-Speed Timer/Counter with PLL (64MHz) and Compare Mode – Four 8-bit PWM Channels – Four PWM Channels with Programmable Resolution from 2 to 16 Bits – Six PWM Channels for High Speed Operation, with Programmable Resolution from 2 to 11 Bits – Output Compare Modulator – 12-channels, 10-bit ADC (features Differential Channels with Programmable Gain) – Programmable Serial USART with Hardware Flow Control – Master/Slave SPI Serial Interface – Byte Oriented 2-wire Serial Interface – Programmable Watchdog Timer with Separate On-chip Oscillator – On-chip Analog Comparator – Interrupt and Wake-up on Pin Change – On-chip Temperature Sensor • Special Microcontroller Features – Power-on Reset and Programmable Brown-out Detection – Internal 8MHz Calibrated Oscillator – Internal clock prescaler and On-the-fly Clock Switching (Int RC / Ext Osc) – External and Internal Interrupt Sources – Six Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, Standby, and Extended Standby • I/O and Packages – All I/O combine CMOS outputs and LVTTL inputs – 26 Programmable I/O Lines – 44-lead TQFP Package, 10x10mm – 44-lead QFN Package, 7x7mm • Operating Voltages – 2.7 - 5.5V • Operating temperature – Industrial (-40°C to +85°C) • Maximum Frequency – 8MHz at 2.7V - Industrial range – 16MHz at 4.5V - Industrial range Note: 1. See “Data Retention” on page8 for details. 2 ATmega16U4/32U4 [DATASHEET] Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

1. Pin Configurations Figure 1-1. Pinout CK) MS) DO) DI) T T T T 0) 1) 4/ 5/ 6/ 7/ C C C C C C D D D D D D AVCC GND AREF PF0 (A PF1 (A PF4 (A PF5 (A PF6 (A PF7 (A GND VCC 4 3 2 1 0 9 8 7 6 5 4 4 4 4 4 4 3 3 3 3 3 3 (INT.6/AIN0) PE6 1 33 PE2 (HWB) UVcc 2 32 PC7 (ICP3/CLK0/OC)4A INDEX CORNER D- 3 31 PC6 O( C3A/OC4A) D+ 4 30 PB6 (PCINT6/OC1B/OC4B/ADC UGnd 5 29 PB5 (PCINT5/OC1A/OC4B/ADC UCap 6 ATmega32U4 28 PB4 (PCINT4/ADC11) ATmega16U4 VBus 7 27 PD7 (T0/OC4D/ADC10) 44-pin QFN/QTFP (SS/PCINT0) PB0 8 26 PD6 (T1/OC4D/ADC9) (PCINT1/SCL) KPB1 9 25 PD4 (ICP1/ADC8) (PDI/PCINT2/MOSI) PB2 10 24 AVCC (PDO/PCINT3/MIS)O PB3 11 23 GND 2 3 4 5 6 7 8 9 0 1 2 1 1 1 1 1 1 1 1 2 2 2 C D 2 1 C1C/RTS) PB7 RESET VC GN XTAL XTAL SCL/INT0) PD0 (SDA/INT1) PD1 RXD1/INT2) PD2 TXD1/INT3) PD3 XCK1/CTS) PD5 O B/ ( ( ( A/ C0 0 O C ( O 7/ T N CI P ( 2. Overview The ATmega16U4/ATmega32U4 is a low-power CMOS 8-bit microcontroller based on the AVR enhanced RISC architecture. By executing powerful instructions in a single clock cycle, the device achieves throughputs approaching 1 MIPS per MHz allowing the system designer to optimize power consumption versus processing speed. ATmega16U4/32U4 [DATASHEET ] 3 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

2.1 Block Diagram Figure 2-1. Block Diagram PF7 - PF4 PF1 PF0 PC7PC6 XTAL1 XTAL2 RESET VCC PORTF DRIVERS PORTC DRIVERS GND DATAREGISTER DATADIR. DATAREGISTER DATADIR. PORTF REG.PORTF PORTC REG.PORTC 8-BIT DATA BUS POR - BOD RESET OISNCTIELRLANTAOLR CALIB. OSC OSCILLATOR WATCHDOG JTAG TAP PCROOUGNRTAEMR PSOTINACTEKR TIMER ON-CHIP DEBUG PRFOLAGSRHAM SRAM MCRUE GCOISNTTERROL TCIMOINNTGR AONLD BOUSNCDAANRY- INRSETGRUISCTTEIRON PGUERNPEORSAEL COTIUMNETRESR/S REGISTERS INTERRUPT X UNIT UVcc PROGLORAGMICMING INDSETRCUOCDTEIRON YZ EEPROM USOBN -PCAHDI P3V REGULATOR UCap TEMPERATURE SENSOR COLNINTERSOL ALU 1uF AVCC HIGH SPEED PLL ADC TIMER/PWM AGND STATUS REGISTER VBUS AREF DP USB 2.0 DM ANALOG TWO-WIRE SERIAL COMPARATOR USART1 SPI INTERFACE DATAREGISTER DATADIR. DATAREGISTER DATADIR. DATAREGISTER DATADIR. PORTE REG.PORTE PORTB REG.PORTB PORTD REG.PORTD PORTE DRIVERS PORTB DRIVERS PORTD DRIVERS PE6 PE2 PB7 - PB0 PD7 - PD0 The AVR core combines a rich instruction set with 32 general purpose working registers. All the 32 registers are directly connected to the Arithmetic Logic Unit (ALU), allowing two independent registers to be accessed in one single instruction executed in one clock cycle. The resulting architecture is more code efficient while achieving throughputs up to ten times faster than conventional CISC microcontrollers. The device provides the following features: 16/32K bytes of In-System Programmable Flash with Read-While- Write capabilities, 512Bytes/1K bytes EEPROM, 1.25/2.5K bytes SRAM, 26 general purpose I/O lines (CMOS outputs and LVTTL inputs), 32 general purpose working registers, four flexible Timer/Counters with compare modes and PWM, one more high-speed Timer/Counter with compare modes and PLL adjustable source, one USART (including CTS/RTS flow control signals), a byte oriented 2-wire Serial Interface, a 12-channels 10-bit ADC with optional differential input stage with programmable gain, an on-chip calibrated temperature sensor, a programmable Watchdog Timer with Internal Oscillator, an SPI serial port, IEEE std. 1149.1 compliant JTAG test interface, also used for accessing the On-chip Debug system and programming and six software selectable 4 ATmega16U4/32U4 [DATASHEET] Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

power saving modes. The Idle mode stops the CPU while allowing the SRAM, Timer/Counters, SPI port, and interrupt system to continue functioning. The Power-down mode saves the register contents but freezes the Oscillator, disabling all other chip functions until the next interrupt or Hardware Reset. The ADC Noise Reduction mode stops the CPU and all I/O modules except ADC, to minimize switching noise during ADC conversions. In Standby mode, the Crystal/Resonator Oscillator is running while the rest of the device is sleeping. This allows very fast start-up combined with low power consumption. The device is manufactured using the Atmel® high-density nonvolatile memory technology. The On-chip ISP Flash allows the program memory to be reprogrammed in-system through an SPI serial interface, by a conventional nonvolatile memory programmer, or by an On-chip Boot program running on the AVR core. The boot program can use any interface to download the application program in the application Flash memory. Software in the Boot Flash section will continue to run while the Application Flash section is updated, providing true Read-While-Write operation. By combining an 8-bit RISC CPU with In-System Self-Programmable Flash on a monolithic chip, the device is a powerful microcontroller that provides a highly flexible and cost effective solution to many embedded control applications. The ATmega16U4/ATmega32U4 AVR is supported with a full suite of program and system development tools including: C compilers, macro assemblers, program debugger/simulators, in-circuit emulators, and evaluation kits. 2.2 Pin Descriptions 2.2.1 VCC Digital supply voltage. 2.2.2 GND Ground. 2.2.3 Port B (PB7..PB0) Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port B output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port B pins that are externally pulled low will source current if the pull-up resistors are activated. The Port B pins are tri- stated when a reset condition becomes active, even if the clock is not running. Port B has better driving capabilities than the other ports. Port B also serves the functions of various special features of the device as listed on page74. 2.2.4 Port C (PC7,PC6) Port C is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port C output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port C pins that are externally pulled low will source current if the pull-up resistors are activated. The Port C pins are tri- stated when a reset condition becomes active, even if the clock is not running. Only bits 6 and 7 are present on the product pinout. Port C also serves the functions of special features of the device as listed on page77. 2.2.5 Port D (PD7..PD0) Port D is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port D output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port D pins that are externally pulled low will source current if the pull-up resistors are activated. The Port D pins are tri- stated when a reset condition becomes active, even if the clock is not running. ATmega16U4/32U4 [DATASHEET ] 5 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Port D also serves the functions of various special features of the ATmega16U4/ATmega32U4 as listed on page78. 2.2.6 Port E (PE6,PE2) Port E is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port E output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port E pins that are externally pulled low will source current if the pull-up resistors are activated. The Port E pins are tri- stated when a reset condition becomes active, even if the clock is not running. Only bits 2 and 6 are present on the product pinout. Port E also serves the functions of various special features of the ATmega16U4/ATmega32U4 as listed on page81. 2.2.7 Port F (PF7..PF4, PF1,PF0) Port F serves as analog inputs to the A/D Converter. Port F also serves as an 8-bit bi-directional I/O port, if the A/D Converter channels are not used. Port pins can provide internal pull-up resistors (selected for each bit). The Port F output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port F pins that are externally pulled low will source current if the pull-up resistors are activated. The Port F pins are tri-stated when a reset condition becomes active, even if the clock is not running. Bits 2 and 3 are not present on the product pinout. Port F also serves the functions of the JTAG interface. If the JTAG interface is enabled, the pull-up resistors on pins PF7(TDI), PF5(TMS), and PF4(TCK) will be activated even if a reset occurs. 2.2.8 D- USB Full speed / Low Speed Negative Data Upstream Port. Should be connected to the USB D- connector pin with a serial 22 resistor. 2.2.9 D+ USB Full speed / Low Speed Positive Data Upstream Port. Should be connected to the USB D+ connector pin with a serial 22 resistor. 2.2.10 UGND USB Pads Ground. 2.2.11 UVCC USB Pads Internal Regulator Input supply voltage. 2.2.12 UCAP USB Pads Internal Regulator Output supply voltage. Should be connected to an external capacitor (1µF). 2.2.13 VBUS USB VBUS monitor input. 6 ATmega16U4/32U4 [DATASHEET] Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

2.2.14 RESET Reset input. A low level on this pin for longer than the minimum pulse length will generate a reset, even if the clock is not running. The minimum pulse length is given in Table8-2 on page53. Shorter pulses are not guaranteed to generate a reset. 2.2.15 XTAL1 Input to the inverting Oscillator amplifier and input to the internal clock operating circuit. 2.2.16 XTAL2 Output from the inverting Oscillator amplifier. 2.2.17 AVCC AVCC is the supply voltage pin (input) for all the A/D Converter channels. If the ADC is not used, it should be externally connected to V . If the ADC is used, it should be connected to V through a low-pass filter. CC CC 2.2.18 AREF This is the analog reference pin (input) for the A/D Converter. ATmega16U4/32U4 [DATASHEET ] 7 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

3. About 3.1 Disclaimer Typical values contained in this datasheet are based on simulations and characterization of other AVR microcontrollers manufactured on the same process technology. Min. and Max. values will be available after the device is characterized. 3.2 Resources A comprehensive set of development tools, application notes and datasheets are available for download on http://www.atmel.com/avr. 3.3 Code Examples This documentation contains simple code examples that briefly show how to use various parts of the device. Be aware that not all C compiler vendors include bit definitions in the header files and interrupt handling in C is compiler dependent. Confirm with the C compiler documentation for more details. These code examples assume that the part specific header file is included before compilation. For I/O registers located in extended I/O map, "IN", "OUT", "SBIS", "SBIC", "CBI", and "SBI" instructions must be replaced with instructions that allow access to extended I/O. Typically "LDS" and "STS" combined with "SBRS", "SBRC", "SBR", and "CBR". 3.4 Data Retention Reliability Qualification results show that the projected data retention failure rate is much less than 1PPM over 20 years at 85°C or 100 years at 25°C. 8 ATmega16U4/32U4 [DATASHEET] Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

4. AVR CPU Core 4.1 Introduction This section discusses the AVR core architecture in general. The main function of the CPU core is to ensure correct program execution. The CPU must therefore be able to access memories, perform calculations, control peripherals, and handle interrupts. 4.2 Architectural Overview Figure 4-1. Block Diagram of the AVR Architecture Data Bus 8-bit Program Status Flash Counter and Control Program Memory Interrupt 32 x 8 Unit Instruction General Register Purpose SPI Registrers Unit Instruction Watchdog Decoder Timer g g n ssin essi ALU Analog Control Lines dre ddr Comparator d A ct A ect Dire Indir I/O Module1 Data I/O Module 2 SRAM I/O Module n EEPROM I/O Lines In order to maximize performance and parallelism, the AVR uses a Harvard architecture – with separate memories and buses for program and data. Instructions in the program memory are executed with a single level pipelining. While one instruction is being executed, the next instruction is pre-fetched from the program memory. This concept enables instructions to be executed in every clock cycle. The program memory is In-System Reprogrammable Flash memory. The fast-access Register File contains 32 x 8-bit general purpose working registers with a single clock cycle access time. This allows single-cycle Arithmetic Logic Unit (ALU) operation. In a typical ALU operation, two ATmega16U4/32U4 [DATASHEET] 9 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

operands are output from the Register File, the operation is executed, and the result is stored back in the Register File – in one clock cycle. Six of the 32 registers can be used as three 16-bit indirect address register pointers for Data Space addressing – enabling efficient address calculations. One of the these address pointers can also be used as an address pointer for look up tables in Flash program memory. These added function registers are the 16-bit X-, Y-, and Z- register, described later in this section. The ALU supports arithmetic and logic operations between registers or between a constant and a register. Single register operations can also be executed in the ALU. After an arithmetic operation, the Status Register is updated to reflect information about the result of the operation. Program flow is provided by conditional and unconditional jump and call instructions, able to directly address the whole address space. Most AVR instructions have a single 16-bit word format. Every program memory address contains a 16- or 32-bit instruction. Program Flash memory space is divided in two sections, the Boot Program section and the Application Program section. Both sections have dedicated Lock bits for write and read/write protection. The SPM instruction that writes into the Application Flash memory section must reside in the Boot Program section. During interrupts and subroutine calls, the return address Program Counter (PC) is stored on the Stack. The Stack is effectively allocated in the general data SRAM, and consequently the Stack size is only limited by the total SRAM size and the usage of the SRAM. All user programs must initialize the SP in the Reset routine (before subroutines or interrupts are executed). The Stack Pointer (SP) is read/write accessible in the I/O space. The data SRAM can easily be accessed through the five different addressing modes supported in the AVR architecture. The memory spaces in the AVR architecture are all linear and regular memory maps. A flexible interrupt module has its control registers in the I/O space with an additional Global Interrupt Enable bit in the Status Register. All interrupts have a separate Interrupt Vector in the Interrupt Vector table. The interrupts have priority in accordance with their Interrupt Vector position. The lower the Interrupt Vector address, the higher the priority. The I/O memory space contains 64 addresses for CPU peripheral functions as Control Registers, SPI, and other I/O functions. The I/O Memory can be accessed directly, or as the Data Space locations following those of the Register File, 0x20 - 0x5F. In addition, the ATmega16U4/ATmega32U4 has Extended I/O space from 0x60 - 0x0FF in SRAM where only the ST/STS/STD and LD/LDS/LDD instructions can be used. 4.3 ALU – Arithmetic Logic Unit The high-performance AVR ALU operates in direct connection with all the 32 general purpose working registers. Within a single clock cycle, arithmetic operations between general purpose registers or between a register and an immediate are executed. The ALU operations are divided into three main categories – arithmetic, logical, and bit-functions. Some implementations of the architecture also provide a powerful multiplier supporting both signed/unsigned multiplication and fractional format. See “Instruction Set Summary” on page418 for a detailed description. 4.4 Status Register The Status Register contains information about the result of the most recently executed arithmetic instruction. This information can be used for altering program flow in order to perform conditional operations. Note that the Status Register is updated after all ALU operations, as specified in the Instruction Set Reference. This will in many cases remove the need for using the dedicated compare instructions, resulting in faster and more compact code. The Status Register is not automatically stored when entering an interrupt routine and restored when returning from an interrupt. This must be handled by software. ATmega16U4/32U4 [DATASHEET] 10 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The AVR Status Register – SREG – is defined as: Bit 7 6 5 4 3 2 1 0 I T H S V N Z C SREG Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 7 – I: Global Interrupt Enable The Global Interrupt Enable bit must be set for the interrupts to be enabled. The individual interrupt enable control is then performed in separate control registers. If the Global Interrupt Enable Register is cleared, none of the interrupts are enabled independent of the individual interrupt enable settings. The I-bit is cleared by hardware after an interrupt has occurred, and is set by the RETI instruction to enable subsequent interrupts. The I-bit can also be set and cleared by the application with the SEI and CLI instructions, as described in the instruction set reference. • Bit 6 – T: Bit Copy Storage The Bit Copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T-bit as source or destination for the operated bit. A bit from a register in the Register File can be copied into T by the BST instruction, and a bit in T can be copied into a bit in a register in the Register File by the BLD instruction. • Bit 5 – H: Half Carry Flag The Half Carry Flag H indicates a Half Carry in some arithmetic operations. Half Carry Is useful in BCD arithmetic. See “Instruction Set Summary” on page418 for detailed information. • Bit 4 – S: Sign Bit, S = N V The S-bit is always an exclusive or between the Negative Flag N and the Two’s Complement Overflow Flag V. See “Instruction Set Summary” on page418 for detailed information. • Bit 3 – V: Two’s Complement Overflow Flag The Two’s Complement Overflow Flag V supports two’s arithmetic complements. See “Instruction Set Summary” on page418 for detailed information. • Bit 2 – N: Negative Flag The Negative Flag N indicates a negative result in an arithmetic or logic operation. See “Instruction Set Summary” on page418 for detailed information. • Bit 1 – Z: Zero Flag The Zero Flag Z indicates a zero result in an arithmetic or logic operation. See “Instruction Set Summary” on page418 for detailed information. • Bit 0 – C: Carry Flag The Carry Flag C indicates a carry in an arithmetic or logic operation. See “Instruction Set Summary” on page418 for detailed information. 4.5 General Purpose Register File The Register File is optimized for the AVR Enhanced RISC instruction set. In order to achieve the required performance and flexibility, the following input/output schemes are supported by the Register File:  One 8-bit output operand and one 8-bit result input  Two 8-bit output operands and one 8-bit result input ATmega16U4/32U4 [DATASHEET] 11 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

 Two 8-bit output operands and one 16-bit result input  One 16-bit output operand and one 16-bit result input Figure 4-2 shows the structure of the 32 general purpose working registers in the CPU. Figure 4-2. AVR CPU General Purpose Working Registers 7 0 Addr. R0 0x00 R1 0x01 R2 0x02 … R13 0x0D General R14 0x0E Purpose R15 0x0F Working R16 0x10 Registers R17 0x11 … R26 0x1A X-register Low Byte R27 0x1B X-register High Byte R28 0x1C Y-register Low Byte R29 0x1D Y-register High Byte R30 0x1E Z-register Low Byte R31 0x1F Z-register High Byte Most of the instructions operating on the Register File have direct access to all registers, and most of them are single cycle instructions. As shown in Figure 4-2, each register is also assigned a data memory address, mapping them directly into the first 32 locations of the user Data Space. Although not being physically implemented as SRAM locations, this memory organization provides great flexibility in access of the registers, as the X-, Y-, and Z-pointer registers can be set to index any register in the file. 4.5.1 The X-register, Y-register, and Z-register The registers R26..R31 have some added functions to their general purpose usage. These registers are 16-bit address pointers for indirect addressing of the data space. The three indirect address registers X, Y, and Z are defined as described in Figure 4-3. ATmega16U4/32U4 [DATASHEET] 12 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 4-3. The X-, Y-, and Z-registers 15 XH XL 0 X-register 7 0 7 0 R27 (0x1B) R26 (0x1A) 15 YH YL 0 Y-register 7 0 7 0 R29 (0x1D) R28 (0x1C) 15 ZH ZL 0 Z-register 7 0 7 0 R31 (0x1F) R30 (0x1E) In the different addressing modes these address registers have functions as fixed displacement, automatic increment, and automatic decrement (See “Instruction Set Summary” on page418 for detailed information). 4.6 Stack Pointer The Stack is mainly used for storing temporary data, for storing local variables and for storing return addresses after interrupts and subroutine calls. The Stack Pointer Register always points to the top of the Stack. Note that the Stack is implemented as growing from higher memory locations to lower memory locations. This implies that a Stack PUSH command decreases the Stack Pointer. The Stack Pointer points to the data SRAM Stack area where the Subroutine and Interrupt Stacks are located. This Stack space in the data SRAM must be defined by the program before any subroutine calls are executed or interrupts are enabled. The Stack Pointer must be set to point above 0x0100. The initial value of the stack pointer is the last address of the internal SRAM. The Stack Pointer is decremented by one when data is pushed onto the Stack with the PUSH instruction, and it is decremented by three when the return address is pushed onto the Stack with subroutine call or interrupt. The Stack Pointer is incremented by one when data is popped from the Stack with the POP instruction, and it is incremented by three when data is popped from the Stack with return from subroutine RET or return from interrupt RETI. The AVR Stack Pointer is implemented as two 8-bit registers in the I/O space. The number of bits actually used is implementation dependent. Note that the data space in some implementations of the AVR architecture is so small that only SPL is needed. In this case, the SPH Register will not be present. Bit 15 14 13 12 11 10 9 8 SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8 SPH SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0 SPL 7 6 5 4 3 2 1 0 Read/Write R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 1 0 0 0 0 0 1 1 1 1 1 1 1 1 ATmega16U4/32U4 [DATASHEET] 13 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

4.6.1 Extended Z-pointer Register for ELPM/SPM - RAMPZ Bit 7 6 5 4 3 2 1 0 RAMPZ7 RAMPZ6 RAMPZ5 RAMPZ4 RAMPZ3 RAMPZ2 RAMPZ1 RAMPZ0 RAMPZ Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 For ELPM/SPM instructions, the Z-pointer is a concatenation of RAMPZ, ZH, and ZL, as shown in Figure 4-4. Note that LPM is not affected by the RAMPZ setting. Figure 4-4. The Z-pointer used by ELPM and SPM Bit (Individually) 7 0 7 0 7 0 RAMPZ ZH ZL Bit (Z-pointer) 23 16 15 8 7 0 The actual number of bits is implementation dependent. Unused bits in an implementation will always read as zero. For compatibility with future devices, be sure to write these bits to zero. 4.7 Instruction Execution Timing This section describes the general access timing concepts for instruction execution. The AVR CPU is driven by the CPU clock clk , directly generated from the selected clock source for the chip. No internal clock division is CPU used. Figure 4-5 shows the parallel instruction fetches and instruction executions enabled by the Harvard architecture and the fast-access Register File concept. This is the basic pipelining concept to obtain up to 1 MIPS per MHz with the corresponding unique results for functions per cost, functions per clocks, and functions per power-unit. Figure 4-5. The Parallel Instruction Fetches and Instruction Executions T1 T2 T3 T4 clk CPU 1st Instruction Fetch 1st Instruction Execute 2nd Instruction Fetch 2nd Instruction Execute 3rd Instruction Fetch 3rd Instruction Execute 4th Instruction Fetch Figure 4-6 shows the internal timing concept for the Register File. In a single clock cycle an ALU operation using two register operands is executed, and the result is stored back to the destination register. ATmega16U4/32U4 [DATASHEET] 14 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 4-6. Single Cycle ALU Operation T1 T2 T3 T4 clk CPU Total Execution Time Register Operands Fetch ALU Operation Execute Result Write Back 4.8 Reset and Interrupt Handling The AVR provides several different interrupt sources. These interrupts and the separate Reset Vector each have a separate program vector in the program memory space. All interrupts are assigned individual enable bits which must be written logic one together with the Global Interrupt Enable bit in the Status Register in order to enable the interrupt. Depending on the Program Counter value, interrupts may be automatically disabled when Boot Lock bits BLB02 or BLB12 are programmed. This feature improves software security. See the section “Memory Programming” on page353 for details. The lowest addresses in the program memory space are by default defined as the Reset and Interrupt Vectors. The complete list of vectors is shown in “Interrupts” on page63. The list also determines the priority levels of the different interrupts. The lower the address the higher is the priority level. RESET has the highest priority, and next is INT0 – the External Interrupt Request 0. The Interrupt Vectors can be moved to the start of the Boot Flash section by setting the IVSEL bit in the MCU Control Register (MCUCR). Refer to “Interrupts” on page63 for more information. The Reset Vector can also be moved to the start of the Boot Flash section by programming the BOOTRST Fuse, see “Memory Programming” on page353. When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts are disabled. The user software can write logic one to the I-bit to enable nested interrupts. All enabled interrupts can then interrupt the current interrupt routine. The I-bit is automatically set when a Return from Interrupt instruction – RETI – is executed. There are basically two types of interrupts. The first type is triggered by an event that sets the Interrupt Flag. For these interrupts, the Program Counter is vectored to the actual Interrupt Vector in order to execute the interrupt handling routine, and hardware clears the corresponding Interrupt Flag. Interrupt Flags can also be cleared by writing a logic one to the flag bit position(s) to be cleared. If an interrupt condition occurs while the corresponding interrupt enable bit is cleared, the Interrupt Flag will be set and remembered until the interrupt is enabled, or the flag is cleared by software. Similarly, if one or more interrupt conditions occur while the Global Interrupt Enable bit is cleared, the corresponding Interrupt Flag(s) will be set and remembered until the Global Interrupt Enable bit is set, and will then be executed by order of priority. The second type of interrupts will trigger as long as the interrupt condition is present. These interrupts do not necessarily have Interrupt Flags. If the interrupt condition disappears before the interrupt is enabled, the interrupt will not be triggered. When the AVR exits from an interrupt, it will always return to the main program and execute one more instruction before any pending interrupt is served. Note that the Status Register is not automatically stored when entering an interrupt routine, nor restored when returning from an interrupt routine. This must be handled by software. ATmega16U4/32U4 [DATASHEET] 15 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

When using the CLI instruction to disable interrupts, the interrupts will be immediately disabled. No interrupt will be executed after the CLI instruction, even if it occurs simultaneously with the CLI instruction. The following example shows how this can be used to avoid interrupts during the timed EEPROM write sequence. Assembly Code Example in r16, SREG ; store SREG value cli ; disable interrupts during timed sequence sbi EECR, EEMPE ; start EEPROM write sbi EECR, EEPE out SREG, r16 ; restore SREG value (I-bit) C Code Example char cSREG; cSREG = SREG; /* store SREG value */ /* disable interrupts during timed sequence */ __disable_interrupt(); EECR |= (1<<EEMPE); /* start EEPROM write */ EECR |= (1<<EEPE); SREG = cSREG; /* restore SREG value (I-bit) */ When using the SEI instruction to enable interrupts, the instruction following SEI will be executed before any pending interrupts, as shown in this example. Assembly Code Example sei ; set Global Interrupt Enable sleep ; enter sleep, waiting for interrupt ; note: will enter sleep before any pending ; interrupt(s) C Code Example __enable_interrupt(); /* set Global Interrupt Enable */ __sleep(); /* enter sleep, waiting for interrupt */ /* note: will enter sleep before any pending interrupt(s) */ 4.8.1 Interrupt Response Time The interrupt execution response for all the enabled AVR interrupts is five clock cycles minimum. After five clock cycles the program vector address for the actual interrupt handling routine is executed. During these five clock cycle period, the Program Counter is pushed onto the Stack. The vector is normally a jump to the interrupt routine, and this jump takes three clock cycles. If an interrupt occurs during execution of a multi-cycle instruction, this instruction is completed before the interrupt is served. If an interrupt occurs when the MCU is in sleep mode, the interrupt execution response time is increased by five clock cycles. This increase comes in addition to the start-up time from the selected sleep mode. ATmega16U4/32U4 [DATASHEET] 16 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

A return from an interrupt handling routine takes five clock cycles. During these five clock cycles, the Program Counter (three bytes) is popped back from the Stack, the Stack Pointer is incremented by three, and the I-bit in SREG is set. ATmega16U4/32U4 [DATASHEET] 17 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

5. AVR Memories This section describes the different memories in the device. The AVR architecture has two main memory spaces, the Data Memory and the Program Memory space. In addition, the device features an EEPROM Memory for data storage. All three memory spaces are linear and regular. Table 5-1. Memory Mapping Memory Mnemonic ATmega32U4 ATmega16U4 Size Flashsize 32KB 16KB - Start Address Flash 0x0000 0x7FFF(1) 0x3FFF(1) End Address Flashend 0x3FFF(2) 0x1FFF(2) Size - 32 bytes 32 bytes 32 Registers Start Address - 0x0000 0x0000 End Address - 0x001F 0x001F Size - 64 bytes 64 bytes I/O Registers Start Address - 0x0020 0x0020 End Address - 0x005F 0x005F Size - 160 bytes 160 bytes Ext I/O Registers Start Address - 0x0060 0x0060 End Address - 0x00FF 0x00FF Size ISRAMsize 2.5KB 1.25KB Internal SRAM Start Address ISRAMstart 0x100 0x100 End Address ISRAMend 0x0AFF 0x05FF External Memory Not Present. Size E2size 1KB 512 bytes EEPROM End Address E2end 0x03FF 0x01FF Notes: 1. Byte address. 2. Word (16-bit) address. 5.1 In-System Reprogrammable Flash Program Memory The device contains 16/32K bytes On-chip In-System Reprogrammable Flash memory for program storage. Since all AVR instructions are 16 or 32 bits wide, the Flash is organized as 16K x 16. For software security, the Flash Program memory space is divided into two sections, Boot Program section and Application Program section. The Flash memory has an endurance of at least 100,000 write/erase cycles. The device Program Counter (PC) is 16 bits wide, thus addressing the 32K program memory locations. The operation of Boot Program section and associated Boot Lock bits for software protection are described in detail in “Memory Programming” on page353. “Memory Programming” on page353 contains a detailed description on Flash data serial downloading using the SPI pins or the JTAG interface. ATmega16U4/32U4 [DATASHEET] 18 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Constant tables can be allocated within the entire program memory address space (see the LPM – Load Program Memory instruction description and ELPM - Extended Load Program Memory instruction description). Timing diagrams for instruction fetch and execution are presented in “Instruction Execution Timing” on page14. Figure 5-1. Program Memory Map Program Memory 0x00000 Application Flash Section Boot Flash Section 0x7FFF (32KBytes) 5.2 SRAM Data Memory Figure 5-2 on page 20 shows how the device SRAM Memory is organized. The device is a complex microcontroller with more peripheral units than can be supported within the 64 location reserved in the Opcode for the IN and OUT instructions. For the Extended I/O space from $060 - $0FF in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used. The first 2,816 Data Memory locations address both the Register File, the I/O Memory, Extended I/O Memory, and the internal data SRAM. The first 32 locations address the Register file, the next 64 location the standard I/O Memory, then 160 locations of Extended I/O memory and the next 2,560 locations address the internal data SRAM. The five different addressing modes for the data memory cover: Direct, Indirect with Displacement, Indirect, Indirect with Pre-decrement, and Indirect with Post-increment. In the Register file, registers R26 to R31 feature the indirect addressing pointer registers. The direct addressing reaches the entire data space. The Indirect with Displacement mode reaches 63 address locations from the base address given by the Y- or Z- register. When using register indirect addressing modes with automatic pre-decrement and post-increment, the address registers X, Y, and Z are decremented or incremented. The 32 general purpose working registers, 64 I/O registers, and the 1.25/2.5Kbytes of internal data SRAM in the device are all accessible through all these addressing modes. The Register File is described in “General Purpose Register File” on page11. ATmega16U4/32U4 [DATASHEET] 19 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 5-2. Data Memory Map Data Memory 32 Registers $0000 - $001F 64 I/O Registers $0020 - $005F 160 E xt I/O Reg. $0060 - $00FF ISRAM start : $0100 Internal S RAM ISRAM end : $05FF / $0AFF $FFFF 5.2.1 Data Memory Access Times This section describes the general access timing concepts for internal memory access. The internal data SRAM access is performed in two clk cycles as described in Figure 5-3. CPU Figure 5-3. On-chip Data SRAM Access Cycles T1 T2 T3 clk CPU Address Compute Address Address valid Data e Writ WR Data d a e R RD Memory Access Instruction Next Instruction 5.3 EEPROM Data Memory The device contains 512Bytes/1K bytes of data EEPROM memory. It is organized as a separate data space, in which single bytes can be read and written. The EEPROM has an endurance of at least 100,000 write/erase cycles. The access between the EEPROM and the CPU is described in the following, specifying the EEPROM Address Registers, the EEPROM Data Register, and the EEPROM Control Register. For a detailed description of SPI, JTAG and Parallel data downloading to the EEPROM, see page367, page371, and page356 respectively. ATmega16U4/32U4 [DATASHEET] 20 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

5.3.1 EEPROM Read/Write Access The EEPROM Access Registers are accessible in the I/O space. The write access time for the EEPROM is given in Table5-3 on page23. A self-timing function, however, lets the user software detect when the next byte can be written. If the user code contains instructions that write the EEPROM, some precautions must be taken. In heavily filtered power supplies, V is likely to rise or fall slowly CC on power-up/down. This causes the device for some period of time to run at a voltage lower than specified as minimum for the clock frequency used. See “Preventing EEPROM Corruption” on page25. for details on how to avoid problems in these situations. In order to prevent unintentional EEPROM writes, a specific write procedure must be followed. Refer to the description of the EEPROM Control Register for details on this. When the EEPROM is read, the CPU is halted for four clock cycles before the next instruction is executed. When the EEPROM is written, the CPU is halted for two clock cycles before the next instruction is executed. 5.3.2 The EEPROM Address Register – EEARH and EEARL Bit 15 14 13 12 11 10 9 8 – – – – EEAR11 EEAR10 EEAR9 EEAR8 EEARH EEAR7 EEAR6 EEAR5 EEAR4 EEAR3 EEAR2 EEAR1 EEAR0 EEARL 7 6 5 4 3 2 1 0 Read/Write R R R R R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 X X X X X X X X X X X X • Bits 15..12 – Res: Reserved Bits These bits are reserved bits and will always read as zero. • Bits 11..0 – EEAR8..0: EEPROM Address The EEPROM Address Registers – EEARH and EEARL specify the EEPROM address in the 512Bytes/1K bytes EEPROM space. The EEPROM data bytes are addressed linearly between 0 and E2_END. The initial value of EEAR is undefined. A proper value must be written before the EEPROM may be accessed. 5.3.3 The EEPROM Data Register – EEDR Bit 7 6 5 4 3 2 1 0 MSB LSB EEDR Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bits 7..0 – EEDR7.0: EEPROM Data For the EEPROM write operation, the EEDR Register contains the data to be written to the EEPROM in the address given by the EEAR Register. For the EEPROM read operation, the EEDR contains the data read out from the EEPROM at the address given by EEAR. 5.3.4 The EEPROM Control Register – EECR Bit 7 6 5 4 3 2 1 0 – – EEPM1 EEPM0 EERIE EEMPE EEPE EERE EECR Read/Write R R R/W R/W R/W R/W R/W R/W Initial Value 0 0 X X 0 0 X 0 ATmega16U4/32U4 [DATASHEET] 21 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bits 7..6 – Res: Reserved Bits These bits are reserved and will always read as zero. • Bits 5, 4 – EEPM1 and EEPM0: EEPROM Programming Mode Bits The EEPROM Programming mode bit setting defines which programming action that will be triggered when writing EEPE. It is possible to program data in one atomic operation (erase the old value and program the new value) or to split the Erase and Write operations in two different operations. The Programming times for the different modes are shown in the table below. While EEPE is set, any write to EEPMn will be ignored. During reset, the EEPMn bits will be reset to 0b00 unless the EEPROM is busy programming. Table 5-2. EEPROM Mode Bits EEPM1 EEPM0 Programming Time Operation 0 0 3.4ms Erase and Write in one operation (Atomic Operation) 0 1 1.8ms Erase Only 1 0 1.8ms Write Only 1 1 – Reserved for future use • Bit 3 – EERIE: EEPROM Ready Interrupt Enable Writing EERIE to one enables the EEPROM Ready Interrupt if the I bit in SREG is set. Writing EERIE to zero disables the interrupt. The EEPROM Ready interrupt generates a constant interrupt when EEPE is cleared. • Bit 2 – EEMPE: EEPROM Master Programming Enable The EEMPE bit determines whether setting EEPE to one causes the EEPROM to be written. When EEMPE is set, setting EEPE within four clock cycles will write data to the EEPROM at the selected address If EEMPE is zero, setting EEPE will have no effect. When EEMPE has been written to one by software, hardware clears the bit to zero after four clock cycles. See the description of the EEPE bit for an EEPROM write procedure. • Bit 1 – EEPE: EEPROM Programming Enable The EEPROM Write Enable Signal EEPE is the write strobe to the EEPROM. When address and data are correctly set up, the EEPE bit must be written to one to write the value into the EEPROM. The EEMPE bit must be written to one before a logical one is written to EEPE, otherwise no EEPROM write takes place. The following procedure should be followed when writing the EEPROM (the order of steps 3 and 4 is not essential): 1. Wait until EEPE becomes zero. 2. Wait until SELFPRGEN in SPMCSR becomes zero. 3. Write new EEPROM address to EEAR (optional). 4. Write new EEPROM data to EEDR (optional). 5. Write a logical one to the EEMPE bit while writing a zero to EEPE in EECR. 6. Within four clock cycles after setting EEMPE, write a logical one to EEPE. The EEPROM can not be programmed during a CPU write to the Flash memory. The software must check that the Flash programming is completed before initiating a new EEPROM write. Step 2 is only relevant if the software contains a Boot Loader allowing the CPU to program the Flash. If the Flash is never being updated by the CPU, step 2 can be omitted. See “Memory Programming” on page353 for details about Boot programming. Caution: An interrupt between step 5 and step 6 will make the write cycle fail, since the EEPROM Master Write Enable will time-out. If an interrupt routine accessing the EEPROM is interrupting another EEPROM access, the ATmega16U4/32U4 [DATASHEET] 22 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

EEAR or EEDR Register will be modified, causing the interrupted EEPROM access to fail. It is recommended to have the Global Interrupt Flag cleared during all the steps to avoid these problems. When the write access time has elapsed, the EEPE bit is cleared by hardware. The user software can poll this bit and wait for a zero before writing the next byte. When EEPE has been set, the CPU is halted for two cycles before the next instruction is executed. • Bit 0 – EERE: EEPROM Read Enable The EEPROM Read Enable Signal EERE is the read strobe to the EEPROM. When the correct address is set up in the EEAR Register, the EERE bit must be written to a logic one to trigger the EEPROM read. The EEPROM read access takes one instruction, and the requested data is available immediately. When the EEPROM is read, the CPU is halted for four cycles before the next instruction is executed. The user should poll the EEPE bit before starting the read operation. If a write operation is in progress, it is neither possible to read the EEPROM, nor to change the EEAR Register. The calibrated Oscillator is used to time the EEPROM accesses. The following table lists the typical programming time for EEPROM access from the CPU. Table 5-3. EEPROM Programming Time Symbol Number of Calibrated RC Oscillator Cycles Typ Programming Time EEPROM write 26,368 3.3ms (from CPU) The following code examples show one assembly and one C function for writing to the EEPROM. The examples assume that interrupts are controlled (e.g. by disabling interrupts globally) so that no interrupts will occur during execution of these functions. The examples also assume that no Flash Boot Loader is present in the software. If such code is present, the EEPROM write function must also wait for any ongoing SPM command to finish. ATmega16U4/32U4 [DATASHEET] 23 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Assembly Code Example(1) EEPROM_write: ; Wait for completion of previous write sbic EECR,EEPE rjmp EEPROM_write ; Set up address (r18:r17) in address register out EEARH, r18 out EEARL, r17 ; Write data (r16) to Data Register out EEDR,r16 ; Write logical one to EEMPE sbi EECR,EEMPE ; Start eeprom write by setting EEPE sbi EECR,EEPE ret C Code Example(1) void EEPROM_write(unsigned int uiAddress, unsigned char ucData) { /* Wait for completion of previous write */ while(EECR & (1<<EEPE)) ; /* Set up address and Data Registers */ EEAR = uiAddress; EEDR = ucData; /* Write logical one to EEMPE */ EECR |= (1<<EEMPE); /* Start eeprom write by setting EEPE */ EECR |= (1<<EEPE); } Note: 1. See “Code Examples” on page 8. The next code examples show assembly and C functions for reading the EEPROM. The examples assume that interrupts are controlled so that no interrupts will occur during execution of these functions. ATmega16U4/32U4 [DATASHEET] 24 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Assembly Code Example(1) EEPROM_read: ; Wait for completion of previous write sbic EECR,EEPE rjmp EEPROM_read ; Set up address (r18:r17) in address register out EEARH, r18 out EEARL, r17 ; Start eeprom read by writing EERE sbi EECR,EERE ; Read data from Data Register in r16,EEDR ret C Code Example(1) unsigned char EEPROM_read(unsigned int uiAddress) { /* Wait for completion of previous write */ while(EECR & (1<<EEPE)) ; /* Set up address register */ EEAR = uiAddress; /* Start eeprom read by writing EERE */ EECR |= (1<<EERE); /* Return data from Data Register */ return EEDR; } Note: 1. See “Code Examples” on page 8. 5.3.5 Preventing EEPROM Corruption During periods of low V the EEPROM data can be corrupted because the supply voltage is too low for the CC, CPU and the EEPROM to operate properly. These issues are the same as for board level systems using EEPROM, and the same design solutions should be applied. An EEPROM data corruption can be caused by two situations when the voltage is too low. First, a regular write sequence to the EEPROM requires a minimum voltage to operate correctly. Secondly, the CPU itself can execute instructions incorrectly, if the supply voltage is too low. EEPROM data corruption can easily be avoided by following this design recommendation: Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can be done by enabling the internal Brown-out Detector (BOD). If the detection level of the internal BOD does not match the needed detection level, an external low V reset Protection circuit can be used. If a reset occurs while a write CC operation is in progress, the write operation will be completed provided that the power supply voltage is sufficient. 5.4 I/O Memory The I/O space definition of the device is shown in “Register Summary” on page414. ATmega16U4/32U4 [DATASHEET] 25 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

All I/Os and peripherals are placed in the I/O space. All I/O locations may be accessed by the LD/LDS/LDD and ST/STS/STD instructions, transferring data between the 32 general purpose working registers and the I/O space. I/O Registers within the address range 0x00 - 0x1F are directly bit-accessible using the SBI and CBI instructions. In these registers, the value of single bits can be checked by using the SBIS and SBIC instructions. Refer to the instruction set section for more details. When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these addresses. The device is a complex microcontroller with more peripheral units than can be supported within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space from 0x60 - 0xFF in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used. For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O memory addresses should never be written. Some of the Status Flags are cleared by writing a logical one to them. Note that, unlike most other AVRs, the CBI and SBI instructions will only operate on the specified bit, and can therefore be used on registers containing such Status Flags. The CBI and SBI instructions work with registers 0x00 to 0x1F only. The I/O and peripherals control registers are explained in later sections. 5.4.1 General Purpose I/O Registers The device contains three General Purpose I/O Registers. These registers can be used for storing any information, and they are particularly useful for storing global variables and Status Flags. General Purpose I/O Registers within the address range 0x00 - 0x1F are directly bit-accessible using the SBI, CBI, SBIS, and SBIC instructions. 5.4.2 General Purpose I/O Register 2 – GPIOR2 Bit 7 6 5 4 3 2 1 0 MSB LSB GPIOR2 Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 5.4.3 General Purpose I/O Register 1 – GPIOR1 Bit 7 6 5 4 3 2 1 0 MSB LSB GPIOR1 Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 5.4.4 General Purpose I/O Register 0 – GPIOR0 Bit 7 6 5 4 3 2 1 0 MSB LSB GPIOR0 Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 ATmega16U4/32U4 [DATASHEET] 26 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

6. System Clock and Clock Options 6.1 Clock Systems and their Distribution Figure 6-1 presents the principal clock systems in the AVR and their distribution. All of the clocks need not be active at a given time. In order to reduce power consumption, the clocks to modules not being used can be halted by using different sleep modes, as described in “Power Management and Sleep Modes” on page43. The clock systems are detailed below. Figure 6-1. Clock Distribution USB HigTh imSpeered GMenoedrualle Is/O ADC CPU Core RAM FElEaPshR aOnMd clkUSB (48MHz) clkTMR clkADC (1) (2) clkI/O AVR Clock clkCPU Control Unit PLL Postcaler clk FLASH clkPLL Reset Logic Watchdog Timer PLL Source clock System Clock Watchdog PLL Input Prescaler clock Multiplexer PLL Clock clkPllPresc Prescaler Clock Multiplexer Clock Switch Crystal Watchdog Calibrated RC Oscillator External Clock Oscillator Oscillator 6.1.1 CPU Clock – clk CPU The CPU clock is routed to parts of the system concerned with operation of the AVR core. Examples of such modules are the General Purpose Register File, the Status Register and the data memory holding the Stack Pointer. Halting the CPU clock inhibits the core from performing general operations and calculations. 6.1.2 I/O Clock – clk I/O The I/O clock is used by the majority of the I/O modules, like Timer/Counters, SPI, and USART. The I/O clock is also used by the External Interrupt module, but note that some external interrupts are detected by asynchronous logic, allowing such interrupts to be detected even if the I/O clock is halted. Also, TWI address recognition is handled in all sleep modes. 6.1.3 Flash Clock – clk FLASH The Flash clock controls operation of the Flash interface. The Flash clock is usually active simultaneously with the CPU clock. ATmega16U4/32U4 [DATASHEET] 27 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

6.1.4 ADC Clock – clk ADC The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks in order to reduce noise generated by digital circuitry. This gives more accurate ADC conversion results. 6.1.5 PLL Prescaler Clock – clk PllPresc The PLL requires a 8MHz input. A prescaler allows user to use either a 8MHz or a 16MHz source (from a crystal or an external source), using a divider (by 2) if necessary. The output of the prescaler goes into the PLL Input multiplexer, that allows the user to select either the prescaler output of the System Clock Multiplexer, or the Internal 8MHz Calibrated Oscillator. 6.1.6 PLL Output Clock – clk Pll When enabled, the PLL outputs one frequency among numerous choices between 32MHz and 96MHz. The output frequency is determined by the PLL clock register. The frequency is independent of the power supply voltage. The PLL Output is connected to a postscaler that allows user to generate two different frequencies (clk and clk ) from the common PLL signal, each on them resulting of a selected division ratio (/1, /1.5, /2). USB TMR 6.1.7 High-Speed Timer Clock– clk TMR When enabled, the PLL outputs one frequency among numerous choices between 32MHz and 96MHz, that goes into the PLL Postcaler. The High Speed Timer frequency input is generated from the PLL Postcaler, that proposes /1, /1.5 and /2 ratios. That can be determined from the PLL clock register. The High Speed Timer maximum frequency input depends on the power supply voltage and reaches its maximum of 64MHz at 5V. 6.1.8 USB Clock – clk USB The USB hardware module needs for a 48MHz clock. This clock is generated from the on-chip PLL. The output of the PLL passes through the PLL Postcaler where the frequency can be either divided by 2 or directly connected to the clk signal. USB 6.2 Clock Sources The device has the following clock source options, selectable by Flash Fuse bits as shown below. The clock from the selected source is input to the AVR clock generator, and routed to the appropriate modules. Table 6-1. Device Clocking Options Select(1) Device Clocking Option CKSEL[3:0] (or EXCKSEL[3:0]) Low Power Crystal Oscillator 1111 - 1000 Reserved 0111 - 0110 Low Frequency Crystal Oscillator 0101 - 0100 Reserved 0011 Calibrated Internal RC Oscillator 0010 External Clock 0000 Reserved 0001 Note: 1. For all fuses “1” means unprogrammed while “0” means programmed. ATmega16U4/32U4 [DATASHEET] 28 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

6.2.1 Default Clock Source ATmega16U4 and ATmega32U4 The device is shipped with Low Power Crystal Oscillator (8.0 - 16MHz) enabled and with the fuse CKDIV8 programmed, resulting in 1.0MHz system clock with an 8MHz crystal. See Table28-5 on page355 for an overview of the default Clock Selection Fuse setting. 6.2.2 Default Clock Source ATmega16U4RC and ATmega32U4RC The device is shipped with Calibrated Internal RC oscillator (8.0MHz) enabled and with the fuse CKDIV8 programmed, resulting in 1.0MHz system clock. See Table28-5 on page355 for an overview of the default Clock Selection Fuse setting. 6.2.3 Clock Startup Sequence Any clock source needs a sufficient V to start oscillating and a minimum number of oscillating cycles before it CC can be considered stable. To ensure sufficient V , the device issues an internal reset with a time-out delay (t ) after the device reset is CC TOUT released by all other reset sources. “On-chip Debug System” on page46 describes the start conditions for the internal reset. The delay (t ) is timed from the Watchdog Oscillator and the number of cycles in the delay is TOUT set by the SUTx and CKSELx fuse bits. The selectable delays are shown in the following table. The frequency of the Watchdog Oscillator is voltage dependent as shown in this table. Table 6-2. Number of Watchdog Oscillator Cycles Typ Time-out (V = 5.0V) Typ Time-out (V = 3.0V) Number of Cycles CC CC 0ms 0ms 0 4.1ms 4.3ms 512 65ms 69ms 8K (8,192) Main purpose of the delay is to keep the AVR in reset until it is supplied with minimum V . The delay will not CC monitor the actual voltage and it will be required to select a delay longer than the V rise time. If this is not CC possible, an internal or external Brown-Out Detection circuit should be used. A BOD circuit will ensure sufficient V before it releases the reset, and the time-out delay can be disabled. Disabling the time-out delay without CC utilizing a Brown-Out Detection circuit is not recommended. The oscillator is required to oscillate for a minimum number of cycles before the clock is considered stable. An internal ripple counter monitors the oscillator output clock, and keeps the internal reset active for a given number of clock cycles. The reset is then released and the device will start to execute. The recommended oscillator start-up time is dependent on the clock type, and varies from six cycles for an externally applied clock to 32K cycles for a low frequency crystal. The start-up sequence for the clock includes both the time-out delay and the start-up time when the device starts up from reset. When starting up from Power-save or Power-down mode, V is assumed to be at a CC sufficient level and only the start-up time is included. 6.3 Low Power Crystal Oscillator Pins XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can be configured for use as an On-chip Oscillator, as shown in Figure 6-2. Either a quartz crystal or a ceramic resonator may be used. This Crystal Oscillator is a low power oscillator, with reduced voltage swing on the XTAL2 output. It gives the lowest power consumption, but is not capable of driving other clock inputs. ATmega16U4/32U4 [DATASHEET] 29 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

C1 and C2 should always be equal for both crystals and resonators. The optimal value of the capacitors depends on the crystal or resonator in use, the amount of stray capacitance, and the electromagnetic noise of the environment. Some initial guidelines for choosing capacitors for use with crystals are given in the below table. For ceramic resonators, the capacitor values given by the manufacturer should be used. Figure 6-2. Crystal Oscillator Connections C2 XTAL2 C1 XTAL1 GND The Low Power Oscillator can operate in three different modes, each optimized for a specific frequency range. The operating mode is selected by the fuses CKSEL[3..1] as shown in this table. Table 6-3. Low Power Crystal Oscillator Operating Modes Frequency Range(1) [MHz] CKSEL3..1 Recommended Range for Capacitors C1 and C2 [pF] 0.4 - 0.9 100(2) – 0.9 - 3.0 101 12 - 22 3.0 - 8.0 110 12 - 22 8.0 - 16.0 111 12 - 22 Notes: 1. This option should not be used with crystals, only with ceramic resonators. 2. If 8 MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8 Fuse can be programmed in order to divide the internal frequency by 8. It must be ensured that the resulting divided clock meets the frequency specification of the device. The CKSEL0 Fuse together with the SUT1..0 Fuses select the start-up times as shown in this table. Table 6-4. Start-up Times for the Low Power Crystal Oscillator Clock Selection Start-up Time from Additional Delay from Oscillator Source / Power-down and Reset Power Conditions Power-save (V = 5.0V) CKSEL0 SUT1..0 CC Ceramic resonator, 258CK 14CK + 4.1ms(1) 0 00 fast rising power Ceramic resonator, 258CK 14CK + 65ms(1) 0 01 slowly rising power Ceramic resonator, 1K CK 14CK(2) 0 10 BOD enabled Ceramic resonator, 1K CK 14CK + 4.1ms(2) 0 11 fast rising power Ceramic resonator, 1K CK 14CK + 65ms(2) 1 00 slowly rising power ATmega16U4/32U4 [DATASHEET] 30 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 6-4. Start-up Times for the Low Power Crystal Oscillator Clock Selection Start-up Time from Additional Delay from Oscillator Source / Power-down and Reset Power Conditions Power-save (V = 5.0V) CKSEL0 SUT1..0 CC Crystal Oscillator, 16K CK 14CK 1 01 BOD enabled Crystal Oscillator, 16K CK 14CK + 4.1ms 1 10 fast rising power Crystal Oscillator, 16K CK 14CK + 65ms 1 11 slowly rising power Notes: 1. These options should only be used when not operating close to the maximum frequency of the device, and only if frequency stability at start-up is not important for the application. These options are not suitable for crystals. 2. These options are intended for use with ceramic resonators and will ensure frequency stability at start-up. They can also be used with crystals when not operating close to the maximum frequency of the device, and if frequency stability at start-up is not important for the application. Table 6-5. Start-up times for the internal calibrated RC Oscillator clock selection Start-up Time from Power- Additional Delay from Power Conditions down and Power-save Reset (V = 5.0V) SUT1..0 CC BOD enabled 6CK 14CK 00 Fast rising power 6CK 14CK + 4.1ms 01 Slowly rising power 6CK 14CK + 65ms(1) 10 Reserved 11 Note: 1. The device is shipped with this option selected. 6.4 Low Frequency Crystal Oscillator The device can utilize a 32.768kHz watch crystal as clock source by a dedicated Low Frequency Crystal Oscillator. The crystal should be connected as shown in Figure 6-2 on page 30. When this Oscillator is selected, start-up times are determined by the SUT Fuses and CKSEL0 as shown in the table below. Table 6-6. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection Start-up Time from Additional Delay from Power-down and Reset Power Conditions Power-save (V = 5.0V) CKSEL0 SUT1..0 CC BOD enabled 1K CK 14CK(1) 0 00 Fast rising power 1K CK 14CK + 4.1ms(1) 0 01 Slowly rising power 1K CK 14CK + 65ms(1) 0 10 Reserved 0 11 BOD enabled 32K CK 14CK 1 00 Fast rising power 32K CK 14CK + 4.1ms 1 01 Slowly rising power 32K CK 14CK + 65ms 1 10 ATmega16U4/32U4 [DATASHEET] 31 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 6-6. Start-up Times for the Low Frequency Crystal Oscillator Clock Selection Start-up Time from Additional Delay from Power-down and Reset Power Conditions Power-save (V = 5.0V) CKSEL0 SUT1..0 CC Reserved 1 11 Note: 1. These options should only be used if frequency stability at start-up is not important for the application. 6.5 Calibrated Internal RC Oscillator The calibrated internal RC Oscillator by default provides a 8.0MHz clock. This frequency is nominal value at 3V and 25C. The device is shipped with the CKDIV8 Fuse programmed. See “System Clock Prescaler” on page35 for more details. This clock may be selected as the system clock by programming the CKSEL Fuses as shown in the table below. If selected, it will operate with no external components. During reset, hardware loads the calibration byte into the OSCCAL Register and thereby automatically calibrates the RC Oscillator. At 3V and 25C, this calibration gives a frequency of 8MHz ±1%. The oscillator can be calibrated to any frequency in the range 7.3 - 8.1MHz within ±1% accuracy, by changing the OSCCAL register. When this Oscillator is used as the chip clock, the Watchdog Oscillator will still be used for the Watchdog Timer and for the Reset Time-out. For more information on the pre-programmed calibration value, see the section “Calibration Byte” on page356 Table 6-7. Internal Calibrated RC Oscillator Operating Modes Frequency Range [MHz] CKSEL[3:0] 7.3 - 8.1 0010 Notes: 1. The device is shipped with this option selected. 2. If 8 MHz frequency exceeds the specification of the device (depends on V ), the CKDIV8 Fuse can be CC programmed in order to divide the internal frequency by 8. When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in the table. Table 6-8. Start-up times for the internal calibrated RC Oscillator clock selection Start-up Time from Power- Additional Delay from Power Conditions down and Power-save Reset (V = 5.0V) SUT1..0 CC BOD enabled 6CK 14CK 00 Fast rising power 6CK 14CK + 4.1ms 01 Slowly rising power 6CK 14CK + 65ms 10 Reserved 11 6.5.1 Oscillator Calibration Register – OSCCAL Bit 7 6 5 4 3 2 1 0 CAL7 CAL6 CAL5 CAL4 CAL3 CAL2 CAL1 CAL0 OSCCAL Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value Device Specific Calibration Value • Bits 7..0 – CAL7..0: Oscillator Calibration Value The Oscillator Calibration Register is used to trim the Calibrated Internal RC Oscillator to remove process variations from the oscillator frequency. The factory-calibrated value is automatically written to this register during chip reset, giving an oscillator frequency of 8.0MHz at 25°C. The application software can write this ATmega16U4/32U4 [DATASHEET] 32 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

register to change the oscillator frequency. The calibration range is ±40% and linear (calibration step ~0.4%). With typical process at 25°C the code should be 127 for 8MHz. Input value of 0x00 gives the lowest frequency, and 0xFF the highest. The temperature sensitivity is quite linear but as said previously depends on the process. To determine its slope, the frequency must be measured at two temperatures. The temperature sensor of the device allows such an operation, that is detailed on “Sensor Calibration” on page304. It is then possible to calibrate the oscillator frequency in function of the temperature measured. Note that this oscillator is used to time EEPROM and Flash write accesses, and these write times will be affected accordingly. If the EEPROM or Flash are written, do not calibrate to more than 8.8 MHz. Otherwise, the EEPROM or Flash write may fail. 6.5.2 Oscillator Control Register – RCCTRL Bit 7 6 5 4 3 2 1 0 - - - - - - - RCFREQ RCCTRL Read/Write R R R R R R R R/W Initial Value 0 0 0 0 0 0 0 0 Bits 7..1 – Reserved Do not set these bits. Bits should be read as ‘0’. Bit 0– RCFREQ: RC Oscillator Frequency Select When this bit is cleared (default value), the RC Oscillator output frequency is set to 8MHz. When the bit is set, the RC output frequency is 1MHz. Note that the OSCCAL value has the same effect on both 8MHz and 1MHz output modes (~0.4% / step). 6.6 External Clock The device can utilize a external clock source as shown in Figure 6-3. To run the device on an external clock, the CKSEL Fuses must be programmed as shown in Table6-1 on page28. Figure 6-3. External Clock Drive Configuration NC XTAL2 EXTERNAL CLOCK XTAL1 SIGNAL GND When this clock source is selected, start-up times are determined by the SUT Fuses as shown in the table below. ATmega16U4/32U4 [DATASHEET] 33 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 6-9. Start-up Times for the External Clock Selection Start-up Time from Power- Additional Delay from Power Conditions down and Power-save Reset (V = 5.0V) SUT1..0 CC BOD enabled 6CK 14CK 00 Fast rising power 6CK 14CK + 4.1ms 01 Slowly rising power 6CK 14CK + 65ms 10 Reserved 11 When applying an external clock, it is required to avoid sudden changes in the applied clock frequency to ensure stable operation of the MCU. A variation in frequency of more than 2% from one clock cycle to the next can lead to unpredictable behavior. If changes of more than 2% is required, ensure that the MCU is kept in Reset during the changes. Note that the System Clock Prescaler can be used to implement run-time changes of the internal clock frequency while still ensuring stable operation. Refer to “System Clock Prescaler” on page35 for details. 6.7 Clock Switch The device includes a Clock Switch controller, that allows user to switch from one clock source to another one by software, in order to control application power and execution time with more accuracy. 6.7.1 Example of use The modification may be needed when the device enters in USB Suspend mode. It then switches from External Clock to Calibrated RC Oscillator in order to reduce consumption and wake-up delay. In such a configuration, the External Clock is disabled. The firmware can then use the watchdog timer to be woken-up from power-down in order to check if there is an event on the application. If an event occurs on the application or if the USB con- troller signals a non-idle state on the USB line (Resume for example), the firmware switches the Clock Multiplexer from the Calibrated RC Oscillator to the External Clock. in order to restart USB operation. This feature can only be used to switch between Calibrated 8MHz RC Oscillator, External Clock and Low Power Crystal Oscillator. The Low Frequency Crystal Oscillator must not be used with this feature. Figure 6-4. Example of Clock Switching with Wake-up from USB Host resume 1 Resume from Host USB non-Idle Idle (Suspend) non-Idle 1 CPU Clock Ext RC Ext External Oscillator RC oscillator 3ms watchdog wake-up from power-down ATmega16U4/32U4 [DATASHEET] 34 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 6-5. Example of Clock Switching with Wake-up from Device upstream-resume 2 Upstream Resume from device USB non-Idle Idle (Suspend) non-Idle 2 CPU Clock Ext RC Ext External Oscillator RC oscillator 3ms watchdog wake-up from power-down 6.8 Clock Output Buffer The device can output the system clock on the CLKO pin. To enable the output, the CKOUT Fuse has to be programmed. This mode is suitable when the chip clock is used to drive other circuits on the system. The clock also will be output during reset, and the normal operation of I/O pin will be overridden when the fuse is programmed. Any clock source, including the internal RC Oscillator, can be selected when the clock is output on CLKO. If the System Clock Prescaler is used, it is the divided system clock that is output. 6.8.1 System Clock Prescaler The AVR USB has a system clock prescaler, and the system clock can be divided by setting the “CLKPR – Clock Prescaler Register” on page39. This feature can be used to decrease the system clock frequency and the power consumption when the requirement for processing power is low. This can be used with all clock source options, and it will affect the clock frequency of the CPU and all synchronous peripherals. clk , clk , I/O ADC clk , and clk are divided by a factor as shown in Table6-10 on page40. CPU FLASH When switching between prescaler settings, the System Clock Prescaler ensures that no glitches occurs in the clock system. It also ensures that no intermediate frequency is higher than neither the clock frequency corresponding to the previous setting, nor the clock frequency corresponding to the new setting. The ripple counter that implements the prescaler runs at the frequency of the undivided clock, which may be faster than the CPU's clock frequency. Hence, it is not possible to determine the state of the prescaler - even if it were readable, and the exact time it takes to switch from one clock division to the other cannot be exactly predicted. From the time the CLKPS values are written, it takes between T1 + T2 and T1 + 2 * T2 before the new clock frequency is active. In this interval, two active clock edges are produced. Here, T1 is the previous clock period, and T2 is the period corresponding to the new prescaler setting. To avoid unintentional changes of clock frequency, a special write procedure must be followed to change the CLKPS bits: 1. Write the Clock Prescaler Change Enable (CLKPCE) bit to one and all other bits in CLKPR to zero. 2. Within four cycles, write the desired value to CLKPS while writing a zero to CLKPCE. Interrupts must be disabled when changing prescaler setting to make sure the write procedure is not interrupted. ATmega16U4/32U4 [DATASHEET] 35 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

6.9 PLL The PLL is used to generate internal high frequency (up to 96MHz) clock for USB interface and/or High Speed Timer module, the PLL input is supplied from an external low-frequency clock (the crystal oscillator or external clock input pin from XTAL1). 6.9.1 Internal PLL The internal PLL in the device generates a clock frequency between 32MHz and 96MHz from nominally 8MHz input. The source of the 8MHz PLL input clock is the output of the internal PLL clock prescaler that generates the 8MHz from the clock source multiplexer output (See “PLL Control and Status Register – PLLCSR” on page40. for PLL interface). The PLL prescaler allows a direct connection (8MHz oscillator) or a divide-by-2 stage for a 16MHz clock input. The PLL output signal enters the PLL Postcaler stage before being distributed to the USB and High Speed Timer modules. Each of these modules can choose an independent division ratio. Figure 6-6. PLL Clocking System CKSEL3:0 PINDIV PINMUX PLOCK PLLE PLLTM1:0 /1.5 10 Lock Detector 01 clkTMR XTAL1 XTAL XTAL2 OSCILLATOR 11 PLL clock 0 Prescaler RC OSCILLATOR clk PLL /2 8 MHz 1 8MHz 1 To System clkUSB Clock Prescaler 0 PDIV3..0 PLLUSB 6.10 Clock switch Algorithm 6.10.1 Switch from External Clock to RC Clock if (Usb_suspend_detected()) // if (UDINT.SUSPI == 1) { Usb_ack_suspend(); // UDINT.SUSPI = 0; Usb_freeze_clock(); // USBCON.FRZCLK = 1; Disable_pll(); // PLLCSR.PLLE = 0; Enable_RC_clock(); // CLKSEL0.RCE = 1; while (!RC_clock_ready()); // while (CLKSTA.RCON != 1); Select_RC_clock(); // CLKSEL0.CLKS = 0; Disable_external_clock(); // CLKSEL0.EXTE = 0; } 6.10.2 Switch from RC Clock to External Clock if (Usb_wake_up_detected()) // if (UDINT.WAKEUPI == 1) { Usb_ack_wake_up(); // UDINT.WAKEUPI = 0; Enable_external_clock(); // CKSEL0.EXTE = 1; while (!External_clock_ready()); // while (CLKSTA.EXTON != 1); ATmega16U4/32U4 [DATASHEET] 36 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Select_external_clock(); // CLKSEL0.CLKS = 1; Enable_pll(); // PLLCSR.PLLE = 1; Disable_RC_clock(); // CLKSEL0.RCE = 0; while (!Pll_ready()); // while (PLLCSR.PLOCK != 1); Usb_unfreeze_clock(); // USBCON.FRZCLK = 0; } ATmega16U4/32U4 [DATASHEET] 37 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

6.11 Register Description 6.11.1 CLKSEL0 – Clock Selection Register 0 Bit 7 6 5 4 3 2 1 0 RCSUT1 RCSUT0 EXSUT1 EXSUT0 RCE EXTE - CLKS CLKSEL0 Read/Write R/W R/W R/W R/W R/W R/W R R/W Initial Value 0 0 0 0 See Bit Description • Bit 7-6 – RCSUT[1:0]: SUT for RC oscillator These two bits are the SUT value for the RC Oscillator. If the RC oscillator is selected by fuse bits, the SUT fuse are copied into these bits. A firmware change will not have any effect because this additional start-up time is only used after a reset and not after a clock switch. • Bit 5-4 – EXSUT[1:0]: SUT for External Clock/ Low Power Crystal Oscillator These two bits are the SUT value for the External Clock / Low Power Crystal Oscillator. If the External Clock / Low Power Crystal Oscillator is selected by fuse bits, the SUT fuses are copied into these bits. The firmware can modify these bits by writing a new value. This value will be used at the next start of the External Clock / Low Power Crystal Oscillator. • Bit 3 – RCE: Enable RC Oscillator The RCE bit must be written to logic one to enable the RC Oscillator. The RCE bit must be written to logic zero to disable the RC Oscillator. • Bit 2 – EXTE: Enable External Clock / Low Power Crystal Oscillator The OSCE bit must be written to logic one to enable External Clock / Low Power Crystal Oscillator. The OSCE bit must be written to logic zero to disable the External Clock / Low Power Crystal Oscillator. • Bit 0 – CLKS: Clock Selector The CLKS bit must be written to logic one to select the External Clock / Low Power Crystal Oscillator as CPU clock. The CLKS bit must be written to logic zero to select the RC Oscillator as CPU clock. After a reset, the CLKS bit is set by hardware if the External Clock / Low Power Crystal Oscillator is selected by the fuse bits con- figuration. The firmware has to check if the clock is correctly started before selected it. 6.11.2 CLKSEL1 – Clock Selection Register 1 Bit 7 6 5 4 3 2 1 0 RCCKSEL3 RCCKSEL2 RCCKSEL1 RCCKSEL0 EXCKSEL3 EXCKSEL2 EXCKSEL1 EXCKSEL0 CLKSEL1 Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 1 0 0 0 0 0 • Bit 7-4 – RCCKSEL[3:0]: CKSEL for RC oscillator Clock configuration for the RC Oscillator. After a reset, this part of the register is loaded with the 0010b value that corresponds to the RC oscillator. Modifying this value by firmware before switching to RC oscillator is pro- hibited because the RC clock will not start. • Bit 3-0 – EXCKSEL[3:0]: CKSEL for External Clock / Low Power Crystal Oscillator Clock configuration for the External Clock / Low Power Crystal Oscillator. After a reset, if the External Clock / Low Power Crystal Oscillator is selected by fuse bits, this part of the register is loaded with the fuse configura- tion. Firmware can modify it to change the start-up time after the clock switch. See Table6-1 on page28 for EXCKSEL[3:0] configuration. Only Low Power Crystal Oscillator, Calibrated Inter- nal RC Oscillator, and External Clock modes are allowed. ATmega16U4/32U4 [DATASHEET] 38 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

6.11.3 CLKSTA – Clock Status Register Bit 7 6 5 4 3 2 1 0 - - - - - - RCON EXTON CLKSTA Read/Write R R R R R R R R Initial Value 0 0 0 0 0 • Bit 7-2 - Reserved bits These bits are reserved and will always read as zero. • Bit 1 – RCON: RC Oscillator On This bit is set by hardware to one if the RC Oscillator is running. This bit is set by hardware to zero if the RC Oscillator is stopped. • Bit 0 – EXTON: External Clock / Low Power Crystal Oscillator On This bit is set by hardware to one if the External Clock / Low Power Crystal Oscillator is running. This bit is set by hardware to zero if the External Clock / Low Power Crystal Oscillator is stopped. 6.11.4 CLKPR – Clock Prescaler Register Bit 7 6 5 4 3 2 1 0 CLKPCE – – – CLKPS3 CLKPS2 CLKPS1 CLKPS0 CLKPR Read/Write R/W R R R R/W R/W R/W R/W Initial Value 0 0 0 0 See Bit Description • Bit 7 – CLKPCE: Clock Prescaler Change Enable The CLKPCE bit must be written to logic one to enable change of the CLKPS bits. The CLKPCE bit is only updated when the other bits in CLKPR are simultaneously written to zero. CLKPCE is cleared by hardware four cycles after it is written or when CLKPS bits are written. Rewriting the CLKPCE bit within this time-out period does neither extend the time-out period, nor clear the CLKPCE bit. • Bits 3..0 – CLKPS[3..0]: Clock Prescaler Select Bits 3 - 0 These bits define the division factor between the selected clock source and the internal system clock. These bits can be written run-time to vary the clock frequency to suit the application requirements. As the divider divides the master clock input to the MCU, the speed of all synchronous peripherals is reduced when a division factor is used. The division factors are given in the table below. The CKDIV8 Fuse determines the initial value of the CLKPS bits. If CKDIV8 is unprogrammed, the CLKPS bits will be reset to “0000”. If CKDIV8 is programmed, CLKPS bits are reset to “0011”, giving a division factor of 8 at start up. This feature should be used if the selected clock source has a higher frequency than the maximum frequency of the device at the present operating conditions. Note that any value can be written to the CLKPS bits regardless of the CKDIV8 Fuse setting. The Application software must ensure that a sufficient division factor is chosen if the selected clock source has a higher frequency than the maximum frequency of the device at the present operating conditions. The device is shipped with the CKDIV8 Fuse programmed. ATmega16U4/32U4 [DATASHEET] 39 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 6-10. Clock Prescaler Select CLKPS3 CLKPS2 CLKPS1 CLKPS0 Clock Division Factor 0 0 0 0 1 0 0 0 1 2 0 0 1 0 4 0 0 1 1 8 0 1 0 0 16 0 1 0 1 32 0 1 1 0 64 0 1 1 1 128 1 0 0 0 256 1 0 0 1 Reserved 1 0 1 0 Reserved 1 0 1 1 Reserved 1 1 0 0 Reserved 1 1 0 1 Reserved 1 1 1 0 Reserved 1 1 1 1 Reserved 6.11.5 PLL Control and Status Register – PLLCSR Bit 7 6 5 4 3 2 1 0 $29 ($29) PINDIV PLLE PLOCK PLLCSR Read/Write R R R R/W R R R/W R Initial Value 0 0 0 0 0 0 0 0 • Bit 7:5 – Res: Reserved Bits These bits are reserved and always read as zero. • Bit 4 – PINDIV PLL Input Prescaler (1:1, 1:2) These bits allow to configure the PLL input prescaler to generate the 8MHz input clock for the PLL from either a 8 or 16MHz input. When using a 8MHz clock source, this bit must be set to 0 before enabling PLL (1:1). When using a 16MHz clock source, this bit must be set to 1 before enabling PLL (1:2). • Bit 3:2 – Res: Reserved Bits These bits are reserved and always read as zero. ATmega16U4/32U4 [DATASHEET] 40 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 1 – PLLE: PLL Enable When the PLLE is set, the PLL is started. Note that the Calibrated 8MHz Internal RC oscillator is automatically enabled when the PLLE bit is set and with PINMUX (see PLLFRQ register) is set. The PLL must be disabled before entering Power down mode in order to stop Internal RC Oscillator and avoid extra-consumption. • Bit 0 – PLOCK: PLL Lock Detector When the PLOCK bit is set, the PLL is locked to the reference clock. After the PLL is enabled, it takes about several ms for the PLL to lock. To clear PLOCK, clear PLLE. 6.11.6 PLL Frequency Control Register – PLLFRQ Bit 7 6 5 4 3 2 1 0 $32 PINMUX PLLUSB PLLTM1 PLLTM0 PDIV3 PDIV2 PDIV1 PDIV0 PLLFRQ Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 1 0 0 • Bit 7– PINMUX: PLL Input Multiplexer This bit selects the clock input of the PLL: ̶ PINMUX = 0: the PLL input is connected to the PLL Prescaler, that has the Primary System Clock as source ̶ PINMUX = 1: the PLL input is directly connected to the Internal Calibrated 8MHz RC Oscillator. This mode allows to work in USB Low Speed mode with no crystal or using a crystal with a value different of 8/16MHz. • Bit 6– PLLUSB: PLL Postcaler for USB Peripheral This bit select the division factor between the PLL output frequency and the USB module input frequency: ̶ PLLUSB = 0: no division, direct connection (if PLL Output = 48MHz) ̶ PLLUSB = 1: PLL Output frequency is divided by two and sent to USB module (ifPLLOutput=96MHz) • Bit 5:4 – PLLTM1:0: PLL Postcaler for High Speed Timer These bits codes for the division factor between the PLL Output Frequency and the High Speed Timer input frequency. Note that the division factor 1.5 will introduce some jitter in the clock, but keeping the error null since the aver- age duty cycle is 50%. See Figures 6-7 for more details. PLLTM1 PLLTM0 PLL Postcaler Factor for High-Speed Timer 0 0 0 (Disconnected) 0 1 1 1 0 1.5 1 1 2 ATmega16U4/32U4 [DATASHEET] 41 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 6-7. PLL Postcaler operation with division factor = 1.5 Fi 2 Fi x --- 3 • Bit 3:0 – PDIV3:0 PLL Lock Frequency These bits configure the PLL internal VCO clock reference according to the required output frequency value. PDIV3 PDIV2 PDIV1 PDIV0 PLL Output Frequency 0 0 0 0 Not allowed 0 0 0 1 Not allowed 0 0 1 0 Not allowed 0 0 1 1 40MHz 0 1 0 0 48MHz 0 1 0 1 56MHz 0 1 1 0 Not allowed 0 1 1 1 72MHz 1 0 0 0 80MHz 1 0 0 1 88MHz 1 0 1 0 96MHz 1 0 1 1 Not allowed 1 1 0 0 Not allowed 1 1 0 1 Not allowed 1 1 1 0 Not allowed 1 1 1 1 Not allowed The optimal PLL configuration at 5V is: PLL output frequency = 96MHz, divided by 1.5 to generate the 64MHz High Speed Timer clock, and divided by 2 to generate the 48MHz USB clock. ATmega16U4/32U4 [DATASHEET] 42 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

7. Power Management and Sleep Modes Sleep modes enable the application to shut down unused modules in the MCU, thereby saving power. The AVR provides various sleep modes allowing the user to tailor the power consumption to the application’s requirements. To enter any of the five sleep modes, the SE bit in SMCR must be written to logic one and a SLEEP instruction must be executed. The SM2, SM1, and SM0 bits in the SMCR Register select which sleep mode (Idle, ADC Noise Reduction, Power-down, Power-save, or Standby) will be activated by the SLEEP instruction. See Table7-1 on page44 for a summary. If an enabled interrupt occurs while the MCU is in a sleep mode, the MCU wakes up. The MCU is then halted for four cycles in addition to the start-up time, executes the interrupt routine, and resumes execution from the instruction following SLEEP. The contents of the Register File and SRAM are unaltered when the device wakes up from sleep. If a reset occurs during sleep mode, the MCU wakes up and executes from the Reset Vector. Figure 6-1 on page 27 presents the different clock systems in the ATmega16U4/ATmega32U4, and their distribution. The figure is helpful in selecting an appropriate sleep mode. 7.1 Idle Mode When the SM2..0 bits are written to 000, the SLEEP instruction makes the MCU enter Idle mode, stopping the CPU but allowing the USB, SPI, USART, Analog Comparator, ADC, 2-wire Serial Interface, Timer/Counters, Watchdog, and the interrupt system to continue operating. This sleep mode basically halts clk and clk , CPU FLASH while allowing the other clocks to run. Idle mode enables the MCU to wake up from external triggered interrupts as well as internal ones like the Timer Overflow and USART Transmit Complete interrupts. If wake-up from the Analog Comparator interrupt is not required, the Analog Comparator can be powered down by setting the ACD bit in the Analog Comparator Control and Status Register – ACSR. This will reduce power consumption in Idle mode. If the ADC is enabled, a conversion starts automatically when this mode is entered. 7.2 ADC Noise Reduction Mode When the SM2..0 bits are written to 001, the SLEEP instruction makes the MCU enter ADC Noise Reduction mode, stopping the CPU but allowing the ADC, the external interrupts, 2-wire Serial Interface address match and the Watchdog to continue operating (if enabled). This sleep mode basically halts clkI/O, clkCPU, and clkFLASH, while allowing the other clocks to run (including clkUSB). This improves the noise environment for the ADC, enabling higher resolution measurements. If the ADC is enabled, a conversion starts automatically when this mode is entered. Apart form the ADC Conversion Complete interrupt, only an External Reset, a Watchdog System Reset, a Watchdog interrupt, a Brown-out Reset, a 2-wire serial interface interrupt, an SPM/EEPROM ready interrupt, an external level interrupt on INT6, an external interrupt on INT3:0 or a pin change interrupt can wake up the MCU from ADC Noise Reduction mode. 7.3 Power-down Mode When the SM2..0 bits are written to 010, the SLEEP instruction makes the MCU enter Power-down mode. In this mode, the external Oscillator is stopped, while the external interrupts, the 2-wire Serial Interface, and the Watchdog continue operating (if enabled). Only an External Reset, a Watchdog Reset, a Brown-out Reset, 2- wire Serial Interface address match, an external level interrupt on INT6, an external interrupt on INT3:0, a pin change interrupt or an asynchronous USB interrupt sources (VBUSTI, WAKEUPI), can wake up the MCU. This sleep mode basically halts all generated clocks, allowing operation of asynchronous modules only. ATmega16U4/32U4 [DATASHEET] 43 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Note that if a level triggered interrupt is used for wake-up from Power-down mode, the changed level must be held for some time to wake up the MCU. Refer to “External Interrupts” on page88 for details. When waking up from Power-down mode, there is a delay from the wake-up condition occurs until the wake-up becomes effective. This allows the clock to restart and become stable after having been stopped. The wake-up period is defined by the same CKSEL Fuses that define the Reset Time-out period, as described in “Clock Sources” on page28. 7.4 Power-save Mode When the SM2..0 bits are written to 011, the SLEEP instruction makes the MCU enter Power-save mode. For compatibility reasons with AT90USB64/128 this mode is still present but since Timer 2 Asynchronous operation is not present here, this mode is identical to Power-down. 7.5 Standby Mode When the SM2..0 bits are 110 and an external crystal/resonator clock option is selected, the SLEEP instruction makes the MCU enter Standby mode. This mode is identical to Power-down with the exception that the Oscillator is kept running. From Standby mode, the device wakes up in six clock cycles. 7.6 Extended Standby Mode When the SM2..0 bits are 111 and an external crystal/resonator clock option is selected, the SLEEP instruction makes the MCU enter Extended Standby mode. For compatibility reasons with AT90USB64/128 this mode is still present but since Timer 2 Asynchronous operation is not present here, this mode is identical to Standby- mode. Table 7-1. Active Clock Domains and Wake-up Sources in the Different Sleep Modes Active Clock Domains Oscillators Wake-up Sources s s u d y ou no Sleep Mode clkCPU clkFLASH clkIO clkADC Main ClockSource Enabled INT6, INT3:0 anPin Change TWI Address Match SPM/EEPROM Read ADC WDT Interrupt Other I/O USB Synchron Interrupts USB Asynchro(3)Interrupts Idle X X X X X X X X X X X ADCNRM X X X(2) X X X X X X Power-down X(2) X X X Power-save X(2) X X X Standby(1) X X(2) X X X Extended X X(2) X X X Standby Notes: 1. Only recommended with external crystal or resonator selected as clock source. 2. For INT6, only level interrupt. 3. Asynchronous USB interrupts are VBUSTI and WAKEUPI. ATmega16U4/32U4 [DATASHEET] 44 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

7.7 Power Reduction Register The Power Reduction Register, PRR, provides a method to stop the clock to individual peripherals to reduce power consumption. The current state of the peripheral is frozen and the I/O registers can not be read or written. Resources used by the peripheral when stopping the clock will remain occupied, hence the peripheral should in most cases be disabled before stopping the clock. Waking up a module, which is done by clearing the bit in PRR, puts the module in the same state as before shutdown. Module shutdown can be used in Idle mode and Active mode to significantly reduce the overall power consumption. In all other sleep modes, the clock is already stopped. 7.8 Minimizing Power Consumption There are several issues to consider when trying to minimize the power consumption in an AVR controlled system. In general, sleep modes should be used as much as possible, and the sleep mode should be selected so that as few as possible of the device’s functions are operating. All functions not needed should be disabled. In particular, the following modules may need special consideration when trying to achieve the lowest possible power consumption. 7.8.1 Analog to Digital Converter If enabled, the ADC will be enabled in all sleep modes. To save power, the ADC should be disabled before entering any sleep mode. When the ADC is turned off and on again, the next conversion will be an extended conversion. Refer to “Analog to Digital Converter - ADC” on page297 for details on ADC operation. 7.8.2 Analog Comparator When entering Idle mode, the Analog Comparator should be disabled if not used. When entering ADC Noise Reduction mode, the Analog Comparator should be disabled. In other sleep modes, the Analog Comparator is automatically disabled. However, if the Analog Comparator is set up to use the Internal Voltage Reference as input, the Analog Comparator should be disabled in all sleep modes. Otherwise, the Internal Voltage Reference will be enabled, independent of sleep mode. Refer to “Analog Comparator” on page293 for details on how to configure the Analog Comparator. 7.8.3 Brown-out Detector If the Brown-out Detector is not needed by the application, this module should be turned off. If the Brown-out Detector is enabled by the BODLEVEL Fuses, it will be enabled in all sleep modes, and hence, always consume power. In the deeper sleep modes, this will contribute significantly to the total current consumption. Refer to “Brown-out Detection” on page52 for details on how to configure the Brown-out Detector. 7.8.4 Internal Voltage Reference The Internal Voltage Reference will be enabled when needed by the Brown-out Detection, the Analog Comparator or the ADC. If these modules are disabled as described in the sections above, the internal voltage reference will be disabled and it will not be consuming power. When turned on again, the user must allow the reference to start up before the output is used. If the reference is kept on in sleep mode, the output can be used immediately. Refer to “Internal Voltage Reference” on page54 for details on the start-up time. 7.8.5 Watchdog Timer If the Watchdog Timer is not needed in the application, the module should be turned off. If the Watchdog Timer is enabled, it will be enabled in all sleep modes, and hence, always consume power. In the deeper sleep modes, this will contribute significantly to the total current consumption. Refer to “Interrupts” on page63 for details on how to configure the Watchdog Timer. ATmega16U4/32U4 [DATASHEET] 45 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

7.8.6 Port Pins When entering a sleep mode, all port pins should be configured to use minimum power. The most important is then to ensure that no pins drive resistive loads. In sleep modes where both the I/O clock (clk ) and the ADC I/O clock (clk ) are stopped, the input buffers of the device will be disabled. This ensures that no power is ADC consumed by the input logic when not needed. In some cases, the input logic is needed for detecting wake-up conditions, and it will then be enabled. Refer to the section “Digital Input Enable and Sleep Modes” on page71 for details on which pins are enabled. If the input buffer is enabled and the input signal is left floating or have an analog signal level close to V /2, the input buffer will use excessive power. CC For analog input pins, the digital input buffer should be disabled at all times. An analog signal level close to V /2 on an input pin can cause significant current even in active mode. Digital input buffers can be disabled by CC writing to the Digital Input Disable Registers (DIDR1 and DIDR0). Refer to “Digital Input Disable Register 1 – DIDR1” on page296 and “Digital Input Disable Register 1 – DIDR1” on page296 for details. 7.8.7 On-chip Debug System If the On-chip debug system is enabled by the OCDEN Fuse and the chip enters sleep mode, the main clock source is enabled, and hence, always consumes power. In the deeper sleep modes, this will contribute significantly to the total current consumption. There are three alternative ways to disable the OCD system:  Disable the OCDEN Fuse  Disable the JTAGEN Fuse  Write one to the JTD bit in MCUCR ATmega16U4/32U4 [DATASHEET] 46 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

7.9 Register Description 7.9.1 Sleep Mode Control Register – SMCR The Sleep Mode Control Register contains control bits for power management. Bit 7 6 5 4 3 2 1 0 – – – – SM2 SM1 SM0 SE SMCR Read/Write R R R R R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bits 3, 2, 1 – SM2..0: Sleep Mode Select Bits 2, 1, and 0 These bits select between the six available sleep modes as shown in Table 7-2. Table 7-2. Sleep Mode Select SM2 SM1 SM0 Sleep Mode 0 0 0 Idle 0 0 1 ADC Noise Reduction 0 1 0 Power-down 0 1 1 Power-save 1 0 0 Reserved 1 0 1 Reserved 1 1 0 Standby(1) 1 1 1 Extended Standby(1) Note: 1. Standby modes are only recommended for use with external crystals or resonators. • Bit 0 – SE: Sleep Enable The SE bit must be written to logic one to make the MCU enter the sleep mode when the SLEEP instruction is executed. To avoid the MCU entering the sleep mode unless it is the programmer’s purpose, it is recommended to write the Sleep Enable (SE) bit to one just before the execution of the SLEEP instruction and to clear it immediately after waking up. 7.9.2 Power Reduction Register 0 - PRR0 Bit 7 6 5 4 3 2 1 0 PRTWI – PRTIM0 – PRTIM1 PRSPI – PRADC PRR0 Read/Write R/W R R/W R R/W R/W R R/W Initial Val- 0 0 0 0 0 0 0 0 ue • Bit 7 - PRTWI: Power Reduction TWI Writing a logic one to this bit shuts down the TWI by stopping the clock to the module. When waking up the TWI again, the TWI should be re initialized to ensure proper operation. • Bit 6 - Res: Reserved bit This bits is reserved and will always read as zero. ATmega16U4/32U4 [DATASHEET] 47 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 5 - PRTIM0: Power Reduction Timer/Counter0 Writing a logic one to this bit shuts down the Timer/Counter0 module. When the Timer/Counter0 is enabled, operation will continue like before the shutdown. • Bit 4 - Res: Reserved bit This bit is reserved and will always read as zero. • Bit 3 - PRTIM1: Power Reduction Timer/Counter1 Writing a logic one to this bit shuts down the Timer/Counter1 module. When the Timer/Counter1 is enabled, operation will continue like before the shutdown. • Bit 2 - PRSPI: Power Reduction Serial Peripheral Interface Writing a logic one to this bit shuts down the Serial Peripheral Interface by stopping the clock to the module. When waking up the SPI again, the SPI should be re initialized to ensure proper operation. • Bit 1 - Res: Reserved bit These bits are reserved and will always read as zero. • Bit 0 - PRADC: Power Reduction ADC Writing a logic one to this bit shuts down the ADC. The ADC must be disabled before shut down. The analog comparator cannot use the ADC input MUX when the ADC is shut down. 7.9.3 Power Reduction Register 1 - PRR1 Bit 7 6 5 4 3 2 1 0 PRUSB – – PRTIM4 PRTIM3 – – PRUSART1 PRR1 Read/Writ R/W R R R R/W R R R/W e Initial Val- 0 0 0 0 0 0 0 0 ue • Bit 7 - PRUSB: Power Reduction USB Writing a logic one to this bit shuts down the USB by stopping the clock to the module. When waking up the USB again, the USB should be re initialized to ensure proper operation. • Bit 6..5 - Res: Reserved bits These bits are reserved and will always read as zero. • Bit 4- PRTIM4: Power Reduction Timer/Counter4 Writing a logic one to this bit shuts down the Timer/Counter4 module. When the Timer/Counter4 is enabled, operation will continue like before the shutdown. • Bit 3 - PRTIM3: Power Reduction Timer/Counter3 Writing a logic one to this bit shuts down the Timer/Counter3 module. When the Timer/Counter3 is enabled, operation will continue like before the shutdown. • Bit 2..1 - Res: Reserved bits These bits are reserved and will always read as zero. ATmega16U4/32U4 [DATASHEET] 48 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 0 - PRUSART1: Power Reduction USART1 Writing a logic one to this bit shuts down the USART1 by stopping the clock to the module. When waking up the USART1 again, the USART1 should be re initialized to ensure proper operation. ATmega16U4/32U4 [DATASHEET] 49 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

8. System Control and Reset 8.1 Resetting the AVR During reset, all I/O Registers are set to their initial values, and the program starts execution from the Reset Vector. The instruction placed at the Reset Vector must be a JMP – Absolute Jump – instruction to the reset handling routine. If the program never enables an interrupt source, the Interrupt Vectors are not used, and regular program code can be placed at these locations. This is also the case if the Reset Vector is in the Application section while the Interrupt Vectors are in the Boot section or vice versa. The circuit diagram in Figure 8-1 on page 51 shows the reset logic. Table29-3 on page386 defines the electrical parameters of the reset circuitry. The I/O ports of the AVR are immediately reset to their initial state when a reset source goes active. This does not require any clock source to be running. After all reset sources have gone inactive, a delay counter is invoked, stretching the internal reset. This allows the power to reach a stable level before normal operation starts. The time-out period of the delay counter is defined by the user through the SUT and CKSEL Fuses. The different selections for the delay period are presented in “Clock Sources” on page28. 8.2 Reset Sources The ATmega16U4/ATmega32U4 has five sources of reset:  Power-on Reset. The MCU is reset when the supply voltage is below the Power-on Reset threshold (V ). POT  External Reset. The MCU is reset when a low level is present on the RESET pin for longer than the minimum pulse length.  Watchdog Reset. The MCU is reset when the Watchdog Timer period expires and the Watchdog is enabled.  Brown-out Reset. The MCU is reset when the supply voltage V is below the Brown-out Reset threshold CC (V ) and the Brown-out Detector is enabled. BOT  JTAG AVR Reset. The MCU is reset as long as there is a logic one in the Reset Register, one of the scan chains of the JTAG system. Refer to the section “IEEE 1149.1 (JTAG) Boundary-scan” on page325 for details.  USB End of Reset. The MCU is reset (excluding the USB controller that remains enabled and attached) on the detection of a USB End of Reset condition on the bus, if this feature is enabled by the user. ATmega16U4/32U4 [DATASHEET] 50 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 8-1. Reset Logic DATA BUS MCU Status Register (MCUSR) FFFFFF RRRRRR OOTDTB Power-on Reset PBEXWJUS Circuit Brown-out BODLEVEL [2..0] Reset Circuit Pull-up Resistor SPIKE FILTER JTAG Reset USB Reset Register Detection Watchdog Oscillator Clock CK Delay Counters Generator TIMEOUT CKSEL[3:0] SUT[1:0] 8.3 Power-on Reset A Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The detection level is defined in Table8-1 on page53. The POR is activated whenever V is below the detection level. The POR circuit can be CC used to trigger the start-up Reset, as well as to detect a failure in supply voltage. A Power-on Reset (POR) circuit ensures that the device is reset from Power-on. Reaching the Power-on Reset threshold voltage invokes the delay counter, which determines how long the device is kept in RESET after V CC rise. The RESET signal is activated again, without any delay, when V decreases below the detection level. CC Figure 8-2. MCU Start-up, RESET Tied to V CC V V V POR POT CC V RESET RST t TIME-OUT TOUT INTERNAL RESET ATmega16U4/32U4 [DATASHEET] 51 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 8-3. MCU Start-up, RESET Extended Externally V V POR POT V CC V RESET RST t TIME-OUT TOUT INTERNAL RESET 8.4 External Reset An External Reset is generated by a low level on the RESET pin. Reset pulses longer than the minimum pulse width (see Table29-3 on page386) will generate a reset, even if the clock is not running. Shorter pulses are not guaranteed to generate a reset. When the applied signal reaches the Reset Threshold Voltage – V – on its RST positive edge, the delay counter starts the MCU after the Time-out period – t –has expired. TOUT Figure 8-4. External Reset During Operation CC 8.5 Brown-out Detection ATmega16U4/ATmega32U4 has an On-chip Brown-out Detection (BOD) circuit for monitoring the V level CC during operation by comparing it to a fixed trigger level. The trigger level for the BOD can be selected by the BODLEVEL Fuses. The trigger level has a hysteresis to ensure spike free Brown-out Detection. The hysteresis on the detection level should be interpreted as V = V + V /2 and V = V - V /2. BOT+ BOT HYST BOT- BOT HYST ATmega16U4/32U4 [DATASHEET] 52 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 8-1. BODLEVEL Fuse Coding BODLEVEL 2..0 Fuses Min. V Typ. V Max. V Units BOT BOT BOT 111 BOD Disabled 110 1.8 2.0 2.2 101 2.0 2.2 2.4 100 2.2 2.4 2.6 011 2.4 2.6 2.8 V 010 3.2 3.4 3.6 001 3.3 3.5 3.7 000 4.0 4.3 4.5 Table 8-2. BOD characteristics Symbol Parameter Min. Typ. Max. Units V Brown-out Detector Hysteresis 50 mV HYST t Min Pulse Width on Brown-out Reset ns BOD When the BOD is enabled, and V decreases to a value below the trigger level (V in Figure 8-5), the CC BOT- Brown-out Reset is immediately activated. When V increases above the trigger level (V in Figure 8-5), the CC BOT+ delay counter starts the MCU after the Time-out period t has expired. TOUT The BOD circuit will only detect a drop in V if the voltage stays below the trigger level for longer than t CC BOD given in Table29-3 on page386. Figure 8-5. Brown-out Reset During Operation VCC V VBOT+ BOT- RESET TIME-OUT tTOUT INTERNAL RESET 8.6 Watchdog Reset When the Watchdog times out, it will generate a short reset pulse of one CK cycle duration. On the falling edge of this pulse, the delay timer starts counting the Time-out period t . For details on operation of the Watchdog TOUT Timer, see “Watchdog Timer” on page 55. ATmega16U4/32U4 [DATASHEET] 53 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 8-6. Watchdog Reset During Operation CC CK 8.7 USB Reset When the USB controller is enabled and configured with the USB Reset CPU feature enabled and if a valid USB Reset signalling is detected on the bus, the CPU core is reset but the USB controller remains enabled and attached. This feature may be used to enhance device reliability. Figure 8-7. USB Reset During Operation CC t End of Reset s) USBRSTMIN e DP n Li USB Traffic USB Traffic B DM S U ( 8.8 Internal Voltage Reference ATmega16U4/ATmega32U4 features an internal bandgap reference. This reference is used for Brown-out Detection, and it can be used as an input to the Analog Comparator or the ADC. 8.8.1 Voltage Reference Enable Signals and Start-up Time The voltage reference has a start-up time that may influence the way it should be used. The start-up time is given in Table8-3 on page55. To save power, the reference is not always turned on. The reference is on during the following situations: 1. When the BOD is enabled (by programming the BODLEVEL [2..0] Fuse). 2. When the bandgap reference is connected to the Analog Comparator (by setting the ACBG bit in ACSR). 3. When the ADC is enabled. Thus, when the BOD is not enabled, after setting the ACBG bit or enabling the ADC, the user must always allow the reference to start up before the output from the Analog Comparator or ADC is used. To reduce power ATmega16U4/32U4 [DATASHEET] 54 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

consumption in Power-down mode, the user can avoid the three conditions above to ensure that the reference is turned off before entering Power-down mode. Table 8-3. Internal Voltage Reference Characteristics Symbol Parameter Condition Min. Typ. Max. Units V =2.7 CC V Bandgap reference voltage 1.0 1.1 1.2 V BG T =25°C A V =2.7 CC t Bandgap reference start-up time 40 70 µs BG T =25°C A V =2.7 CC I Bandgap reference current consumption 10 µA BG T =25°C A 8.9 Watchdog Timer ATmega16U4/ATmega32U4 has an Enhanced Watchdog Timer (WDT). The main features are: • Clocked from separate On-chip Oscillator • Three Operating modes – Interrupt – System Reset – Interrupt and System Reset • Selectable Time-out period from 16ms to 8s • Possible Hardware fuse Watchdog always on (WDTON) for fail-safe mode Figure 8-8. Watchdog Timer 128kHz OSCILLATOR KKKKKKKKKK 2486248624 OSC/OSC/OSC/OSC/1OSC/3OSC/6OSC/12OSC/25OSC/51SC/102 O WDP0 WDP1 WATCHDOG WDP2 RESET WDP3 WDE MCU RESET WDIF INTERRUPT WDIE The Watchdog Timer (WDT) is a timer counting cycles of a separate on-chip 128kHz oscillator. The WDT gives an interrupt or a system reset when the counter reaches a given time-out value. In normal operation mode, it is required that the system uses the WDR - Watchdog Timer Reset - instruction to restart the counter before the time-out value is reached. If the system doesn't restart the counter, an interrupt or system reset will be issued. In Interrupt mode, the WDT gives an interrupt when the timer expires. This interrupt can be used to wake the device from sleep-modes, and also as a general system timer. One example is to limit the maximum time allowed for certain operations, giving an interrupt when the operation has run longer than expected. In System Reset mode, the WDT gives a reset when the timer expires. This is typically used to prevent system hang-up in ATmega16U4/32U4 [DATASHEET] 55 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

case of runaway code. The third mode, Interrupt and System Reset mode, combines the other two modes by first giving an interrupt and then switch to System Reset mode. This mode will for instance allow a safe shutdown by saving critical parameters before a system reset. The Watchdog always on (WDTON) fuse, if programmed, will force the Watchdog Timer to System Reset mode. With the fuse programmed the System Reset mode bit (WDE) and Interrupt mode bit (WDIE) are locked to 1 and 0 respectively. To further ensure program security, alterations to the Watchdog set-up must follow timed sequences. The sequence for clearing WDE and changing time-out configuration is as follows: 1. In the same operation, write a logic one to the Watchdog change enable bit (WDCE) and WDE. A logic one must be written to WDE regardless of the previous value of the WDE bit. 2. Within the next four clock cycles, write the WDE and Watchdog prescaler bits (WDP) as desired, but with the WDCE bit cleared. This must be done in one operation. The following code example shows one assembly and one C function for turning off the Watchdog Timer. The example assumes that interrupts are controlled (e.g. by disabling interrupts globally) so that no interrupts will occur during the execution of these functions. ATmega16U4/32U4 [DATASHEET] 56 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Assembly Code Example(1) WDT_off: ; Turn off global interrupt cli ; Reset Watchdog Timer wdr ; Clear WDRF in MCUSR in r16, MCUSR andi r16, (0xff & (0<<WDRF)) out MCUSR, r16 ; Write logical one to WDCE and WDE ; Keep old prescaler setting to prevent unintentional time-out in r16, WDTCSR ori r16, (1<<WDCE) | (1<<WDE) out WDTCSR, r16 ; Turn off WDT ldi r16, (0<<WDE) out WDTCSR, r16 ; Turn on global interrupt sei ret C Code Example(1) void WDT_off(void) { __disable_interrupt(); __watchdog_reset(); /* Clear WDRF in MCUSR */ MCUSR &= ~(1<<WDRF); /* Write logical one to WDCE and WDE */ /* Keep old prescaler setting to prevent unintentional time-out */ WDTCSR |= (1<<WDCE) | (1<<WDE); /* Turn off WDT */ WDTCSR = 0x00; __enable_interrupt(); } Note: 1. The example code assumes that the part specific header file is included. Note: If the Watchdog is accidentally enabled, for example by a runaway pointer or brown-out condition, the device will be reset and the Watchdog Timer will stay enabled. If the code is not set up to handle the Watchdog, this might lead to an eternal loop of time-out resets. To avoid this situation, the application software should always clear the Watchdog System Reset Flag (WDRF) and the WDE control bit in the initialization routine, even if the Watchdog is not in use. The following code example shows one assembly and one C function for changing the time-out value of the Watchdog Timer. ATmega16U4/32U4 [DATASHEET] 57 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Assembly Code Example(1) WDT_Prescaler_Change: ; Turn off global interrupt cli ; Reset Watchdog Timer wdr ; Start timed sequence in r16, WDTCSR ori r16, (1<<WDCE) | (1<<WDE) out WDTCSR, r16 ; -- Got four cycles to set the new values from here - ; Set new prescaler(time-out) value = 64K cycles (~0.5 s) ldi r16, (1<<WDE) | (1<<WDP2) | (1<<WDP0) out WDTCSR, r16 ; -- Finished setting new values, used 2 cycles - ; Turn on global interrupt sei ret C Code Example(1) void WDT_Prescaler_Change(void) { __disable_interrupt(); __watchdog_reset(); /* Start timed sequence */ WDTCSR |= (1<<WDCE) | (1<<WDE); /* Set new prescaler(time-out) value = 64K cycles (~0.5 s) */ WDTCSR = (1<<WDE) | (1<<WDP2) | (1<<WDP0); __enable_interrupt(); } Note: 1. The example code assumes that the part specific header file is included. Note: The Watchdog Timer should be reset before any change of the WDP bits, since a change in the WDP bits can result in a time-out when switching to a shorter time-out period. ATmega16U4/32U4 [DATASHEET] 58 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

8.10 Register Description 8.11 MCU Status Register – MCUSR The MCU Status Register provides information on which reset source caused an MCU reset. Bit 7 6 5 4 3 2 1 0 – – USBRF JTRF WDRF BORF EXTRF PORF MCUSR Read/Write R R R R/W R/W R/W R/W R/W Initial Value 0 0 0 See Bit Description • Bit 7..6 - Reserved These bits are reserved and should be read as 0. Do not set these bits. • Bit 5– USBRF: USB Reset Flag This bit is set if a reset is being caused by a logic one in the JTAG Reset Register selected by the JTAG instruction AVR_RESET. This bit is reset by a Power-on Reset, or by writing a logic zero to the flag. • Bit 4 – JTRF: JTAG Reset Flag This bit is set if a reset is being caused by a logic one in the JTAG Reset Register selected by the JTAG instruction AVR_RESET. This bit is reset by a Power-on Reset, or by writing a logic zero to the flag. • Bit 3 – WDRF: Watchdog Reset Flag This bit is set if a Watchdog Reset occurs. The bit is reset by a Power-on Reset, or by writing a logic zero to the flag. • Bit 2 – BORF: Brown-out Reset Flag This bit is set if a Brown-out Reset occurs. The bit is reset by a Power-on Reset, or by writing a logic zero to the flag. • Bit 1 – EXTRF: External Reset Flag This bit is set if an External Reset occurs. The bit is reset by a Power-on Reset, or by writing a logic zero to the flag. • Bit 0 – PORF: Power-on Reset Flag This bit is set if a Power-on Reset occurs. The bit is reset only by writing a logic zero to the flag. To make use of the Reset Flags to identify a reset condition, the user should read and then Reset the MCUSR as early as possible in the program. If the register is cleared before another reset occurs, the source of the reset can be found by examining the Reset Flags. ATmega16U4/32U4 [DATASHEET] 59 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

8.11.1 Watchdog Timer Control Register - WDTCSR Bit 7 6 5 4 3 2 1 0 WDIF WDIE WDP3 WDCE WDE WDP2 WDP1 WDP0 WDTCSR Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 X 0 0 0 • Bit 7 - WDIF: Watchdog Interrupt Flag This bit is set when a time-out occurs in the Watchdog Timer and the Watchdog Timer is configured for interrupt. WDIF is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, WDIF is cleared by writing a logic one to the flag. When the I-bit in SREG and WDIE are set, the Watchdog Time-out Interrupt is executed. • Bit 6 - WDIE: Watchdog Interrupt Enable When this bit is written to one and the I-bit in the Status Register is set, the Watchdog Interrupt is enabled. If WDE is cleared in combination with this setting, the Watchdog Timer is in Interrupt Mode, and the corresponding interrupt is executed if time-out in the Watchdog Timer occurs. If WDE is set, the Watchdog Timer is in Interrupt and System Reset Mode. The first time-out in the Watchdog Timer will set WDIF. Executing the corresponding interrupt vector will clear WDIE and WDIF automatically by hardware (the Watchdog goes to System Reset Mode). This is useful for keeping the Watchdog Timer security while using the interrupt. To stay in Interrupt and System Reset Mode, WDIE must be set after each interrupt. This should however not be done within the interrupt service routine itself, as this might compromise the safety- function of the Watchdog System Reset mode. If the interrupt is not executed before the next time-out, a System Reset will be applied. Table 8-4. Watchdog Timer Configuration WDTON WDE WDIE Mode Action on Time-out 0 0 0 Stopped None 0 0 1 Interrupt Mode Interrupt 0 1 0 System Reset Mode Reset Interrupt and System Reset Interrupt, then go to System 0 1 1 Mode Reset Mode 1 x x System Reset Mode Reset • Bit 4 - WDCE: Watchdog Change Enable This bit is used in timed sequences for changing WDE and prescaler bits. To clear the WDE bit, and/or change the prescaler bits, WDCE must be set. Once written to one, hardware will clear WDCE after four clock cycles. • Bit 3 - WDE: Watchdog System Reset Enable WDE is overridden by WDRF in MCUSR. This means that WDE is always set when WDRF is set. To clear WDE, WDRF must be cleared first. This feature ensures multiple resets during conditions causing failure, and a safe start-up after the failure. ATmega16U4/32U4 [DATASHEET] 60 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 5, 2..0 - WDP3..0: Watchdog Timer Prescaler 3, 2, 1, and 0 The WDP3..0 bits determine the Watchdog Timer prescaling when the Watchdog Timer is running. The different prescaling values and their corresponding time-out periods are shown in the following table. Table 8-5. Watchdog Timer Prescale Select Number of WDT Oscillator Typical Time-out at WDP3 WDP2 WDP1 WDP0 Cycles V = 5.0V CC 0 0 0 0 2K (2048) cycles 16ms 0 0 0 1 4K (4096) cycles 32ms 0 0 1 0 8K (8192) cycles 64ms 0 0 1 1 16K (16384) cycles 0.125s 0 1 0 0 32K (32768) cycles 0.25s 0 1 0 1 64K (65536) cycles 0.5s 0 1 1 0 128K (131072) cycles 1.0s 0 1 1 1 256K (262144) cycles 2.0s 1 0 0 0 512K (524288) cycles 4.0s 1 0 0 1 1024K (1048576) cycles 8.0s 1 0 1 0 1 0 1 1 1 1 0 0 Reserved 1 1 0 1 1 1 1 0 1 1 1 1 ATmega16U4/32U4 [DATASHEET] 61 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

ATmega16U4/32U4 [DATASHEET] 62 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

9. Interrupts This chapter describes the specifics of the interrupt handling as performed in ATmega16U4/ATmega32U4. For a general explanation of the AVR interrupt handling, refer to “Reset and Interrupt Handling” on page15. 9.1 Interrupt Vectors in ATmega16U4/ATmega32U4 Table 9-1. Reset and Interrupt Vectors(cont’d) Vector Program No. Address(2) Source Interrupt Definition External Pin, Power-on Reset, Brown-out Reset, Watchdog 1 $0000(1) RESET Reset, and JTAG AVR Reset 2 $0002 INT0 External Interrupt Request 0 3 $0004 INT1 External Interrupt Request 1 4 $0006 INT2 External Interrupt Request 2 5 $0008 INT3 External Interrupt Request 3 6 $000A Reserved Reserved 7 $000C Reserved Reserved 8 $000E INT6 External Interrupt Request 6 9 $0010 Reserved Reserved 10 $0012 PCINT0 Pin Change Interrupt Request 0 11 $0014 USB General USB General Interrupt request 12 $0016 USB Endpoint USB Endpoint Interrupt request 13 $0018 WDT Watchdog Time-out Interrupt 14 $001A Reserved Reserved 15 $001C Reserved Reserved 16 $001E Reserved Reserved 17 $0020 TIMER1 CAPT Timer/Counter1 Capture Event 18 $0022 TIMER1 COMPA Timer/Counter1 Compare Match A 19 $0024 TIMER1 COMPB Timer/Counter1 Compare Match B 20 $0026 TIMER1 COMPC Timer/Counter1 Compare Match C 21 $0028 TIMER1 OVF Timer/Counter1 Overflow 22 $002A TIMER0 COMPA Timer/Counter0 Compare Match A 23 $002C TIMER0 COMPB Timer/Counter0 Compare match B 24 $002E TIMER0 OVF Timer/Counter0 Overflow 25 $0030 SPI (STC) SPI Serial Transfer Complete 26 $0032 USART1 RX USART1 Rx Complete 27 $0034 USART1 UDRE USART1 Data Register Empty ATmega16U4/32U4 [DATASHEET] 63 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 9-1. Reset and Interrupt Vectors(cont’d) Vector Program No. Address(2) Source Interrupt Definition 28 $0036 USART1TX USART1 Tx Complete 29 $0038 ANALOG COMP Analog Comparator 30 $003A ADC ADC Conversion Complete 31 $003C EE READY EEPROM Ready 32 $003E TIMER3 CAPT Timer/Counter3 Capture Event 33 $0040 TIMER3 COMPA Timer/Counter3 Compare Match A 34 $0042 TIMER3 COMPB Timer/Counter3 Compare Match B 35 $0044 TIMER3 COMPC Timer/Counter3 Compare Match C 36 $0046 TIMER3 OVF Timer/Counter3 Overflow 37 $0048 TWI 2-wire Serial Interface 38 $004A SPM READY Store Program Memory Ready 39 $004C TIMER4 COMPA Timer/Counter4 Compare Match A 40 $004E TIMER4 COMPB Timer/Counter4 Compare Match B 41 $0050 TIMER4 COMPD Timer/Counter4 Compare Match D 42 $0052 TIMER4 OVF Timer/Counter4 Overflow 43 $0054 TIMER4 FPF Timer/Counter4 Fault Protection Interrupt Notes: 1. When the BOOTRST Fuse is programmed, the device will jump to the Boot Loader address at reset, see “Memory Programming” on page353. 2. When the IVSEL bit in MCUCR is set, Interrupt Vectors will be moved to the start of the Boot Flash Section. The address of each Interrupt Vector will then be the address in this table added to the start address of the Boot Flash Section. The table shows reset and Interrupt Vectors placement for the various combinations of BOOTRST and IVSEL settings. If the program never enables an interrupt source, the Interrupt Vectors are not used, and regular program code can be placed at these locations. This is also the case if the Reset Vector is in the Application section while the Interrupt Vectors are in the Boot section or vice versa. Table 9-2. Reset and Interrupt Vectors Placement BOOTRST IVSEL Reset Address Interrupt Vectors Start Address 1 0 0x0000 0x0002 1 1 0x0000 Boot Reset Address + 0x0002 0 0 Boot Reset Address 0x0002 0 1 Boot Reset Address Boot Reset Address + 0x0002 Note: 1. The Boot Reset Address is shown in Table27-4 on page340. For the BOOTRST Fuse “1” means unprogrammed while “0” means programmed. 9.1.1 Moving Interrupts Between Application and Boot Space The General Interrupt Control Register controls the placement of the Interrupt Vector table. ATmega16U4/32U4 [DATASHEET] 64 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

9.2 Register Description 9.2.1 MCU Control Register – MCUCR Bit 7 6 5 4 3 2 1 0 JTD – – PUD – – IVSEL IVCE MCUCR Read/Write R/W R R R/W R R R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 1 – IVSEL: Interrupt Vector Select When the IVSEL bit is cleared (zero), the Interrupt Vectors are placed at the start of the Flash memory. When this bit is set (one), the Interrupt Vectors are moved to the beginning of the Boot Loader section of the Flash. The actual address of the start of the Boot Flash Section is determined by the BOOTSZ Fuses. Refer to the section “Memory Programming” on page353 for details. To avoid unintentional changes of Interrupt Vector tables, a special write procedure must be followed to change the IVSEL bit: 1. Write the Interrupt Vector Change Enable (IVCE) bit to one. 2. Within four cycles, write the desired value to IVSEL while writing a zero to IVCE. Interrupts will automatically be disabled while this sequence is executed. Interrupts are disabled in the cycle IVCE is set, and they remain disabled until after the instruction following the write to IVSEL. If IVSEL is not written, interrupts remain disabled for four cycles. The I-bit in the Status Register is unaffected by the automatic disabling. Note: If Interrupt Vectors are placed in the Boot Loader section and Boot Lock bit BLB02 is programmed, interrupts are disabled while executing from the Application section. If Interrupt Vectors are placed in the Application section and Boot Lock bit BLB12 is programed, interrupts are disabled while executing from the Boot Loader section. Refer to the section “Memory Programming” on page353 for details on Boot Lock bits. • Bit 0 – IVCE: Interrupt Vector Change Enable The IVCE bit must be written to logic one to enable change of the IVSEL bit. IVCE is cleared by hardware four cycles after it is written or when IVSEL is written. Setting the IVCE bit will disable interrupts, as explained in the IVSEL description above. See Code Example below. ATmega16U4/32U4 [DATASHEET] 65 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Assembly Code Example Move_interrupts: ; Enable change of Interrupt Vectors ldi r16, (1<<IVCE) out MCUCR, r16 ; Move interrupts to Boot Flash section ldi r16, (1<<IVSEL) out MCUCR, r16 ret C Code Example void Move_interrupts(void) { /* Enable change of Interrupt Vectors */ MCUCR = (1<<IVCE); /* Move interrupts to Boot Flash section */ MCUCR = (1<<IVSEL); } ATmega16U4/32U4 [DATASHEET] 66 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

10. I/O-Ports 10.1 Introduction All AVR ports have true Read-Modify-Write functionality when used as general digital I/O ports. This means that the direction of one port pin can be changed without unintentionally changing the direction of any other pin with the SBI and CBI instructions. The same applies when changing drive value (if configured as output) or enabling/disabling of pull-up resistors (if configured as input). Each output buffer has symmetrical drive characteristics with both high sink and source capability. The pin driver is strong enough to drive LED displays directly. All port pins have individually selectable pull-up resistors with a supply-voltage invariant resistance. All I/O pins have protection diodes to both V and Ground as indicated in Figure 10-1. Refer to “Electrical CC Characteristics” on page383 for a complete list of parameters. Figure 10-1. I/O Pin Equivalent Schematic All registers and bit references in this section are written in general form. A lower case “x” represents the numbering letter for the port, and a lower case “n” represents the bit number. However, when using the register or bit defines in a program, the precise form must be used. For example, PORTB3 for bit no. 3 in Port B, here documented generally as PORTxn. The physical I/O Registers and bit locations are listed in “” on page83. Three I/O memory address locations are allocated for each port, one each for the Data Register – PORTx, Data Direction Register – DDRx, and the Port Input Pins – PINx. The Port Input Pins I/O location is read only, while the Data Register and the Data Direction Register are read/write. However, writing a logic one to a bit in the PINx Register, will result in a toggle in the corresponding bit in the Data Register. In addition, the Pull-up Disable – PUD bit in MCUCR disables the pull-up function for all pins in all ports when set. Using the I/O port as General Digital I/O is described in “Ports as General Digital I/O” on page67. Most port pins are multiplexed with alternate functions for the peripheral features on the device. How each alternate function interferes with the port pin is described in “Alternate Port Functions” on page72. Refer to the individual module sections for a full description of the alternate functions. Note that enabling the alternate function of some of the port pins does not affect the use of the other pins in the port as general digital I/O. 10.2 Ports as General Digital I/O The ports are bi-directional I/O ports with optional internal pull-ups. Figure 10-2 shows a functional description of one I/O-port pin, here generically called Pxn. ATmega16U4/32U4 [DATASHEET] 67 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 10-2. General Digital I/O(1) PUD Q D DDxn QCLR WDx RESET RDx S U 1 B Pxn PQORTxDn 0 TA QCLR DA RESET WRx WPx SLEEP RRx SYNCHRONIZER RPx D Q D Q PINxn L Q Q clk I/O WDx: WRITE DDRx PUD: PULLUP DISABLE RDx: READ DDRx SLEEP: SLEEP CONTROL WRx: WRITE PORTx clk : I/O CLOCK RRx: READ PORTx REGISTER I/O RPx: READ PORTx PIN WPx: WRITE PINx REGISTER Note: 1. WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clk , SLEEP, and PUD I/O are common to all ports. 10.2.1 Configuring the Pin Each port pin consists of three register bits: DDxn, PORTxn, and PINxn. As shown in “” on page83, the DDxn bits are accessed at the DDRx I/O address, the PORTxn bits at the PORTx I/O address, and the PINxn bits at the PINx I/O address. The DDxn bit in the DDRx Register selects the direction of this pin. If DDxn is written logic one, Pxn is configured as an output pin. If DDxn is written logic zero, Pxn is configured as an input pin. If PORTxn is written logic one when the pin is configured as an input pin, the pull-up resistor is activated. To switch the pull-up resistor off, PORTxn has to be written logic zero or the pin has to be configured as an output pin. The port pins are tri-stated when reset condition becomes active, even if no clocks are running. If PORTxn is written logic one when the pin is configured as an output pin, the port pin is driven high (one). If PORTxn is written logic zero when the pin is configured as an output pin, the port pin is driven low (zero). 10.2.2 Toggling the Pin Writing a logic one to PINxn toggles the value of PORTxn, independent on the value of DDRxn. Note that the SBI instruction can be used to toggle one single bit in a port. 10.2.3 Switching Between Input and Output When switching between tri-state ({DDxn, PORTxn} = 0b00) and output high ({DDxn, PORTxn} = 0b11), an intermediate state with either pull-up enabled {DDxn, PORTxn} = 0b01) or output low ({DDxn, PORTxn} = 0b10) occurs. Normally, the pull-up enabled state is fully acceptable, as a high-impedance environment will not notice ATmega16U4/32U4 [DATASHEET] 68 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

the difference between a strong high driver and a pull-up. If this is not the case, the PUD bit in the MCUCR Register can be set to disable all pull-ups in all ports. Switching between input with pull-up and output low generates the same problem. The user must use either the tri-state ({DDxn, PORTxn} = 0b00) or the output high state ({DDxn, PORTxn} = 0b11) as an intermediate step. Table 10-1 summarizes the control signals for the pin value Table 10-1. Port Pin Configurations PUD DDxn PORTxn (in MCUCR) I/O Pull-up Comment 0 0 X Input No Tri-state (Hi-Z) 0 1 0 Input Yes Pxn will source current if ext. pulled low 0 1 1 Input No Tri-state (Hi-Z) 1 0 X Output No Output Low (Sink) 1 1 X Output No Output High (Source) 10.2.4 Reading the Pin Value Independent of the setting of Data Direction bit DDxn, the port pin can be read through the PINxn Register bit. As shown in Figure 10-2 on page 68, the PINxn Register bit and the preceding latch constitute a synchronizer. This is needed to avoid metastability if the physical pin changes value near the edge of the internal clock, but it also introduces a delay. Figure 10-3 on page 69 shows a timing diagram of the synchronization when reading an externally applied pin value. The maximum and minimum propagation delays are denoted t and t pd,max pd,min respectively. Figure 10-3. Synchronization when Reading an Externally Applied Pin Value SYSTEM CLK INSTRUCTIONS XXX XXX in r17, PINx SYNC LATCH PINxn r17 0x00 0xFF t pd, max tpd, min Consider the clock period starting shortly after the first falling edge of the system clock. The latch is closed when the clock is low, and goes transparent when the clock is high, as indicated by the shaded region of the “SYNC LATCH” signal. The signal value is latched when the system clock goes low. It is clocked into the PINxn Register at the succeeding positive clock edge. As indicated by the two arrows tpd max. and tpd min., a single signal transition on the pin will be delayed between ½ and 1½ system clock period depending upon the time of assertion. ATmega16U4/32U4 [DATASHEET] 69 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

When reading back a software assigned pin value, a nop instruction must be inserted as indicated in Figure 10- 4. The out instruction sets the “SYNC LATCH” signal at the positive edge of the clock. In this case, the delay tpd through the synchronizer is one system clock period. Figure 10-4. Synchronization when Reading a Software Assigned Pin Value SYSTEM CLK r16 0xFF INSTRUCTIONS out PORTx, r16 nop in r17, PINx SYNC LATCH PINxn r17 0x00 0xFF t pd The following code example shows how to set port B pins 0 and 1 high, 2 and 3 low, and define the port pins from 4 to 7 as input with pull-ups assigned to port pins 6 and 7. The resulting pin values are read back again, but as previously discussed, a nop instruction is included to be able to read back the value recently assigned to some of the pins. ATmega16U4/32U4 [DATASHEET] 70 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Assembly Code Example(1) ... ; Define pull-ups and set outputs high ; Define directions for port pins ldi r16,(1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0) ldi r17,(1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0) out PORTB,r16 out DDRB,r17 ; Insert nop for synchronization nop ; Read port pins in r16,PINB ... C Code Example unsigned char i; ... /* Define pull-ups and set outputs high */ /* Define directions for port pins */ PORTB = (1<<PB7)|(1<<PB6)|(1<<PB1)|(1<<PB0); DDRB = (1<<DDB3)|(1<<DDB2)|(1<<DDB1)|(1<<DDB0); /* Insert nop for synchronization*/ __no_operation(); /* Read port pins */ i = PINB; ... Note: 1. For the assembly program, two temporary registers are used to minimize the time from pull-ups are set on pins 0, 1, 6, and 7, until the direction bits are correctly set, defining bit 2 and 3 as low and redefining bits 0 and 1 as strong high drivers. 10.2.5 Digital Input Enable and Sleep Modes As shown in Figure 10-2, the digital input signal can be clamped to ground at the input of the Schmidt-trigger. The signal denoted SLEEP in the figure, is set by the MCU Sleep Controller in Power-down mode, Power-save mode, and Standby mode to avoid high power consumption if some input signals are left floating, or have an analog signal level close to V /2. CC SLEEP is overridden for port pins enabled as external interrupt pins. If the external interrupt request is not enabled, SLEEP is active also for these pins. SLEEP is also overridden by various other alternate functions as described in “Alternate Port Functions” on page72. If a logic high level (“one”) is present on an asynchronous external interrupt pin configured as “Interrupt on Rising Edge, Falling Edge, or Any Logic Change on Pin” while the external interrupt is not enabled, the corresponding External Interrupt Flag will be set when resuming from the above mentioned Sleep mode, as the clamping in these sleep mode produces the requested logic change. 10.2.6 Unconnected Pins If some pins are unused, it is recommended to ensure that these pins have a defined level. Even though most of the digital inputs are disabled in the deep sleep modes as described above, floating inputs should be avoided to ATmega16U4/32U4 [DATASHEET] 71 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

reduce current consumption in all other modes where the digital inputs are enabled (Reset, Active mode, and Idle mode). The simplest method to ensure a defined level of an unused pin, is to enable the internal pull-up. In this case, the pull-up will be disabled during reset. If low power consumption during reset is important, it is recommended to use an external pull-up or pull-down. Connecting unused pins directly to V or GND is not recommended, CC since this may cause excessive currents if the pin is accidentally configured as an output. 10.3 Alternate Port Functions Most port pins have alternate functions in addition to being general digital I/Os. Figure 10-5 shows how the port pin control signals from the simplified Figure 10-2 on page 68 can be overridden by alternate functions. The overriding signals may not be present in all port pins, but the figure serves as a generic description applicable to all port pins in the AVR microcontroller family. Figure 10-5. Alternate Port Functions(1) PUOExn PUOVxn 1 PUD 0 DDOExn DDOVxn 1 0 Q D DDxn QCLR WDx PVOExn RESET RDx PVOVxn S 1 1 U Pxn B 0 Q D 0 A PORTxn PTOExn T DIEOExn QCLR DA WPx DIEOVxn RESET 1 WRx RRx 0 SLEEP SYNCHRONIZER RPx DSETQ D Q PINxn LCLRQ CLRQ clkI/O DIxn AIOxn PUOExn: Pxn PULL-UP OVERRIDE ENABLE PUD: PULLUP DISABLE PUOVxn: Pxn PULL-UP OVERRIDE VALUE WDx: WRITE DDRx DDOExn: Pxn DATA DIRECTION OVERRIDE ENABLE RDx: READ DDRx DDOVxn: Pxn DATA DIRECTION OVERRIDE VALUE RRx: READ PORTx REGISTER PVOExn: Pxn PORT VALUE OVERRIDE ENABLE WRx: WRITE PORTx PVOVxn: Pxn PORT VALUE OVERRIDE VALUE RPx: READ PORTx PIN DIEOExn: Pxn DIGITAL INPUT-ENABLE OVERRIDE ENABLE WPx: WRITE PINx DIEOVxn: Pxn DIGITAL INPUT-ENABLE OVERRIDE VALUE clk : I/O CLOCK SLEEP: SLEEP CONTROL DIxI/nO: DIGITAL INPUT PIN n ON PORTx PTOExn: Pxn, PORT TOGGLE OVERRIDE ENABLE AIOxn: ANALOG INPUT/OUTPUT PIN n ON PORTx Note: 1. WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clk , SLEEP, and PUD I/O are common to all ports. All other signals are unique for each pin. The table summarizes the function of the overriding signals. The pin and port indexes from Figure 10-5 on page 72 are not shown in the succeeding tables. The overriding signals are generated internally in the modules having the alternate function. ATmega16U4/32U4 [DATASHEET] 72 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 10-2. Generic Description of Overriding Signals for Alternate Functions Signal Name Full Name Description If this signal is set, the pull-up enable is controlled by the PUOV Pull-up Override PUOE signal. If this signal is cleared, the pull-up is enabled when {DDxn, Enable PORTxn, PUD} = 0b010. If PUOE is set, the pull-up is enabled/disabled when PUOV is Pull-up Override PUOV set/cleared, regardless of the setting of the DDxn, PORTxn, and Value PUD Register bits. If this signal is set, the Output Driver Enable is controlled by the Data Direction DDOE DDOV signal. If this signal is cleared, the Output driver is enabled Override Enable by the DDxn Register bit. Data Direction If DDOE is set, the Output Driver is enabled/disabled when DDOV DDOV Override Value is set/cleared, regardless of the setting of the DDxn Register bit. If this signal is set and the Output Driver is enabled, the port value Port Value is controlled by the PVOV signal. If PVOE is cleared, and the PVOE Override Enable Output Driver is enabled, the port Value is controlled by the PORTxn Register bit. Port Value If PVOE is set, the port value is set to PVOV, regardless of the PVOV Override Value setting of the PORTxn Register bit. Port Toggle PTOE If PTOE is set, the PORTxn Register bit is inverted. Override Enable Digital Input If this bit is set, the Digital Input Enable is controlled by the DIEOV DIEOE Enable Override signal. If this signal is cleared, the Digital Input Enable is Enable determined by MCU state (Normal mode, sleep mode). Digital Input If DIEOE is set, the Digital Input is enabled/disabled when DIEOV DIEOV Enable Override is set/cleared, regardless of the MCU state (Normal mode, sleep Value mode). This is the Digital Input to alternate functions. In the figure, the signal is connected to the output of the schmitt trigger but before DI Digital Input the synchronizer. Unless the Digital Input is used as a clock source, the module with the alternate function will use its own synchronizer. This is the Analog Input/output to/from alternate functions. The Analog AIO signal is connected directly to the pad, and can be used bi- Input/Output directionally. The following subsections shortly describe the alternate functions for each port, and relate the overriding signals to the alternate function. Refer to the alternate function description for further details. ATmega16U4/32U4 [DATASHEET] 73 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

10.3.1 Alternate Functions of Port B The Port B pins with alternate functions are shown below. Table 10-3. Port B Pins Alternate Functions Port Pin Alternate Functions OC0A/OC1C/PCINT7/RTS (Output Compare and PWM Output A for Timer/Counter0, PB7 Output Compare and PWM Output C for Timer/Counter1 or Pin Change Interrupt 7 or UART flow control RTS signal) OC1B/PCINT6/OC.4B/ADC13 (Output Compare and PWM Output B for Timer/Counter1 or PB6 Pin Change Interrupt 6 or Timer 4 Output Compare B / PWM output or Analog to Digital Converter channel 13) OC1A/PCINT5/OC.4B/ADC12 (Output Compare and PWM Output A for Timer/Counter1 or PB5 Pin Change Interrupt 5 or Timer 4 Complementary Output Compare B / PWM output or Analog to Digital Converter channel 12) PB4 PCINT4/ADC11 (Pin Change Interrupt 4 or Analog to Digital Converter channel 11) PDO/MISO/PCINT3 (Programming Data Output or SPI Bus Master Input/Slave Output or PB3 Pin Change Interrupt 3) PDI/MOSI/PCINT2 (Programming Data Input or SPI Bus Master Output/Slave Input or Pin PB2 Change Interrupt 2) PB1 SCK/PCINT1 (SPI Bus Serial Clock or Pin Change Interrupt 1) PB0 SS/PCINT0 (SPI Slave Select input or Pin Change Interrupt 0) The alternate pin configuration is as follows: • OC0A/OC1C/PCINT7/RTS, Bit 7 OC0A, Output Compare Match A output: The PB7 pin can serve as an external output for the Timer/Counter0 Output Compare. The pin has to be configured as an output (DDB7 set “one”) to serve this function. The OC0A pin is also the output pin for the PWM mode timer function. OC1C, Output Compare Match C output: The PB7 pin can serve as an external output for the Timer/Counter1 Output Compare C. The pin has to be configured as an output (DDB7 set “one”) to serve this function. The OC1C pin is also the output pin for the PWM mode timer function. PCINT7, Pin Change Interrupt source 7: The PB7 pin can serve as an external interrupt source. RTS: RTS flow control signal used by enhanced UART. • OC1B/PCINT6/OC.4B/ADC12, Bit 6 OC1B, Output Compare Match B output: The PB6 pin can serve as an external output for the Timer/Counter1 Output Compare B. The pin has to be configured as an output (DDB6 set “one”) to serve this function. The OC1B pin is also the output pin for the PWM mode timer function. PCINT6, Pin Change Interrupt source 6: The PB7 pin can serve as an external interrupt source. OC.4B: Timer 4 Output Compare B. This pin can be used to generate a high-speed PWM signal from Timer 4 module. The pin has to be configured as an output (DDB6 set “one”) to serve this function. ADC13: Analog to Digital Converter, channel 13. ATmega16U4/32U4 [DATASHEET] 74 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• OC1A/PCINT5/OC.4B/ADC12, Bit 5 OC1A, Output Compare Match A output: The PB5 pin can serve as an external output for the Timer/Counter1 Output Compare A. The pin has to be configured as an output (DDB5 set (one)) to serve this function. The OC1A pin is also the output pin for the PWM mode timer function. PCINT5, Pin Change Interrupt source 5: The PB7 pin can serve as an external interrupt source. OC.4B: Timer 4 Output Compare B. This pin can be used to generate a high-speed PWM signal from Timer 4 module, complementary to OC.4B (PB5) signal. The pin has to be configured as an output (DDB5 set (one)) to serve this function. ADC12: Analog to Digital Converter, channel 12. • PCINT4/ADC11, Bit 4 PCINT4, Pin Change Interrupt source 4: The PB7 pin can serve as an external interrupt source. ADC11, Analog to Digital Converter channel 11. • PDO/MISO/PCINT3 – Port B, Bit 3 PDO, SPI Serial Programming Data Output. During Serial Program Downloading, this pin is used as data output line for the ATmega16U4/ATmega32U4. MISO: Master Data input, Slave Data output pin for SPI channel. When the SPI is enabled as a master, this pin is configured as an input regardless of the setting of DDB3. When the SPI is enabled as a slave, the data direction of this pin is controlled by DDB3. When the pin is forced to be an input, the pull-up can still be controlled by the PORTB3 bit. PCINT3, Pin Change Interrupt source 3: The PB7 pin can serve as an external interrupt source. • PDI/MOSI/PCINT2 – Port B, Bit 2 PDI, SPI Serial Programming Data Input. During Serial Program Downloading, this pin is used as data input line for the ATmega16U4/ATmega32U4. MOSI: SPI Master Data output, Slave Data input for SPI channel. When the SPI is enabled as a slave, this pin is configured as an input regardless of the setting of DDB2. When the SPI is enabled as a master, the data direction of this pin is controlled by DDB2. When the pin is forced to be an input, the pull-up can still be controlled by the PORTB2 bit. PCINT2, Pin Change Interrupt source 2: The PB7 pin can serve as an external interrupt source. • SCK/PCINT1 – Port B, Bit 1 SCK: Master Clock output, Slave Clock input pin for SPI channel. When the SPI is enabled as a slave, this pin is configured as an input regardless of the setting of DDB1. When the SPI0 is enabled as a master, the data direction of this pin is controlled by DDB1. When the pin is forced to be an input, the pull-up can still be controlled by the PORTB1 bit. PCINT1, Pin Change Interrupt source 1: The PB7 pin can serve as an external interrupt source. • SS/PCINT0 – Port B, Bit 0 SS: Slave Port Select input. When the SPI is enabled as a slave, this pin is configured as an input regardless of the setting of DDB0. As a slave, the SPI is activated when this pin is driven low. When the SPI is enabled as a master, the data direction of this pin is controlled by DDB0. When the pin is forced to be an input, the pull-up can still be controlled by the PORTB0 bit. Table 10-4 and Table10-5 on page76 relate the alternate functions of Port B to the overriding signals shown in Figure 10-5 on page 72. SPI MSTR INPUT and SPI SLAVE OUTPUT constitute the MISO signal, while MOSI is divided into SPI MSTR OUTPUT and SPI SLAVE INPUT. ATmega16U4/32U4 [DATASHEET] 75 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

PCINT0, Pin Change Interrupt source 0: The PB7 pin can serve as an external interrupt source Table 10-4. Overriding Signals for Alternate Functions in PB7..PB4 Signal PB7/PCINT7/OC0A/O PB6/PCINT6/OC1B/ PB5/PCINT5/OC1A/ PB4/PCINT4/ADC Name C1C/RTS OC.4B/ADC13 OC.4B/ADC12 11 PUOE 0 0 0 0 PUOV 0 0 0 0 DDOE 0 0 0 0 DDOV 0 0 0 0 PVOE OC0/OC1C ENABLE OC1B ENABLE OC1A ENABLE 0 PVOV OC0/OC1C OC1B OC1A 0 DIEOE PCINT7 • PCIE0 PCINT6 • PCIE0 PCINT5 • PCIE0 PCINT4 • PCIE0 DIEOV 1 1 1 1 DI PCINT7 INPUT PCINT6 INPUT PCINT5 INPUT PCINT4 INPUT AIO – – – – Table 10-5. Overriding Signals for Alternate Functions in PB3..PB0 Signal PB3/PD0/PCINT3/ PB2/PDI/PCINT2/ PB1/PCINT1/ PB0/PCINT0/ Name MISO MOSI SCK SS PUOE SPE • MSTR SPE • MSTR SPE • MSTR SPE • MSTR PUOV PORTB3 • PUD PORTB2 • PUD PORTB1 • PUD PORTB0 • PUD DDOE SPE • MSTR SPE • MSTR SPE • MSTR SPE • MSTR DDOV 0 0 0 0 PVOE SPE • MSTR SPE • MSTR SPE • MSTR 0 PVOV SPI SLAVE OUTPUT SPI MSTR OUTPUT SCK OUTPUT 0 DIEOE PCINT3 • PCIE0 PCINT2 • PCIE0 PCINT1 • PCIE0 PCINT0 • PCIE0 DIEOV 1 1 1 1 SPI MSTR INPUT SPI SLAVE INPUT SCK INPUT SPI SS DI PCINT3 INPUT PCINT2 INPUT PCINT1 INPUT PCINT0 INPUT AIO – – – – ATmega16U4/32U4 [DATASHEET] 76 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

10.3.2 Alternate Functions of Port C The Port C pins with alternate functions are shown below. Table 10-6. Port C Pins Alternate Functions Port Pin Alternate Function ICP3/CLKO/OC4A(Input Capture Timer 3 or CLK0 (Divided System PC7 Clock) or Output Compare and direct PWM output A for Timer 4) OC.3A/OC4A (Output Compare and PWM output A for Timer/Counter3 PC6 or Output Compare and complementary PWM output A for Timer 4) PC5 PC4 PC3 Not present on pin-out. PC2 PC1 PC0 • ICP3/CLKO/OC.4A – Port C, Bit 7 ICP3: If Timer 3 is correctly configured, this pin can serve as Input Capture feature. CLKO: When the corresponding fuse is enabled, this pin outputs the internal microcontroller working frequency. If the clock prescaler is used, this will affect this output frequency. OC.4A: Timer 4 Output Compare A. This pin can be used to generate a high-speed PWM signal from Timer 4 module. The pin has to be configured as an output (DDC7 set “one”) to serve this function. • OC.3A/OC.4A – Port C, Bit 6 OC.3A: Timer 3 Output Compare A. This pin can be used to generate a PWM signal from Timer 3 module. OC.4A: Timer 4 Output Compare A. This pin can be used to generate a high-speed PWM signal from Timer 4 module, complementary to OC.4A (PC7) signal. The pin has to be configured as an output (DDC6 set “one”) to serve this function. ATmega16U4/32U4 [DATASHEET] 77 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The following table shows the alternate functions of Port C to the overriding signals shown in Figure 10-5 on page 72. Table 10-7. Overriding Signals for Alternate Functions in PC7..PC6 Signal Name PC7/ICP3/CLKO/OC.4A PC6/OC.3A/OC.4A PUOE SRE • (XMM<1) SRE • (XMM<2)|OC3A enable PUOV 0 0 DDOE SRE • (XMM<1) SRE • (XMM<2) DDOV 1 1 PVOE SRE • (XMM<1) SRE • (XMM<2) PVOV A15 if (SRE.XMM<2) then A14 else OC3A DIEOE 0 0 DIEOV 0 0 DI ICP3 input – AIO – – 10.3.3 Alternate Functions of Port D The Port D pins with alternate functions are shown below. Table 10-8. Port D Pins Alternate Functions Port Pin Alternate Function T0/OC.4D/ADC10 (Timer/Counter0 Clock Input or Timer 4 Output Compare D / PWM PD7 output or Analog to Digital Converter channel 10) T1/OC.4D/ADC9 (Timer/Counter1 Clock Input or Timer 4 Output Complementary PD6 Compare D / PWM output or Analog to Digital Converter channel 9) PD5 XCK1/CTS (USART1 External Clock Input/Output or UART flow control CTS signal) ICP1/ADC8 (Timer/Counter1 Input Capture Trigger or Analog to Digital Converter PD4 channel 8) PD3 INT3/TXD1 (External Interrupt3 Input or USART1 Transmit Pin) PD2 INT2/RXD1 (External Interrupt2 Input or USART1 Receive Pin) PD1 INT1/SDA (External Interrupt1 Input or TWI Serial DAta) INT0/SCL/OC0B (External Interrupt0 Input or TWI Serial CLock or Output Compare for PD0 Timer/Counter0) The alternate pin configuration is as follows: • T0/OC.4D/ADC10 – Port D, Bit 7 T0, Timer/Counter0 counter source. OC.4D: Timer 4 Output Compare D. This pin can be used to generate a high-speed PWM signal from Timer 4 module. The pin has to be configured as an output (DDD7 set “one”) to serve this function. ADC10: Analog to Digital Converter, Channel 10. ATmega16U4/32U4 [DATASHEET] 78 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• T1/OC.4D/ADC9 – Port D, Bit 6 T1, Timer/Counter1 counter source. OC.4D: Timer 4 Output Compare D. This pin can be used to generate a high-speed PWM signal from Timer 4 module, complementary to OC.4D (PD7) signal. The pin has to be configured as an output (DDD6 set “one”) to serve this function. ADC9: Analog to Digital Converter, Channel 9. • XCK1/CTS – Port D, Bit 5 XCK1, USART1 External clock. The Data Direction Register (DDD5) controls whether the clock is output (DDD5 set) or input (DDD5 cleared). The XCK1 pin is active only when the USART1 operates in Synchronous mode. CTS: Clear-To-Send flow control signal used by enhanced UART module. • ICP1/ADC8 – Port D, Bit 4 ICP1 – Input Capture Pin 1: The PD4 pin can act as an input capture pin for Timer/Counter1. ADC8: Analog to Digital Converter, Channel 8. • INT3/TXD1 – Port D, Bit 3 INT3, External Interrupt source 3: The PD3 pin can serve as an external interrupt source to the MCU. TXD1, Transmit Data (Data output pin for the USART1). When the USART1 Transmitter is enabled, this pin is configured as an output regardless of the value of DDD3. • INT2/RXD1 – Port D, Bit 2 INT2, External Interrupt source 2. The PD2 pin can serve as an External Interrupt source to the MCU. RXD1, Receive Data (Data input pin for the USART1). When the USART1 receiver is enabled this pin is configured as an input regardless of the value of DDD2. When the USART forces this pin to be an input, the pull-up can still be controlled by the PORTD2 bit. • INT1/SDA – Port D, Bit 1 INT1, External Interrupt source 1. The PD1 pin can serve as an external interrupt source to the MCU. SDA, 2-wire Serial Interface Data: When the TWEN bit in TWCR is set (one) to enable the 2-wire Serial Interface, pin PD1 is disconnected from the port and becomes the Serial Data I/O pin for the 2-wire Serial Interface. In this mode, there is a spike filter on the pin to suppress spikes shorter than 50ns on the input signal, and the pin is driven by an open drain driver with slew-rate limitation. • INT0/SCL/OC0B – Port D, Bit 0 INT0, External Interrupt source 0. The PD0 pin can serve as an external interrupt source to the MCU. SCL, 2-wire Serial Interface Clock: When the TWEN bit in TWCR is set (one) to enable the 2-wire Serial Interface, pin PD0 is disconnected from the port and becomes the Serial Clock I/O pin for the 2-wire Serial Interface. In this mode, there is a spike filter on the pin to suppress spikes shorter than 50ns on the input signal, and the pin is driven by an open drain driver with slew-rate limitation. OC.0B: Timer 0 Output Compare B. This pin can be used to generate a PWM signal from the Timer 0 module. The two following tables relate the alternate functions of Port D to the overriding signals shown in Figure 10-5 on page 72. ATmega16U4/32U4 [DATASHEET] 79 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 10-9. Overriding Signals for Alternate Functions PD7..PD4 Signal Name PD7/T0/OC4D/ADC10 PD6/T1/OC4D/ADC9 PD5/XCK1/CTS PD4/ICP1/ADC8 PUOE 0 0 0 0 PUOV 0 0 0 0 DDOE 0 0 XCK1 OUTPUT ENABLE 0 DDOV 0 0 1 0 PVOE 0 0 XCK1 OUTPUT ENABLE 0 PVOV 0 0 XCK1 OUTPUT 0 DIEOE 0 0 0 0 DIEOV 0 0 0 0 DI T0 INPUT T1 INPUT XCK1 INPUT ICP1 INPUT AIO – – – – Table 10- 10. Overriding Signals for Alternate Functions in PD3..PD0(1) Signal Name PD3/INT3/TXD1 PD2/INT2/RXD1 PD1/INT1/SDA PD0/INT0/SCL/OC0B PUOE TXEN1 RXEN1 TWEN TWEN PUOV 0 PORTD2 • PUD PORTD1 • PUD PORTD0 • PUD DDOE TXEN1 RXEN1 TWEN TWEN DDOV 1 0 SDA_OUT SCL_OUT PVOE TXEN1 0 TWEN ENABLE TWEN | OC0B ENABLE PVOV TXD1 0 0 OC0B DIEOE INT3 ENABLE INT2 ENABLE INT1 ENABLE INT0 ENABLE DIEOV 1 1 1 1 DI INT3 INPUT INT2 INPUT/RXD1 INT1 INPUT INT0 INPUT AIO – – SDA INPUT SCL INPUT Note: 1. When enabled, the 2-wire Serial Interface enables Slew-Rate controls on the output pins PD0 and PD1. This is not shown in this table. In addition, spike filters are connected between the AIO outputs shown in the port figure and the digital logic of the TWI module. ATmega16U4/32U4 [DATASHEET] 80 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

10.3.4 Alternate Functions of Port E The Port E pins with alternate functions are shown below. Table 10-11. Port E Pins Alternate Functions Port Pin Alternate Function PE7 Not present on pin-out. PE6 INT6/AIN0 (External Interrupt 6 Input or Analog Comparator Positive Input) PE5 PE4 Not present on pin-out. PE3 PE2 HWB (Hardware bootloader activation) PE1 Not present on pin-out. PE0 • INT6/AIN0 – Port E, Bit 6 INT6, External Interrupt source 6: The PE6 pin can serve as an external interrupt source. AIN0 – Analog Comparator Negative input. This pin is directly connected to the negative input of the Analog Comparator. • HWB – Port E, Bit 2 HWB allows to execute the bootloader section after reset when tied to ground during external reset pulse. The HWB mode of this pin is active only when the HWBE fuse is enable. During normal operation (excluded Reset), this pin acts as a general purpose I/O. Table 10-12. Overriding Signals for Alternate Functions PE6, PE2 Signal Name PE6/INT6/AIN0 PE2/HWB PUOE 0 0 PUOV 0 0 DDOE 0 0 DDOV 0 1 PVOE 0 0 PVOV 0 0 DIEOE INT6 ENABLE 0 DIEOV 1 0 DI INT6 INPUT HWB AIO AIN0 INPUT - 10.3.5 Alternate Functions of Port F The Port F has an alternate function as analog input for the ADC as shown in Table10-13 on page82. If some Port F pins are configured as outputs, it is essential that these do not switch when a conversion is in progress. ATmega16U4/32U4 [DATASHEET] 81 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

This might corrupt the result of the conversion. If the JTAG interface is enabled, the pull-up resistors on pins PF7(TDI), PF5(TMS), and PF4(TCK) will be activated even if a Reset occurs. Table 10-13. Port F Pins Alternate Functions Port Pin Alternate Function PF7 ADC7/TDI (ADC input channel 7 or JTAG Test Data Input) PF6 ADC6/TDO (ADC input channel 6 or JTAG Test Data Output) PF5 ADC5/TMS (ADC input channel 5 or JTAG Test Mode Select) PF4 ADC4/TCK (ADC input channel 4 or JTAG Test ClocK) PF3 Not present on pin-out PF2 PF1 ADC1 (ADC input channel 1) PF0 ADC0 (ADC input channel 0) • TDI, ADC7 – Port F, Bit 7 ADC7, Analog to Digital Converter, Channel 7. TDI, JTAG Test Data In: Serial input data to be shifted in to the Instruction Register or Data Register (scan chains). When the JTAG interface is enabled, this pin can not be used as an I/O pin. • TDO, ADC6 – Port F, Bit 6 ADC6, Analog to Digital Converter, Channel 6. TDO, JTAG Test Data Out: Serial output data from Instruction Register or Data Register. When the JTAG interface is enabled, this pin can not be used as an I/O pin. The TDO pin is tri-stated unless TAP states that shift out data are entered. • TMS, ADC5 – Port F, Bit 5 ADC5, Analog to Digital Converter, Channel 5. TMS, JTAG Test Mode Select: This pin is used for navigating through the TAP-controller state machine. When the JTAG interface is enabled, this pin can not be used as an I/O pin. • TCK, ADC4 – Port F, Bit 4 ADC4, Analog to Digital Converter, Channel 4. TCK, JTAG Test Clock: JTAG operation is synchronous to TCK. When the JTAG interface is enabled, this pin can not be used as an I/O pin. • ADC3 – ADC0 – Port F, Bit 1..0 Analog to Digital Converter, Channel 1.0 ATmega16U4/32U4 [DATASHEET] 82 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 10-14. Overriding Signals for Alternate Functions in PF7..PF4 Signal Name PF7/ADC7/TDI PF6/ADC6/TDO PF5/ADC5/TMS PF4/ADC4/TCK PUOE JTAGEN JTAGEN JTAGEN JTAGEN PUOV 1 0 1 1 DDOE JTAGEN JTAGEN JTAGEN JTAGEN SHIFT_IR + DDOV 0 0 0 SHIFT_DR PVOE 0 JTAGEN 0 0 PVOV 0 TDO 0 0 DIEOE JTAGEN JTAGEN JTAGEN JTAGEN DIEOV 0 0 0 0 DI – – – – AIO TDI/ADC7 INPUT ADC6 INPUT TMS/ADC5 INPUT TCK/ADC4 INPUT Table 10-15. Overriding Signals for Alternate Functions in PF1..PF0 Signal Name PF1/ADC1 PF0/ADC0 PUOE 0 0 PUOV 0 0 DDOE 0 0 DDOV 0 0 PVOE 0 0 PVOV 0 0 DIEOE 0 0 DIEOV 0 0 DI – – AIO ADC1 INPUT ADC0 INPUT ATmega16U4/32U4 [DATASHEET] 83 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

10.4 Register Description for I/O-Ports 10.4.1 MCU Control Register – MCUCR Bit 7 6 5 4 3 2 1 0 JTD – – PUD – – IVSEL IVCE MCUCR Read/Writ R/W R R R/W R R R/W R/W e Initial Val- 0 0 0 0 0 0 0 0 ue • Bit 4 – PUD: Pull-up Disable When this bit is written to one, the pull-ups in the I/O ports are disabled even if the DDxn and PORTxn Registers are configured to enable the pull-ups ({DDxn, PORTxn} = 0b01). See “Configuring the Pin” on page68 for more details about this feature. 10.4.2 Port B Data Register – PORTB Bit 7 6 5 4 3 2 1 0 PORTB7 PORTB6 PORTB5 PORTB4 PORTB3 PORTB2 PORTB1 PORTB0 PORTB Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 10.4.3 Port B Data Direction Register – DDRB Bit 7 6 5 4 3 2 1 0 DDB7 DDB6 DDB5 DDB4 DDB3 DDB2 DDB1 DDB0 DDRB Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 10.4.4 Port B Input Pins Address – PINB Bit 7 6 5 4 3 2 1 0 PINB7 PINB6 PINB5 PINB4 PINB3 PINB2 PINB1 PINB0 PINB Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value N/A N/A N/A N/A N/A N/A N/A N/A 10.4.5 Port C Data Register – PORTC Bit 7 6 5 4 3 2 1 0 PORTC7 PORTC6 - - - - - - PORTC Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 10.4.6 Port C Data Direction Register – DDRC Bit 7 6 5 4 3 2 1 0 DDC7 DDC6 - - - - - - DDRC Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 ATmega16U4/32U4 [DATASHEET] 84 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

10.4.7 Port C Input Pins Address – PINC Bit 7 6 5 4 3 2 1 0 PINC7 PINC6 - - - - - - PINC Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value N/A N/A N/A N/A N/A N/A N/A N/A 10.4.8 Port D Data Register – PORTD Bit 7 6 5 4 3 2 1 0 PORTD7 PORTD6 PORTD5 PORTD4 PORTD3 PORTD2 PORTD1 PORTD0 PORTD Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 10.4.9 Port D Data Direction Register – DDRD Bit 7 6 5 4 3 2 1 0 DDD7 DDD6 DDD5 DDD4 DDD3 DDD2 DDD1 DDD0 DDRD Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 10.4.10Port D Input Pins Address – PIND Bit 7 6 5 4 3 2 1 0 PIND7 PIND6 PIND5 PIND4 PIND3 PIND2 PIND1 PIND0 PIND Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value N/A N/A N/A N/A N/A N/A N/A N/A 10.4.11 Port E Data Register – PORTE Bit 7 6 5 4 3 2 1 0 - PORTE6 - - - PORTE2 - - PORTE Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 10.4.12Port E Data Direction Register – DDRE Bit 7 6 5 4 3 2 1 0 - DDE6 - - - DDE2 - - DDRE Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 10.4.13Port E Input Pins Address – PINE Bit 7 6 5 4 3 2 1 0 - PINE6 - - - PINE2 - - PINE Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value N/A N/A N/A N/A N/A N/A N/A N/A ATmega16U4/32U4 [DATASHEET] 85 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

10.4.14Port F Data Register – PORTF Bit 7 6 5 4 3 2 1 0 PORTF7 PORTF6 PORTF5 PORTF4 - - PORTF1 PORTF0 PORTF Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 10.4.15Port F Data Direction Register – DDRF Bit 7 6 5 4 3 2 1 0 DDF7 DDF6 DDF5 DDF4 - - DDF1 DDF0 DDRF Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 10.4.16Port F Input Pins Address – PINF Bit 7 6 5 4 3 2 1 0 PINF7 PINF6 PINF5 PINF4 - - PINF1 PINF0 PINF Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value N/A N/A N/A N/A N/A N/A N/A N/A ATmega16U4/32U4 [DATASHEET] 86 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

ATmega16U4/32U4 [DATASHEET] 87 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

11. External Interrupts The External Interrupts are triggered by the INT6, INT3:0 pin or any of the PCINT7..0 pins. Observe that, if enabled, the interrupts will trigger even if the INT[6;3:0] or PCINT7..0 pins are configured as outputs. This feature provides a way of generating a software interrupt. The Pin change interrupt PCI0 will trigger if any enabled PCINT7:0 pin toggles. PCMSK0 Register control which pins contribute to the pin change interrupts. Pin change interrupts on PCINT7 ..0 are detected asynchronously. This implies that these interrupts can be used for waking the part also from sleep modes other than Idle mode. The External Interrupts can be triggered by a falling or rising edge or a low level. This is set up as indicated in the specification for the External Interrupt Control Registers – EICRA (INT3:0) and EICRB (INT6). When the external interrupt is enabled and is configured as level triggered, the interrupt will trigger as long as the pin is held low. Note that recognition of falling or rising edge interrupts on INT6 requires the presence of an I/O clock, described in “System Clock and Clock Options” on page27. Low level interrupts and the edge interrupt on INT3:0 are detected asynchronously. This implies that these interrupts can be used for waking the part also from sleep modes other than Idle mode. The I/O clock is halted in all sleep modes except Idle mode. Note that if a level triggered interrupt is used for wake-up from Power-down, the required level must be held long enough for the MCU to complete the wake-up to trigger the level interrupt. If the level disappears before the end of the Start-up Time, the MCU will still wake up, but no interrupt will be generated. The start-up time is defined by the SUT and CKSEL Fuses as described in “System Clock and Clock Options” on page27. ATmega16U4/32U4 [DATASHEET] 88 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

11.1 Register Description 11.1.1 External Interrupt Control Register A – EICRA The External Interrupt Control Register A contains control bits for interrupt sense control. Bit 7 6 5 4 3 2 1 0 ISC31 ISC30 ISC21 ISC20 ISC11 ISC10 ISC01 ISC00 EICRA Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bits 7..0 – ISC31, ISC30 – ISC00, ISC00: External Interrupt 3 - 0 Sense Control Bits The External Interrupts 3 - 0 are activated by the external pins INT3:0 if the SREG I-flag and the corresponding interrupt mask in the EIMSK is set. The level and edges on the external pins that activate the interrupts are defined in the below table. Edges on INT3..INT0 are registered asynchronously. Pulses on INT3:0 pins wider than the minimum pulse width given in the below table will generate an interrupt. Shorter pulses are not guaranteed to generate an interrupt. If low level interrupt is selected, the low level must be held until the completion of the currently executing instruction to generate an interrupt. If enabled, a level triggered interrupt will generate an interrupt request as long as the pin is held low. When changing the ISCn bit, an interrupt can occur. Therefore, it is recommended to first disable INTn by clearing its Interrupt Enable bit in the EIMSK Register. Then, the ISCn bit can be changed. Finally, the INTn interrupt flag should be cleared by writing a logical one to its Interrupt Flag bit (INTFn) in the EIFR Register before the interrupt is re-enabled. ISCn1 ISCn0 Description 0 0 The low level of INTn generates an interrupt request. 0 1 Any edge of INTn generates asynchronously an interrupt request. 1 0 The falling edge of INTn generates asynchronously an interrupt request. 1 1 The rising edge of INTn generates asynchronously an interrupt request. Note: 1. n = 3, 2, 1, or 0. When changing the ISCn1/ISCn0 bits, the interrupt must be disabled by clearing its Interrupt Enable bit in the EIMSK Register. Otherwise an interrupt can occur when the bits are changed. Symbol Parameter Condition Min. Typ. Max. Units Minimum pulse width for asynchronous t 50 ns INT external interrupt 11.1.2 External Interrupt Control Register B – EICRB Bit 7 6 5 4 3 2 1 0 - - ISC61 ISC60 - - - - EICRB Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 7..6 – Reserved Bits These bits are reserved bits in the ATmega16U4/ATmega32U4 and always read as zero. • Bits 5, 4 – ISC61, ISC60: External Interrupt 6 Sense Control Bits The External Interrupt 6 is activated by the external pin INT6 if the SREG I-flag and the corresponding interrupt mask in the EIMSK is set. The level and edges on the external pin that activate the interrupt are defined in the following table. The value on the INT6 pin are sampled before detecting edges. If edge or toggle interrupt is ATmega16U4/32U4 [DATASHEET] 89 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

selected, pulses that last longer than one clock period will generate an interrupt. Shorter pulses are not guaranteed to generate an interrupt. Observe that CPU clock frequency can be lower than the XTAL frequency if the XTAL divider is enabled. If low level interrupt is selected, the low level must be held until the completion of the currently executing instruction to generate an interrupt. If enabled, a level triggered interrupt will generate an interrupt request as long as the pin is held low. ISC61 ISC60 Description 0 0 The low level of INT6 generates an interrupt request. 0 1 Any logical change on INT6 generates an interrupt request 1 0 The falling edge between two samples of INT6 generates an interrupt request. 1 1 The rising edge between two samples of INT6 generates an interrupt request. Note: 1. When changing the ISC61/ISC60 bits, the interrupt must be disabled by clearing its Interrupt Enable bit in the EIMSK Register. Otherwise an interrupt can occur when the bits are changed. • Bit 3..0 – Reserved Bits These bits are reserved bits and always read as zero. 11.1.3 External Interrupt Mask Register – EIMSK Bit 7 6 5 4 3 2 1 0 - INT6 - - INT3 INT2 INT1 IINT0 EIMSK Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bits 7..0 – INT6, INT3 – INT0: External Interrupt Request 6, 3 - 0 Enable When an INT[6;3:0] bit is written to one and the I-bit in the Status Register (SREG) is set (one), the corresponding external pin interrupt is enabled. The Interrupt Sense Control bits in the External Interrupt Control Registers – EICRA and EICRB – defines whether the external interrupt is activated on rising or falling edge or level sensed. Activity on any of these pins will trigger an interrupt request even if the pin is enabled as an output. This provides a way of generating a software interrupt. 11.1.4 External Interrupt Flag Register – EIFR Bit 7 6 5 4 3 2 1 0 - INTF6 - - INTF3 INTF2 INTF1 IINTF0 EIFR Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bits 7..0 – INTF6, INTF3 - INTF0: External Interrupt Flags 6, 3 - 0 When an edge or logic change on the INT[6;3:0] pin triggers an interrupt request, INTF7:0 becomes set (one). If the I-bit in SREG and the corresponding interrupt enable bit, INT[6;3:0] in EIMSK, are set (one), the MCU will jump to the interrupt vector. The flag is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing a logical one to it. These flags are always cleared when INT[6;3:0] are configured as level interrupt. Note that when entering sleep mode with the INT3:0 interrupts disabled, the input buffers on these pins will be disabled. This may cause a logic change in internal signals which will set the INTF3:0 flags. See “Digital Input Enable and Sleep Modes” on page71 for more information. ATmega16U4/32U4 [DATASHEET] 90 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

11.1.5 Pin Change Interrupt Control Register - PCICR Bit 7 6 5 4 3 2 1 0 – – – – – PCIE0 PCICR Read/Write R R R R R R R R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 0 – PCIE0: Pin Change Interrupt Enable 0 When the PCIE0 bit is set (one) and the I-bit in the Status Register (SREG) is set (one), pin change interrupt 0 is enabled. Any change on any enabled PCINT7..0 pin will cause an interrupt. The corresponding interrupt of Pin Change Interrupt Request is executed from the PCI0 Interrupt Vector. PCINT7..0 pins are enabled individually by the PCMSK0 Register. 11.1.6 Pin Change Interrupt Flag Register – PCIFR Bit 7 6 5 4 3 2 1 0 – – – – – PCIF0 PCIFR Read/Write R R R R R R R R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 0 – PCIF0: Pin Change Interrupt Flag 0 When a logic change on any PCINT7..0 pin triggers an interrupt request, PCIF0 becomes set (one). If the I-bit in SREG and the PCIE0 bit in EIMSK are set (one), the MCU will jump to the corresponding Interrupt Vector. The flag is cleared when the interrupt routine is executed. Alternatively, the flag can be cleared by writing a logical one to it. 11.1.7 Pin Change Mask Register 0 – PCMSK0 Bit 7 6 5 4 3 2 1 0 PCINT7 PCINT6 PCINT5 PCINT4 PCINT3 PCINT2 PCINT1 PCINT0 PCMSK0 Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Val- 0 0 0 0 0 0 0 0 ue • Bit 7..0 – PCINT7..0: Pin Change Enable Mask 7..0 Each PCINT7..0 bit selects whether pin change interrupt is enabled on the corresponding I/O pin. If PCINT7..0 is set and the PCIE0 bit in PCICR is set, pin change interrupt is enabled on the corresponding I/O pin. If PCINT7..0 is cleared, pin change interrupt on the corresponding I/O pin is disabled. ATmega16U4/32U4 [DATASHEET] 91 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

12. Timer/Counter0, Timer/Counter1, and Timer/Counter3 Prescalers Timer/Counter0, 1, and 3 share the same prescaler module, but the Timer/Counters can have different prescaler settings. The description below applies to all Timer/Counters. Tn is used as a general name, n = 0, 1, or 3. 12.1 Internal Clock Source The Timer/Counter can be clocked directly by the system clock (by setting the CSn2:0 = 1). This provides the fastest operation, with a maximum Timer/Counter clock frequency equal to system clock frequency (f ). CLK_I/O Alternatively, one of four taps from the prescaler can be used as a clock source. The prescaled clock has a fre- quency of either f /8, f /64, f /256, or f /1024. CLK_I/O CLK_I/O CLK_I/O CLK_I/O 12.2 Prescaler Reset The prescaler is free running, i.e., operates independently of the Clock Select logic of the Timer/Counter, and it is shared by the Timer/Counter Tn. Since the prescaler is not affected by the Timer/Counter’s clock select, the state of the prescaler will have implications for situations where a prescaled clock is used. One example of pres- caling artifacts occurs when the timer is enabled and clocked by the prescaler (6 > CSn2:0 > 1). The number of system clock cycles from when the timer is enabled to the first count occurs can be from 1 to N+1 system clock cycles, where N equals the prescaler divisor (8, 64, 256, or 1024). It is possible to use the prescaler reset for synchronizing the Timer/Counter to program execution. However, care must be taken if the other Timer/Counter that shares the same prescaler also uses prescaling. A prescaler reset will affect the prescaler period for all Timer/Counters it is connected to. 12.3 External Clock Source An external clock source applied to the Tn pin can be used as Timer/Counter clock (clk ). The Tn pin is sam- Tn pled once every system clock cycle by the pin synchronization logic. The synchronized (sampled) signal is then passed through the edge detector. Figure 12-1 shows a functional equivalent block diagram of the Tn synchro- nization and edge detector logic. The registers are clocked at the positive edge of the internal system clock (clk ). The latch is transparent in the high period of the internal system clock. I/O The edge detector generates one clk pulse for each positive (CSn2:0 = 7) or negative (CSn2:0 = 6) edge it Tn detects. Figure 12-1. Tn/T0 Pin Sampling Tn D Q D Q D Q Tn_sync (To Clock Select Logic) LE clk I/O Synchronization Edge Detector The synchronization and edge detector logic introduces a delay of 2.5 to 3.5 system clock cycles from an edge has been applied to the Tn pin to the counter is updated. Enabling and disabling of the clock input must be done when Tn has been stable for at least one system clock cycle, otherwise it is a risk that a false Timer/Counter clock pulse is generated. Each half period of the external clock applied must be longer than one system clock cycle to ensure correct sampling. The external clock must be guaranteed to have less than half the system clock frequency (f < ExtClk f /2) given a 50/50% duty cycle. Since the edge detector uses sampling, the maximum frequency of an clk_I/O external clock it can detect is half the sampling frequency (Nyquist sampling theorem). However, due to varia- tion of the system clock frequency and duty cycle caused by Oscillator source (crystal, resonator, and ATmega16U4/32U4 [DATASHEET] 92 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

capacitors) tolerances, it is recommended that maximum frequency of an external clock source is less than f /2.5. clk_I/O An external clock source can not be prescaled. Figure 12-2. Prescaler for Synchronous Timer/Counters clk I/O Clear PSR10 Tn Synchronization Tn Synchronization CSn0 CSn0 CSn1 CSn1 CSn2 CSn2 TIMER/COUNTERn CLOCK SOURCE TIMER/COUNTERn CLOCK SOURCE clk clk Tn Tn Note: T3 input is not available on the ATmega16U4/ATmega32U4 products. “Tn” only refers to either T0 or T1 inputs. 12.4 Register Description 12.4.1 General Timer/Counter Control Register – GTCCR Bit 7 6 5 4 3 2 1 0 TSM – – – – – PSRASY PSRSYNC GTCCR Read/Write R/W R R R R R R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 7 – TSM: Timer/Counter Synchronization Mode Writing the TSM bit to one activates the Timer/Counter Synchronization mode. In this mode, the value that is written to the PSRASY and PSRSYNC bits is kept, hence keeping the corresponding prescaler reset signals asserted. This ensures that the corresponding Timer/Counters are halted and can be configured to the same value without the risk of one of them advancing during configuration. When the TSM bit is written to zero, the PSRASY and PSRSYNC bits are cleared by hardware, and the Timer/Counters start counting simultaneously. • Bit 0 – PSRSYNC: Prescaler Reset for Synchronous Timer/Counters When this bit is one, Timer/Counter0 and Timer/Counter1 and Timer/Counter3 prescaler will be Reset. This bit is normally cleared immediately by hardware, except if the TSM bit is set. Note that Timer/Counter0, Timer/Counter1 and Timer/Counter3 share the same prescaler and a reset of this prescaler will affect all timers. ATmega16U4/32U4 [DATASHEET] 93 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

13. 8-bit Timer/Counter0 with PWM Timer/Counter0 is a general purpose 8-bit Timer/Counter module, with two independent Output Compare Units, and with PWM support. It allows accurate program execution timing (event management) and wave generation. The main features are: • Two Independent Output Compare Units • Double Buffered Output Compare Registers • Clear Timer on Compare Match (Auto Reload) • Glitch Free, Phase Correct Pulse Width Modulator (PWM) • Variable PWM Period • Frequency Generator • Three Independent Interrupt Sources (TOV0, OCF0A, and OCF0B) 13.1 Overview A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 13-1. For the actual placement of I/O pins, refer to “Pinout” on page3. CPU accessible I/O Registers, including I/O bits and I/O pins, are shown in bold. The device-specific I/O Register and bit locations are listed in the “8-bit Timer/Counter Register Description” on page104. Figure 13-1. 8-bit Timer/Counter Block Diagram Count TOVn Clear (Int.Req.) Control Logic Direction clkTn Clock Select DeEtdegcetor Tn TOP BOTTOM ( From Prescaler ) Timer/Counter TCNTn = = 0 OCnA (Int.Req.) = GWeanveerfaotrimon OCnA OCRnA S VFTaiOlxuePed O(InCt.nRBeq.) A BU = GWeanveerfaotrimon OCnB T A D OCRnB TCCRnA TCCRnB 13.1.1 Registers The Timer/Counter (TCNT0) and Output Compare Registers (OCR0A and OCR0B) are 8-bit registers. Interrupt request (abbreviated to Int.Req. in the figure) signals are all visible in the Timer Interrupt Flag Register (TIFR0). All interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK0). TIFR0 and TIMSK0 are not shown in the figure. The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on the T0 pin. The Clock Select logic block controls which clock source and edge the Timer/Counter uses to increment (or decrement) its value. The Timer/Counter is inactive when no clock source is selected. The output from the Clock Select logic is referred to as the timer clock (clk ). T0 ATmega16U4/32U4 [DATASHEET] 94 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The double buffered Output Compare Registers (OCR0A and OCR0B) are compared with the Timer/Counter value at all times. The result of the compare can be used by the Waveform Generator to generate a PWM or variable frequency output on the Output Compare pins (OC0A and OC0B). See “Output Compare Unit” on page96. for details. The Compare Match event will also set the Compare Flag (OCF0A or OCF0B) which can be used to generate an Output Compare interrupt request. 13.1.2 Definitions Many register and bit references in this section are written in general form. A lower case “n” replaces the Timer/Counter number, in this case 0. A lower case “x” replaces the Output Compare Unit, in this case Compare Unit A or Compare Unit B. However, when using the register or bit defined in a program, the precise form must be used, i.e., TCNT0 for accessing Timer/Counter0 counter value and so on. The definitions in the table are also used extensively throughout the document. BOTTOM The counter reaches the BOTTOM when it becomes 0x00. MAX The counter reaches its MAXimum when it becomes 0xFF (decimal 255). The counter reaches the TOP when it becomes equal to the highest value in the count TOP sequence. The TOP value can be assigned to be the fixed value 0xFF (MAX) or the value stored in the OCR0A Register. The assignment is dependent on the mode of operation. 13.2 Timer/Counter Clock Sources The Timer/Counter can be clocked by an internal or an external clock source. The clock source is selected by the Clock Select logic which is controlled by the Clock Select (CS02:0) bits located in the Timer/Counter Control Register (TCCR0B). For details on clock sources and prescaler, see “Timer/Counter0, Timer/Counter1, and Timer/Counter3 Prescalers” on page92. 13.3 Counter Unit The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure 13-2 shows a block diagram of the counter and its surroundings. Figure 13-2. Counter Unit Block Diagram TOVn DATA BUS (Int.Req.) Clock Select count Edge Tn TCNTn clear Control Logic clkTn Detector direction ( From Prescaler ) bottom top Signal description (internal signals): count: Increment or decrement TCNT0 by 1. direction: Select between increment and decrement. clear: Clear TCNT0 (set all bits to zero). clk Timer/Counter clock, referred to as clk in the following. Tn: T0 top: Signalize that TCNT0 has reached maximum value. ATmega16U4/32U4 [DATASHEET] 95 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

bottom: Signalize that TCNT0 has reached minimum value (zero). Depending of the mode of operation used, the counter is cleared, incremented, or decremented at each timer clock (clk ). clk can be generated from an external or internal clock source, selected by the Clock Select bits T0 T0 (CS02:0). When no clock source is selected (CS02:0 = 0) the timer is stopped. However, the TCNT0 value can be accessed by the CPU, regardless of whether clk is present or not. A CPU write overrides (has priority over) T0 all counter clear or count operations. The counting sequence is determined by the setting of the WGM01 and WGM00 bits located in the Timer/Counter Control Register (TCCR0A) and the WGM02 bit located in the Timer/Counter Control Register B (TCCR0B). There are close connections between how the counter behaves (counts) and how waveforms are generated on the Output Compare outputs OC0A and OC0B. For more details about advanced counting sequences and waveform generation, see “Modes of Operation” on page98. The Timer/Counter Overflow Flag (TOV0) is set according to the mode of operation selected by the WGM02:0 bits. TOV0 can be used for generating a CPU interrupt. 13.4 Output Compare Unit The 8-bit comparator continuously compares TCNT0 with the Output Compare Registers (OCR0A and OCR0B). Whenever TCNT0 equals OCR0A or OCR0B, the comparator signals a match. A match will set the Output Compare Flag (OCF0A or OCF0B) at the next timer clock cycle. If the corresponding interrupt is enabled, the Output Compare Flag generates an Output Compare interrupt. The Output Compare Flag is automatically cleared when the interrupt is executed. Alternatively, the flag can be cleared by software by writing a logical one to its I/O bit location. The Waveform Generator uses the match signal to generate an output according to operating mode set by the WGM02:0 bits and Compare Output mode (COM0x1:0) bits. The max. and bottom signals are used by the Waveform Generator for handling the special cases of the extreme values in some modes of operation (“Modes of Operation” on page98). Figure 13-3 on page 96 shows a block diagram of the Output Compare unit. Figure 13-3. Output Compare Unit, Block Diagram DATA BUS OCRnx TCNTn = (8-bit Comparator ) OCFnx (Int.Req.) top bottom Waveform Generator OCnx FOCn WGMn1:0 COMnX1:0 The OCR0x Registers are double buffered when using any of the Pulse Width Modulation (PWM) modes. For the normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is disabled. The double buffering synchronizes the update of the OCR0x Compare Registers to either top or bottom of the ATmega16U4/32U4 [DATASHEET] 96 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

counting sequence. The synchronization prevents the occurrence of odd-length, non-symmetrical PWM pulses, thereby making the output glitch-free. The OCR0x Register access may seem complex, but this is not case. When the double buffering is enabled, the CPU has access to the OCR0x Buffer Register, and if double buffering is disabled the CPU will access the OCR0x directly. 13.4.1 Force Output Compare In non-PWM waveform generation modes, the match output of the comparator can be forced by writing a one to the Force Output Compare (FOC0x) bit. Forcing Compare Match will not set the OCF0x Flag or reload/clear the timer, but the OC0x pin will be updated as if a real Compare Match had occurred (the COM0x1:0 bits settings define whether the OC0x pin is set, cleared or toggled). 13.4.2 Compare Match Blocking by TCNT0 Write All CPU write operations to the TCNT0 Register will block any Compare Match that occur in the next timer clock cycle, even when the timer is stopped. This feature allows OCR0x to be initialized to the same value as TCNT0 without triggering an interrupt when the Timer/Counter clock is enabled. 13.4.3 Using the Output Compare Unit Since writing TCNT0 in any mode of operation will block all Compare Matches for one timer clock cycle, there are risks involved when changing TCNT0 when using the Output Compare Unit, independently of whether the Timer/Counter is running or not. If the value written to TCNT0 equals the OCR0x value, the Compare Match will be missed, resulting in incorrect waveform generation. Similarly, do not write the TCNT0 value equal to BOTTOM when the counter is down-counting. The setup of the OC0x should be performed before setting the Data Direction Register for the port pin to output. The easiest way of setting the OC0x value is to use the Force Output Compare (FOC0x) strobe bits in Normal mode. The OC0x Registers keep their values even when changing between Waveform Generation modes. Be aware that the COM0x1:0 bits are not double buffered together with the compare value. Changing the COM0x1:0 bits will take effect immediately. 13.5 Compare Match Output Unit The Compare Output mode (COM0x1:0) bits have two functions. The Waveform Generator uses the COM0x1:0 bits for defining the Output Compare (OC0x) state at the next Compare Match. Also, the COM0x1:0 bits control the OC0x pin output source. Figure 13-4 shows a simplified schematic of the logic affected by the COM0x1:0 bit setting. The I/O Registers, I/O bits, and I/O pins in the figure are shown in bold. Only the parts of the general I/O Port Control Registers (DDR and PORT) that are affected by the COM0x1:0 bits are shown. When referring to the OC0x state, the reference is for the internal OC0x Register, not the OC0x pin. If a system reset occur, the OC0x Register is reset to “0”. ATmega16U4/32U4 [DATASHEET] 97 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 13-4. Compare Match Output Unit, Schematic COMnx1 COMnx0 Waveform D Q FOCn Generator 1 OCnx OCnx Pin 0 D Q S U B PORT A T A D D Q DDR clk I/O The general I/O port function is overridden by the Output Compare (OC0x) from the Waveform Generator if either of the COM0x1:0 bits are set. However, the OC0x pin direction (input or output) is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction Register bit for the OC0x pin (DDR_OC0x) must be set as output before the OC0x value is visible on the pin. The port override function is independent of the Waveform Generation mode. The design of the Output Compare pin logic allows initialization of the OC0x state before the output is enabled. Note that some COM0x1:0 bit settings are reserved for certain modes of operation. See “8-bit Timer/Counter Register Description” on page104. 13.5.1 Compare Output Mode and Waveform Generation The Waveform Generator uses the COM0x1:0 bits differently in Normal, CTC, and PWM modes. For all modes, setting the COM0x1:0 = 0 tells the Waveform Generator that no action on the OC0x Register is to be performed on the next Compare Match. For compare output actions in the non-PWM modes refer to Table13-1 on page104. For fast PWM mode, refer to Table13-2 on page104, and for phase correct PWM refer to Table13- 3 on page105. A change of the COM0x1:0 bits state will have effect at the first Compare Match after the bits are written. For non-PWM modes, the action can be forced to have immediate effect by using the FOC0x strobe bits. 13.6 Modes of Operation The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is defined by the combination of the Waveform Generation mode (WGM02:0) and Compare Output mode (COM0x1:0) bits. The Compare Output mode bits do not affect the counting sequence, while the Waveform Generation mode bits do. The COM0x1:0 bits control whether the PWM output generated should be inverted or not (inverted or non- inverted PWM). For non-PWM modes the COM0x1:0 bits control whether the output should be set, cleared, or toggled at a Compare Match (See “Compare Match Output Unit” on page97.). For detailed timing information see “Timer/Counter Timing Diagrams” on page102. 13.6.1 Normal Mode The simplest mode of operation is the Normal mode (WGM02:0 = 0). In this mode the counting direction is always up (incrementing), and no counter clear is performed. The counter simply overruns when it passes its ATmega16U4/32U4 [DATASHEET] 98 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

maximum 8-bit value (TOP = 0xFF) and then restarts from the bottom (0x00). In normal operation the Timer/Counter Overflow Flag (TOV0) will be set in the same timer clock cycle as the TCNT0 becomes zero. The TOV0 Flag in this case behaves like a ninth bit, except that it is only set, not cleared. However, combined with the timer overflow interrupt that automatically clears the TOV0 Flag, the timer resolution can be increased by software. There are no special cases to consider in the Normal mode, a new counter value can be written anytime. The Output Compare Unit can be used to generate interrupts at some given time. Using the Output Compare to generate waveforms in Normal mode is not recommended, since this will occupy too much of the CPU time. 13.6.2 Clear Timer on Compare Match (CTC) Mode In Clear Timer on Compare or CTC mode (WGM02:0 = 2), the OCR0A Register is used to manipulate the counter resolution. In CTC mode the counter is cleared to zero when the counter value (TCNT0) matches the OCR0A. The OCR0A defines the top value for the counter, hence also its resolution. This mode allows greater control of the Compare Match output frequency. It also simplifies the operation of counting external events. The timing diagram for the CTC mode is shown in Figure 13-5 on page 99. The counter value (TCNT0) increases until a Compare Match occurs between TCNT0 and OCR0A, and then counter (TCNT0) is cleared. Figure 13-5. CTC Mode, Timing Diagram OCnx Interrupt Flag Set TCNTn OCn (COMnx1:0 = 1) (Toggle) Period 1 2 3 4 An interrupt can be generated each time the counter value reaches the TOP value by using the OCF0A Flag. If the interrupt is enabled, the interrupt handler routine can be used for updating the TOP value. However, changing TOP to a value close to BOTTOM when the counter is running with none or a low prescaler value must be done with care since the CTC mode does not have the double buffering feature. If the new value written to OCR0A is lower than the current value of TCNT0, the counter will miss the Compare Match. The counter will then have to count to its maximum value (0xFF) and wrap around starting at 0x00 before the Compare Match can occur. For generating a waveform output in CTC mode, the OC0A output can be set to toggle its logical level on each Compare Match by setting the Compare Output mode bits to toggle mode (COM0A1:0 = 1). The OC0A value will not be visible on the port pin unless the data direction for the pin is set to output. The waveform generated will have a maximum frequency of f = f /2 when OCR0A is set to zero (0x00). The waveform frequency is OC0 clk_I/O defined by the following equation: f clk_I/O f = -------------------------------------------------- OCnx 2N1+OCRnx The N variable represents the prescaler factor (1, 8, 64, 256, or 1024). As for the Normal mode of operation, the TOV0 Flag is set in the same timer clock cycle that the counter counts from MAX to 0x00. ATmega16U4/32U4 [DATASHEET] 99 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

13.6.3 Fast PWM Mode The fast Pulse Width Modulation or fast PWM mode (WGM02:0 = 3 or 7) provides a high frequency PWM waveform generation option. The fast PWM differs from the other PWM option by its single-slope operation. The counter counts from BOTTOM to TOP then restarts from BOTTOM. TOP is defined as 0xFF when WGM2:0 = 3, and OCR0A when WGM2:0 = 7. In non-inverting Compare Output mode, the Output Compare (OC0x) is cleared on the Compare Match between TCNT0 and OCR0x, and set at BOTTOM. In inverting Compare Output mode, the output is set on Compare Match and cleared at BOTTOM. Due to the single-slope operation, the operating frequency of the fast PWM mode can be twice as high as the phase correct PWM mode that use dual-slope operation. This high frequency makes the fast PWM mode well suited for power regulation, rectification, and DAC applications. High frequency allows physically small sized external components (coils, capacitors), and therefore reduces total system cost. In fast PWM mode, the counter is incremented until the counter value matches the TOP value. The counter is then cleared at the following timer clock cycle. The timing diagram for the fast PWM mode is shown in Figure 13-6. The TCNT0 value is in the timing diagram shown as a histogram for illustrating the single-slope operation. The diagram includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNT0 slopes represent Compare Matches between OCR0x and TCNT0. Figure 13-6. Fast PWM Mode, Timing Diagram OCRnx Interrupt Flag Set OCRnx Update and TOVn Interrupt Flag Set TCNTn OCnx (COMnx1:0 = 2) OCnx (COMnx1:0 = 3) Period 1 2 3 4 5 6 7 The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches TOP. If the interrupt is enabled, the interrupt handler routine can be used for updating the compare value. In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC0x pins. Setting the COM0x1:0 bits to two will produce a non-inverted PWM and an inverted PWM output can be generated by setting the COM0x1:0 to three: Setting the COM0A1:0 bits to one allows the OC0A pin to toggle on Compare Matches if the WGM02 bit is set. This option is not available for the OC0B pin (See Table13-2 on page104). The actual OC0x value will only be visible on the port pin if the data direction for the port pin is set as output. The PWM waveform is generated by setting (or clearing) the OC0x Register at the Compare Match between OCR0x and TCNT0, and clearing (or setting) the OC0x Register at the timer clock cycle the counter is cleared (changes from TOP to BOTTOM). The PWM frequency for the output can be calculated by the following equation: f clk_I/O f = ------------------ OCnxPWM N256 The N variable represents the prescaler factor (1, 8, 64, 256, or 1024). ATmega16U4/32U4 [DATASHEET] 100 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The extreme values for the OCR0A Register represents special cases when generating a PWM waveform output in the fast PWM mode. If the OCR0A is set equal to BOTTOM, the output will be a narrow spike for each MAX+1 timer clock cycle. Setting the OCR0A equal to MAX will result in a constantly high or low output (depending on the polarity of the output set by the COM0A1:0 bits.) A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OC0x to toggle its logical level on each Compare Match (COM0x1:0 = 1). The waveform generated will have a maximum frequency of f = f /2 when OCR0A is set to zero. This feature is similar to the OC0A toggle in CTC mode, OC0 clk_I/O except the double buffer feature of the Output Compare unit is enabled in the fast PWM mode. 13.6.4 Phase Correct PWM Mode The phase correct PWM mode (WGM02:0 = 1 or 5) provides a high resolution phase correct PWM waveform generation option. The phase correct PWM mode is based on a dual-slope operation. The counter counts repeatedly from BOTTOM to TOP and then from TOP to BOTTOM. TOP is defined as 0xFF when WGM2:0 = 1, and OCR0A when WGM2:0 = 5. In non-inverting Compare Output mode, the Output Compare (OC0x) is cleared on the Compare Match between TCNT0 and OCR0x while up counting, and set on the Compare Match while down-counting. In inverting Output Compare mode, the operation is inverted. The dual-slope operation has lower maximum operation frequency than single slope operation. However, due to the symmetric feature of the dual-slope PWM modes, these modes are preferred for motor control applications. In phase correct PWM mode the counter is incremented until the counter value matches TOP. When the counter reaches TOP, it changes the count direction. The TCNT0 value will be equal to TOP for one timer clock cycle. The timing diagram for the phase correct PWM mode is shown on Figure 13-7. The TCNT0 value is in the timing diagram shown as a histogram for illustrating the dual-slope operation. The diagram includes non- inverted and inverted PWM outputs. The small horizontal line marks on the TCNT0 slopes represent Compare Matches between OCR0x and TCNT0. Figure 13-7. Phase Correct PWM Mode, Timing Diagram OCnx Interrupt Flag Set OCRnx Update TOVn Interrupt Flag Set TCNTn OCnx (COMnx1:0 = 2) OCnx (COMnx1:0 = 3) Period 1 2 3 The Timer/Counter Overflow Flag (TOV0) is set each time the counter reaches BOTTOM. The Interrupt Flag can be used to generate an interrupt each time the counter reaches the BOTTOM value. In phase correct PWM mode, the compare unit allows generation of PWM waveforms on the OC0x pins. Setting the COM0x1:0 bits to two will produce a non-inverted PWM. An inverted PWM output can be generated by ATmega16U4/32U4 [DATASHEET] 101 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

setting the COM0x1:0 to three: Setting the COM0A0 bits to one allows the OC0A pin to toggle on Compare Matches if the WGM02 bit is set. This option is not available for the OC0B pin. The actual OC0x value will only be visible on the port pin if the data direction for the port pin is set as output. The PWM waveform is generated by clearing (or setting) the OC0x Register at the Compare Match between OCR0x and TCNT0 when the counter increments, and setting (or clearing) the OC0x Register at Compare Match between OCR0x and TCNT0 when the counter decrements. The PWM frequency for the output when using phase correct PWM can be calculated by the following equation: f clk_I/O f = ------------------ OCnxPCPWM N510 The N variable represents the prescaler factor (1, 8, 64, 256, or 1024). The extreme values for the OCR0A Register represent special cases when generating a PWM waveform output in the phase correct PWM mode. If the OCR0A is set equal to BOTTOM, the output will be continuously low and if set equal to MAX the output will be continuously high for non-inverted PWM mode. For inverted PWM the output will have the opposite logic values. At the very start of period 2 in Figure 13-7 on page 101 OCnx has a transition from high to low even though there is no Compare Match. The point of this transition is to guarantee symmetry around BOTTOM. There are two cases that give a transition without Compare Match.  OCR0A changes its value from MAX, like in Figure 13-7 on page 101. When the OCR0A value is MAX the OCn pin value is the same as the result of a down-counting Compare Match. To ensure symmetry around BOTTOM the OCn value at MAX must correspond to the result of an up-counting Compare Match.  The timer starts counting from a value higher than the one in OCR0A, and for that reason misses the Compare Match and hence the OCn change that would have happened on the way up. 13.7 Timer/Counter Timing Diagrams The Timer/Counter is a synchronous design and the timer clock (clk ) is therefore shown as a clock enable T0 signal in the following figures. The figures include information on when Interrupt Flags are set. Figure 13-8 contains timing data for basic Timer/Counter operation. The figure shows the count sequence close to the MAX value in all modes other than phase correct PWM mode. Figure 13-8. Timer/Counter Timing Diagram, no Prescaling clk I/O clk Tn (clk /1) I/O TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1 TOVn Figure 13-9 on page 103 shows the same timing data, but with the prescaler enabled. ATmega16U4/32U4 [DATASHEET] 102 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 13-9. Timer/Counter Timing Diagram, with Prescaler (f /8) clk_I/O clk I/O clk Tn (clk /8) I/O TCNTn MAX - 1 MAX BOTTOM BOTTOM + 1 TOVn Figure 13-10 shows the setting of OCF0B in all modes and OCF0A in all modes except CTC mode and PWM mode, where OCR0A is TOP. Figure 13-10. Timer/Counter Timing Diagram, Setting of OCF0x, with Prescaler (f /8) clk_I/O clk I/O clk Tn (clk /8) I/O TCNTn OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2 OCRnx OCRnx Value OCFnx Figure 13-11 shows the setting of OCF0A and the clearing of TCNT0 in CTC mode and fast PWM mode where OCR0A is TOP. Figure 13-11. Timer/Counter Timing Diagram, Clear Timer on Compare Match mode, with Prescaler (f /8) clk_I/O clk I/O clk Tn (clk /8) I/O TCNTn TOP - 1 TOP BOTTOM BOTTOM + 1 (CTC) OCRnx TOP OCFnx ATmega16U4/32U4 [DATASHEET] 103 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

13.8 8-bit Timer/Counter Register Description 13.8.1 Timer/Counter Control Register A – TCCR0A Bit 7 6 5 4 3 2 1 0 COM0A1 COM0A0 COM0B1 COM0B0 – – WGM01 WGM00 TCCR0A Read/Write R/W R/W R/W R/W R R R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bits 7:6 – COM01A:0: Compare Match Output A Mode These bits control the Output Compare pin (OC0A) behavior. If one or both of the COM0A1:0 bits are set, the OC0A output overrides the normal port functionality of the I/O pin it is connected to. However, note that the Data Direction Register (DDR) bit corresponding to the OC0A pin must be set in order to enable the output driver. When OC0A is connected to the pin, the function of the COM0A1:0 bits depends on the WGM02:0 bit setting. The table shows the COM0A1:0 bit functionality when the WGM02:0 bits are set to a normal or CTC mode (non- PWM). Table 13-1. Compare Output Mode, non-PWM Mode COM0A1 COM0A0 Description 0 0 Normal port operation, OC0A disconnected 0 1 Toggle OC0A on Compare Match 1 0 Clear OC0A on Compare Match 1 1 Set OC0A on Compare Match The table shows the COM0A1:0 bit functionality when the WGM01:0 bits are set to fast PWM mode Table 13-2. Compare Output Mode, Fast PWM Mode(1) COM0A1 COM0A0 Description 0 0 Normal port operation, OC0A disconnected WGM02 = 0: Normal Port Operation, OC0A Disconnected 0 1 WGM02 = 1: Toggle OC0A on Compare Match 1 0 Clear OC0A on Compare Match, set OC0A at TOP 1 1 Set OC0A on Compare Match, clear OC0A at TOP Note: 1. A special case occurs when OCR0A equals TOP and COM0A1 is set. In this case, the Compare Match is ignored, but the set or clear is done at TOP. See “Fast PWM Mode” on page100 for more details. The table shows the COM0A1:0 bit functionality when the WGM02:0 bits are set to phase correct PWM mode. ATmega16U4/32U4 [DATASHEET] 104 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 13-3. Compare Output Mode, Phase Correct PWM Mode(1) COM0A1 COM0A0 Description 0 0 Normal port operation, OC0A disconnected WGM02 = 0: Normal Port Operation, OC0A Disconnected 0 1 WGM02 = 1: Toggle OC0A on Compare Match Clear OC0A on Compare Match when up-counting. Set OC0A on Compare 1 0 Match when down-counting. Set OC0A on Compare Match when up-counting. Clear OC0A on Compare 1 1 Match when down-counting. Note: 1. A special case occurs when OCR0A equals TOP and COM0A1 is set. In this case, the Compare Match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on page101 for more details. • Bits 5:4 – COM0B1:0: Compare Match Output B Mode These bits control the Output Compare pin (OC0B) behavior. If one or both of the COM0B1:0 bits are set, the OC0B output overrides the normal port functionality of the I/O pin it is connected to. However, note that the Data Direction Register (DDR) bit corresponding to the OC0B pin must be set in order to enable the output driver. When OC0B is connected to the pin, the function of the COM0B1:0 bits depends on the WGM02:0 bit setting. The table shows the COM0A1:0 bit functionality when the WGM02:0 bits are set to a normal or CTC mode (non- PWM). Table 13-4. Compare Output Mode, non-PWM Mode COM01 COM00 Description 0 0 Normal port operation, OC0B disconnected 0 1 Toggle OC0B on Compare Match 1 0 Clear OC0B on Compare Match 1 1 Set OC0B on Compare Match The table shows the COM0B1:0 bit functionality when the WGM02:0 bits are set to fast PWM mode Table 13-5. Compare Output Mode, Fast PWM Mode(1) COM01 COM00 Description 0 0 Normal port operation, OC0B disconnected 0 1 Reserved 1 0 Clear OC0B on Compare Match, set OC0B at TOP 1 1 Set OC0B on Compare Match, clear OC0B at TOP Note: 1. A special case occurs when OCR0B equals TOP and COM0B1 is set. In this case, the Compare Match is ignored, but the set or clear is done at TOP. See “Fast PWM Mode” on page100 for more details. ATmega16U4/32U4 [DATASHEET] 105 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The table shows the COM0B1:0 bit functionality when the WGM02:0 bits are set to phase correct PWM mode. Table 13-6. Compare Output Mode, Phase Correct PWM Mode(1) COM0A1 COM0A0 Description 0 0 Normal port operation, OC0B disconnected 0 1 Reserved Clear OC0B on Compare Match when up-counting. Set OC0B on Compare 1 0 Match when down-counting. Set OC0B on Compare Match when up-counting. Clear OC0B on Compare 1 1 Match when down-counting. Note: 1. A special case occurs when OCR0B equals TOP and COM0B1 is set. In this case, the Compare Match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on page101 for more details. • Bits 3, 2 – Res: Reserved Bits These bits are reserved bits in the ATmega16U4/ATmega32U4 and will always read as zero. • Bits 1:0 – WGM01:0: Waveform Generation Mode Combined with the WGM02 bit found in the TCCR0B Register, these bits control the counting sequence of the counter, the source for maximum (TOP) counter value, and what type of waveform generation to be used as shown in the table. Modes of operation supported by the Timer/Counter unit are: Normal mode (counter), Clear Timer on Compare Match (CTC) mode, and two types of Pulse Width Modulation (PWM) modes (see “Modes of Operation” on page98). Table 13-7. Waveform Generation Mode Bit Description Timer/Counter Mode of Update of TOV Flag Mode WGM2 WGM1 WGM0 Operation TOP OCRx at Set on(1)(2) 0 0 0 0 Normal 0xFF Immediate MAX 1 0 0 1 PWM, Phase Correct 0xFF TOP BOTTOM 2 0 1 0 CTC OCRA Immediate MAX 3 0 1 1 Fast PWM 0xFF TOP MAX 4 1 0 0 Reserved – – – 5 1 0 1 PWM, Phase Correct OCRA TOP BOTTOM 6 1 1 0 Reserved – – – 7 1 1 1 Fast PWM OCRA TOP TOP Notes: 1. MAX = 0xFF 2. BOTTOM = 0x00 ATmega16U4/32U4 [DATASHEET] 106 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

13.8.2 Timer/Counter Control Register B – TCCR0B Bit 7 6 5 4 3 2 1 0 FOC0A FOC0B – – WGM02 CS02 CS01 CS00 TCCR0B Read/Write W W R R R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 7 – FOC0A: Force Output Compare A The FOC0A bit is only active when the WGM bits specify a non-PWM mode. However, for ensuring compatibility with future devices, this bit must be set to zero when TCCR0B is written when operating in PWM mode. When writing a logical one to the FOC0A bit, an immediate Compare Match is forced on the Waveform Generation unit. The OC0A output is changed according to its COM0A1:0 bits setting. Note that the FOC0A bit is implemented as a strobe. Therefore it is the value present in the COM0A1:0 bits that determines the effect of the forced compare. A FOC0A strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR0A as TOP. The FOC0A bit is always read as zero. • Bit 6 – FOC0B: Force Output Compare B The FOC0B bit is only active when the WGM bits specify a non-PWM mode. However, for ensuring compatibility with future devices, this bit must be set to zero when TCCR0B is written when operating in PWM mode. When writing a logical one to the FOC0B bit, an immediate Compare Match is forced on the Waveform Generation unit. The OC0B output is changed according to its COM0B1:0 bits setting. Note that the FOC0B bit is implemented as a strobe. Therefore it is the value present in the COM0B1:0 bits that determines the effect of the forced compare. A FOC0B strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR0B as TOP. The FOC0B bit is always read as zero. • Bits 5:4 – Res: Reserved Bits These bits are reserved bits and will always read as zero. • Bit 3 – WGM02: Waveform Generation Mode See the description in the “Timer/Counter Control Register A – TCCR0A” on page104. ATmega16U4/32U4 [DATASHEET] 107 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bits 2:0 – CS02:0: Clock Select The three Clock Select bits select the clock source to be used by the Timer/Counter. Table 13-8. Clock Select Bit Description CS02 CS01 CS00 Description 0 0 0 No clock source (Timer/Counter stopped) 0 0 1 clk /(No prescaling) I/O 0 1 0 clk /8 (From prescaler) I/O 0 1 1 clk /64 (From prescaler) I/O 1 0 0 clk /256 (From prescaler) I/O 1 0 1 clk /1024 (From prescaler) I/O 1 1 0 External clock source on T0 pin. Clock on falling edge. 1 1 1 External clock source on T0 pin. Clock on rising edge. If external pin modes are used for the Timer/Counter0, transitions on the T0 pin will clock the counter even if the pin is configured as an output. This feature allows software control of the counting. 13.8.3 Timer/Counter Register – TCNT0 Bit 7 6 5 4 3 2 1 0 TCNT0[7:0] TCNT0 Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 The Timer/Counter Register gives direct access, both for read and write operations, to the Timer/Counter unit 8-bit counter. Writing to the TCNT0 Register blocks (removes) the Compare Match on the following timer clock. Modifying the counter (TCNT0) while the counter is running, introduces a risk of missing a Compare Match between TCNT0 and the OCR0x Registers. 13.8.4 Output Compare Register A – OCR0A Bit 7 6 5 4 3 2 1 0 OCR0A[7:0] OCR0A Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 The Output Compare Register A contains an 8-bit value that is continuously compared with the counter value (TCNT0). A match can be used to generate an Output Compare interrupt, or to generate a waveform output on the OC0A pin. 13.8.5 Output Compare Register B – OCR0B Bit 7 6 5 4 3 2 1 0 OCR0B[7:0] OCR0B Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 ATmega16U4/32U4 [DATASHEET] 108 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The Output Compare Register B contains an 8-bit value that is continuously compared with the counter value (TCNT0). A match can be used to generate an Output Compare interrupt, or to generate a waveform output on the OC0B pin. 13.8.6 Timer/Counter Interrupt Mask Register – TIMSK0 Bit 7 6 5 4 3 2 1 0 – – – – – OCIE0B OCIE0A TOIE0 TIMSK0 Read/Write R R R R R R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bits 7..3, 0 – Res: Reserved Bits These bits are reserved bits and will always read as zero. • Bit 2 – OCIE0B: Timer/Counter Output Compare Match B Interrupt Enable When the OCIE0B bit is written to one, and the I-bit in the Status Register is set, the Timer/Counter Compare Match B interrupt is enabled. The corresponding interrupt is executed if a Compare Match in Timer/Counter occurs, i.e., when the OCF0B bit is set in the Timer/Counter Interrupt Flag Register – TIFR0. • Bit 1 – OCIE0A: Timer/Counter0 Output Compare Match A Interrupt Enable When the OCIE0A bit is written to one, and the I-bit in the Status Register is set, the Timer/Counter0 Compare Match A interrupt is enabled. The corresponding interrupt is executed if a Compare Match in Timer/Counter0 occurs, i.e., when the OCF0A bit is set in the Timer/Counter 0 Interrupt Flag Register – TIFR0. • Bit 0 – TOIE0: Timer/Counter0 Overflow Interrupt Enable When the TOIE0 bit is written to one, and the I-bit in the Status Register is set, the Timer/Counter0 Overflow interrupt is enabled. The corresponding interrupt is executed if an overflow in Timer/Counter0 occurs, i.e., when the TOV0 bit is set in the Timer/Counter 0 Interrupt Flag Register – TIFR0. 13.8.7 Timer/Counter 0 Interrupt Flag Register – TIFR0 Bit 7 6 5 4 3 2 1 0 – – – – – OCF0B OCF0A TOV0 TIFR0 Read/Write R R R R R R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bits 7..3, 0 – Res: Reserved Bits These bits are reserved bits in the ATmega16U4/ATmega32U4 and will always read as zero. • Bit 2 – OCF0B: Timer/Counter 0 Output Compare B Match Flag The OCF0B bit is set when a Compare Match occurs between the Timer/Counter and the data in OCR0B – Output Compare Register0 B. OCF0B is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, OCF0B is cleared by writing a logic one to the flag. When the I-bit in SREG, OCIE0B (Timer/Counter Compare B Match Interrupt Enable), and OCF0B are set, the Timer/Counter Compare Match Interrupt is executed. • Bit 1 – OCF0A: Timer/Counter 0 Output Compare A Match Flag The OCF0A bit is set when a Compare Match occurs between the Timer/Counter0 and the data in OCR0A – Output Compare Register0. OCF0A is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, OCF0A is cleared by writing a logic one to the flag. When the I-bit in SREG, ATmega16U4/32U4 [DATASHEET] 109 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

OCIE0A (Timer/Counter0 Compare Match Interrupt Enable), and OCF0A are set, the Timer/Counter0 Compare Match Interrupt is executed. • Bit 0 – TOV0: Timer/Counter0 Overflow Flag The bit TOV0 is set when an overflow occurs in Timer/Counter0. TOV0 is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, TOV0 is cleared by writing a logic one to the flag. When the SREG I-bit, TOIE0 (Timer/Counter0 Overflow Interrupt Enable), and TOV0 are set, the Timer/Counter0 Overflow interrupt is executed. The setting of this flag is dependent of the WGM02:0 bit setting. Refer to “Waveform Generation Mode Bit Description” on page106. ATmega16U4/32U4 [DATASHEET] 110 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

14. 16-bit Timers/Counters (Timer/Counter1 and Timer/Counter3) The 16-bit Timer/Counter unit allows accurate program execution timing (event management), wave generation, and signal timing measurement. The main features are: • True 16-bit Design (i.e., Allows 16-bit PWM) • Three independent Output Compare Units • Double Buffered Output Compare Registers • One Input Capture Unit • Input Capture Noise Canceler • Clear Timer on Compare Match (Auto Reload) • Glitch-free, Phase Correct Pulse Width Modulator (PWM) • Variable PWM Period • Frequency Generator • External Event Counter • Ten independent interrupt sources (TOV1, OCF1A, OCF1B, OCF1C, ICF1, TOV3, OCF3A, OCF3B, OCF3C, and ICF3) 14.1 Overview Most register and bit references in this section are written in general form. A lower case “n” replaces the Timer/Counter number, and a) lower case “x” replaces the Output Compare unit channel. However, when using the register or bit defines in a program, the precise form must be used, i.e., TCNT1 for accessing Timer/Counter1 counter value and so on. A simplified block diagram of the 16-bit Timer/Counter is shown in Figure 14-1 on page 112. For the actual placement of I/O pins, see “Pinout” on page3. CPU accessible I/O Registers, including I/O bits and I/O pins, are shown in bold. The device-specific I/O Register and bit locations are listed in the “16-bit Timers/Counters (Timer/Counter1 and Timer/Counter3)” on page111. The Power Reduction Timer/Counter1 bit, PRTIM1, in “Power Reduction Register 0 - PRR0” on page47 must be written to zero to enable Timer/Counter1 module. The Power Reduction Timer/Counter3 bit, PRTIM3, in “Power Reduction Register 1 - PRR1” on page48 must be written to zero to enable Timer/Counter3 module. ATmega16U4/32U4 [DATASHEET] 111 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 14-1. 16-bit Timer/Counter Block Diagram(1) Count TOVn Clear (Int.Req.) Control Logic Direction TCLK Clock Select (2) Edge Tn Detector TOP BOTTOM ( From Prescaler ) Timer/Counter TCNTn = = 0 OCFnA (Int.Req.) = Waveform OCnA Generation OCRnA Fixed OCFnB TOP (Int.Req.) Values = Waveform OCnB Generation S U OCRnB B A OCFnC AT (Int.Req.) D = Waveform OCnC Generation OCRnC ( From Analog Comparator Ouput ) ICFn (Int.Req.) ICRn Edge Noise Detector Canceler ICPn TCCRnA TCCRnB TCCRnC Note: 1. Refer to “Pinout” on page3, Table10-3 on page74, and Table10-6 on page77 for Timer/Counter1 and 3 and 3 pin placement and description. 2. Tn only refers to T1 since T3 input is not available on the product. 14.1.1 Registers The Timer/Counter (TCNTn), Output Compare Registers (OCRnA/B/C), and Input Capture Register (ICRn) are all 16-bit registers. Special procedures must be followed when accessing the 16-bit registers. These procedures are described in the section “Accessing 16-bit Registers” on page113. The Timer/Counter Control Registers (TCCRnA/B/C) are 8-bit registers and have no CPU access restrictions. Interrupt requests (shorten as Int.Req.) signals are all visible in the Timer Interrupt Flag Register (TIFRn). All interrupts are individually masked with the Timer Interrupt Mask Register (TIMSKn). TIFRn and TIMSKn are not shown in the figure since these registers are shared by other timer units. The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on the Tn pin. The Clock Select logic block controls which clock source and edge the Timer/Counter uses to increment (or decrement) its value. The Timer/Counter is inactive when no clock source is selected. The output from the clock select logic is referred to as the timer clock (clk ). Tn The double buffered Output Compare Registers (OCRnA/B/C) are compared with the Timer/Counter value at all time. The result of the compare can be used by the Waveform Generator to generate a PWM or variable frequency output on the Output Compare pin (OCnA/B/C). See “Output Compare Units” on page119. The compare match event will also set the Compare Match Flag (OCFnA/B/C) which can be used to generate an Output Compare interrupt request. ATmega16U4/32U4 [DATASHEET] 112 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The Input Capture Register can capture the Timer/Counter value at a given external (edge triggered) event on either the Input Capture pin (ICPn) or on the Analog Comparator pins (See “Analog Comparator” on page293.) The Input Capture unit includes a digital filtering unit (Noise Canceler) for reducing the chance of capturing noise spikes. The TOP value, or maximum Timer/Counter value, can in some modes of operation be defined by either the OCRnA Register, the ICRn Register, or by a set of fixed values. When using OCRnA as TOP value in a PWM mode, the OCRnA Register can not be used for generating a PWM output. However, the TOP value will in this case be double buffered allowing the TOP value to be changed in run time. If a fixed TOP value is required, the ICRn Register can be used as an alternative, freeing the OCRnA to be used as PWM output. 14.1.2 Definitions The following definitions are used extensively throughout the document: BOTTOM The counter reaches the BOTTOM when it becomes 0x0000. MAX The counter reaches its MAXimum when it becomes 0xFFFF (decimal 65535). The counter reaches the TOP when it becomes equal to the highest value in the count sequence. The TOP value can be assigned to be one of the fixed values: 0x00FF, TOP 0x01FF, or 0x03FF, or to the value stored in the OCRnA or ICRn Register. The assignment is dependent of the mode of operation. 14.2 Accessing 16-bit Registers The TCNTn, OCRnA/B/C, and ICRn are 16-bit registers that can be accessed by the AVR CPU via the 8-bit data bus. The 16-bit register must be byte accessed using two read or write operations. Each 16-bit timer has a single 8-bit register for temporary storing of the high byte of the 16-bit access. The same Temporary Register is shared between all 16-bit registers within each 16-bit timer. Accessing the low byte triggers the 16-bit read or write operation. When the low byte of a 16-bit register is written by the CPU, the high byte stored in the Temporary Register, and the low byte written are both copied into the 16-bit register in the same clock cycle. When the low byte of a 16-bit register is read by the CPU, the high byte of the 16-bit register is copied into the Temporary Register in the same clock cycle as the low byte is read. Not all 16-bit accesses uses the Temporary Register for the high byte. Reading the OCRnA/B/C 16-bit registers does not involve using the Temporary Register. To do a 16-bit write, the high byte must be written before the low byte. For a 16-bit read, the low byte must be read before the high byte. The following code examples show how to access the 16-bit timer registers assuming that no interrupts updates the temporary register. The same principle can be used directly for accessing the OCRnA/B/C and ICRn Registers. Note that when using “C”, the compiler handles the 16-bit access. ATmega16U4/32U4 [DATASHEET] 113 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Assembly Code Examples(1) ... ; Set TCNTn to 0x01FF ldi r17,0x01 ldi r16,0xFF out TCNTnH,r17 out TCNTnL,r16 ; Read TCNTn into r17:r16 in r16,TCNTnL in r17,TCNTnH ... C Code Examples(1) unsigned int i; ... /* Set TCNTn to 0x01FF */ TCNTn = 0x1FF; /* Read TCNTn into i */ i = TCNTn; ... Note: 1. See “Code Examples” on page 8. The assembly code example returns the TCNTn value in the r17:r16 register pair. It is important to notice that accessing 16-bit registers are atomic operations. If an interrupt occurs between the two instructions accessing the 16-bit register, and the interrupt code updates the temporary register by accessing the same or any other of the 16-bit Timer Registers, then the result of the access outside the interrupt will be corrupted. Therefore, when both the main code and the interrupt code update the temporary register, the main code must disable the interrupts during the 16-bit access. The following code examples show how to do an atomic read of the TCNTn Register contents. Reading any of the OCRnA/B/C or ICRn Registers can be done by using the same principle. ATmega16U4/32U4 [DATASHEET] 114 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Assembly Code Example(1) TIM16_ReadTCNTn: ; Save global interrupt flag in r18,SREG ; Disable interrupts cli ; Read TCNTn into r17:r16 in r16,TCNTnL in r17,TCNTnH ; Restore global interrupt flag out SREG,r18 ret C Code Example(1) unsigned int TIM16_ReadTCNTn( void ) { unsigned char sreg; unsigned int i; /* Save global interrupt flag */ sreg = SREG; /* Disable interrupts */ __disable_interrupt(); /* Read TCNTn into i */ i = TCNTn; /* Restore global interrupt flag */ SREG = sreg; return i; } Note: 1. See “Code Examples” on page 8. The assembly code example returns the TCNTn value in the r17:r16 register pair. The following code examples show how to do an atomic write of the TCNTn Register contents. Writing any of the OCRnA/B/C or ICRn Registers can be done by using the same principle. ATmega16U4/32U4 [DATASHEET] 115 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Assembly Code Example(1) TIM16_WriteTCNTn: ; Save global interrupt flag in r18,SREG ; Disable interrupts cli ; Set TCNTn to r17:r16 out TCNTnH,r17 out TCNTnL,r16 ; Restore global interrupt flag out SREG,r18 ret C Code Example(1) void TIM16_WriteTCNTn( unsigned int i ) { unsigned char sreg; unsigned int i; /* Save global interrupt flag */ sreg = SREG; /* Disable interrupts */ __disable_interrupt(); /* Set TCNTn to i */ TCNTn = i; /* Restore global interrupt flag */ SREG = sreg; } Note: 1. See “Code Examples” on page 8. The assembly code example requires that the r17:r16 register pair contains the value to be written to TCNTn. 14.2.1 Reusing the Temporary High Byte Register If writing to more than one 16-bit register where the high byte is the same for all registers written, then the high byte only needs to be written once. However, note that the same rule of atomic operation described previously also applies in this case. 14.3 Timer/Counter Clock Sources The Timer/Counter can be clocked by an internal or an external clock source. The clock source is selected by the Clock Select logic which is controlled by the Clock Select (CSn2:0) bits located in the Timer/Counter control Register B (TCCRnB). For details on clock sources and prescaler, see “Timer/Counter0, Timer/Counter1, and Timer/Counter3 Prescalers” on page92. 14.4 Counter Unit The main part of the 16-bit Timer/Counter is the programmable 16-bit bi-directional counter unit. Figure 14-2 shows a block diagram of the counter and its surroundings. ATmega16U4/32U4 [DATASHEET] 116 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 14-2. Counter Unit Block Diagram DATA BUS (8-bit) TOVn (Int.Req.) TEMP (8-bit) Clock Select Count Edge Tn TCNTnH (8-bit) TCNTnL (8-bit) Clear clk Detector Control Logic Tn Direction TCNTn (16-bit Counter) ( From Prescaler ) TOP BOTTOM Signal description (internal signals): Count: Increment or decrement TCNTn by 1. Direction: Select between increment and decrement. Clear: Clear TCNTn (set all bits to zero). clk : Timer/Counter clock. Tn TOP: Signalize that TCNTn has reached maximum value. BOTTOM: Signalize that TCNTn has reached minimum value (zero). The 16-bit counter is mapped into two 8-bit I/O memory locations: Counter High (TCNTnH) containing the upper eight bits of the counter, and Counter Low (TCNTnL) containing the lower eight bits. The TCNTnH Register can only be indirectly accessed by the CPU. When the CPU does an access to the TCNTnH I/O location, the CPU accesses the high byte temporary register (TEMP). The temporary register is updated with the TCNTnH value when the TCNTnL is read, and TCNTnH is updated with the temporary register value when TCNTnL is written. This allows the CPU to read or write the entire 16-bit counter value within one clock cycle via the 8-bit data bus. It is important to notice that there are special cases of writing to the TCNTn Register when the counter is counting that will give unpredictable results. The special cases are described in the sections where they are of importance. Depending on the mode of operation used, the counter is cleared, incremented, or decremented at each timer clock (clk ). The clk can be generated from an external or internal clock source, selected by the Clock Select Tn Tn bits (CSn2:0). When no clock source is selected (CSn2:0 = 0) the timer is stopped. However, the TCNTn value can be accessed by the CPU, independent of whether clk is present or not. A CPU write overrides (has priority Tn over) all counter clear or count operations. The counting sequence is determined by the setting of the Waveform Generation mode bits (WGMn3:0) located in the Timer/Counter Control Registers A and B (TCCRnA and TCCRnB). There are close connections between how the counter behaves (counts) and how waveforms are generated on the Output Compare outputs OCnx. For more details about advanced counting sequences and waveform generation, see “Modes of Operation” on page122. The Timer/Counter Overflow Flag (TOVn) is set according to the mode of operation selected by the WGMn3:0 bits. TOVn can be used for generating a CPU interrupt. 14.5 Input Capture Unit The Timer/Counter incorporates an input capture unit that can capture external events and give them a time- stamp indicating time of occurrence. The external signal indicating an event, or multiple events, can be applied via the ICPn pin or alternatively, for the Timer/Counter1 only, via the Analog Comparator unit. The time-stamps can then be used to calculate frequency, duty-cycle, and other features of the signal applied. Alternatively the time-stamps can be used for creating a log of the events. ATmega16U4/32U4 [DATASHEET] 117 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The Input Capture unit is illustrated by the block diagram shown in Figure 14-3. The elements of the block diagram that are not directly a part of the input capture unit are gray shaded. The small “n” in register and bit names indicates the Timer/Counter number. Figure 14-3. Input Capture Unit Block Diagram DATA BUS (8-bit) TEMP (8-bit) ICRnH (8-bit) ICRnL (8-bit) TCNTnH (8-bit) TCNTnL (8-bit) WRITE ICRn (16-bit Register) TCNTn (16-bit Counter) ACO* ACIC* ICNC ICES Analog Comparator Noise Edge ICFn (Int.Req.) Canceler Detector ICPn Note: The Analog Comparator Output (ACO) can only trigger the Timer/Counter1 ICP – not Timer/Counter3, 4, or 5. When a change of the logic level (an event) occurs on the Input Capture Pin (ICPn), alternatively on the analog Comparator output (ACO), and this change confirms to the setting of the edge detector, a capture will be triggered. When a capture is triggered, the 16-bit value of the counter (TCNTn) is written to the Input Capture Register (ICRn). The Input Capture Flag (ICFn) is set at the same system clock as the TCNTn value is copied into ICRn Register. If enabled (TICIEn = 1), the input capture flag generates an input capture interrupt. The ICFn flag is automatically cleared when the interrupt is executed. Alternatively the ICFn flag can be cleared by software by writing a logical one to its I/O bit location. Reading the 16-bit value in the Input Capture Register (ICRn) is done by first reading the low byte (ICRnL) and then the high byte (ICRnH). When the low byte is read the high byte is copied into the high byte Temporary Register (TEMP). When the CPU reads the ICRnH I/O location it will access the TEMP Register. The ICRn Register can only be written when using a Waveform Generation mode that utilizes the ICRn Register for defining the counter’s TOP value. In these cases the Waveform Generation mode (WGMn3:0) bits must be set before the TOP value can be written to the ICRn Register. When writing the ICRn Register the high byte must be written to the ICRnH I/O location before the low byte is written to ICRnL. For more information on how to access the 16-bit registers refer to “Accessing 16-bit Registers” on page113. 14.5.1 Input Capture Trigger Source The main trigger source for the input capture unit is the Input Capture Pin (ICPn). Timer/Counter1 can alternatively use the analog comparator output as trigger source for the input capture unit. The Analog Comparator is selected as trigger source by setting the analog Comparator Input Capture (ACIC) bit in the Analog Comparator Control and Status Register (ACSR). Be aware that changing trigger source can trigger a capture. The input capture flag must therefore be cleared after the change. ATmega16U4/32U4 [DATASHEET] 118 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Both the Input Capture Pin (ICPn) and the Analog Comparator output (ACO) inputs are sampled using the same technique as for the Tn pin (Figure 12-1 on page 92). The edge detector is also identical. However, when the noise canceler is enabled, additional logic is inserted before the edge detector, which increases the delay by four system clock cycles. Note that the input of the noise canceler and edge detector is always enabled unless the Timer/Counter is set in a Waveform Generation mode that uses ICRn to define TOP. An input capture can be triggered by software by controlling the port of the ICPn pin. 14.5.2 Noise Canceler The noise canceler improves noise immunity by using a simple digital filtering scheme. The noise canceler input is monitored over four samples, and all four must be equal for changing the output that in turn is used by the edge detector. The noise canceler is enabled by setting the Input Capture Noise Canceler (ICNCn) bit in Timer/Counter Control Register B (TCCRnB). When enabled the noise canceler introduces additional four system clock cycles of delay from a change applied to the input, to the update of the ICRn Register. The noise canceler uses the system clock and is therefore not affected by the prescaler. 14.5.3 Using the Input Capture Unit The main challenge when using the Input Capture unit is to assign enough processor capacity for handling the incoming events. The time between two events is critical. If the processor has not read the captured value in the ICRn Register before the next event occurs, the ICRn will be overwritten with a new value. In this case the result of the capture will be incorrect. When using the Input Capture interrupt, the ICRn Register should be read as early in the interrupt handler routine as possible. Even though the Input Capture interrupt has relatively high priority, the maximum interrupt response time is dependent on the maximum number of clock cycles it takes to handle any of the other interrupt requests. Using the Input Capture unit in any mode of operation when the TOP value (resolution) is actively changed during operation, is not recommended. Measurement of an external signal’s duty cycle requires that the trigger edge is changed after each capture. Changing the edge sensing must be done as early as possible after the ICRn Register has been read. After a change of the edge, the Input Capture Flag (ICFn) must be cleared by software (writing a logical one to the I/O bit location). For measuring frequency only, the clearing of the ICFn Flag is not required (if an interrupt handler is used). 14.6 Output Compare Units The 16-bit comparator continuously compares TCNTn with the Output Compare Register (OCRnx). If TCNT equals OCRnx the comparator signals a match. A match will set the Output Compare Flag (OCFnx) at the next timer clock cycle. If enabled (OCIEnx = 1), the Output Compare Flag generates an Output Compare interrupt. The OCFnx Flag is automatically cleared when the interrupt is executed. Alternatively the OCFnx Flag can be cleared by software by writing a logical one to its I/O bit location. The Waveform Generator uses the match signal to generate an output according to operating mode set by the Waveform Generation mode (WGMn3:0) bits and Compare Output mode (COMnx1:0) bits. The TOP and BOTTOM signals are used by the Waveform Generator for handling the special cases of the extreme values in some modes of operation (See “Modes of Operation” on page122.) A special feature of Output Compare unit A allows it to define the Timer/Counter TOP value (i.e., counter resolution). In addition to the counter resolution, the TOP value defines the period time for waveforms generated by the Waveform Generator. ATmega16U4/32U4 [DATASHEET] 119 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 14-4 shows a block diagram of the Output Compare unit. The small “n” in the register and bit names indicates the device number (n = n for Timer/Counter n), and the “x” indicates Output Compare unit (A/B/C). The elements of the block diagram that are not directly a part of the Output Compare unit are gray shaded. Figure 14-4. Output Compare Unit, Block Diagram DATA BUS (8-bit) TEMP (8-bit) OCRnxH Buf. (8-bit) OCRnxL Buf. (8-bit) TCNTnH (8-bit) TCNTnL (8-bit) OCRnx Buffer (16-bit Register) TCNTn (16-bit Counter) OCRnxH (8-bit) OCRnxL (8-bit) OCRnx (16-bit Register) = (16-bit Comparator ) OCFnx (Int.Req.) TOP Waveform Generator OCnx BOTTOM WGMn3:0 COMnx1:0 The OCRnx Register is double buffered when using any of the twelve Pulse Width Modulation (PWM) modes. For the Normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is disabled. The double buffering synchronizes the update of the OCRnx Compare Register to either TOP or BOTTOM of the counting sequence. The synchronization prevents the occurrence of odd-length, non-symmetrical PWM pulses, thereby making the output glitch-free. The OCRnx Register access may seem complex, but this is not case. When the double buffering is enabled, the CPU has access to the OCRnx Buffer Register, and if double buffering is disabled the CPU will access the OCRnx directly. The content of the OCR1x (Buffer or Compare) Register is only changed by a write operation (the Timer/Counter does not update this register automatically as the TCNT1 and ICR1 Register). Therefore OCR1x is not read via the high byte temporary register (TEMP). However, it is a good practice to read the low byte first as when accessing other 16-bit registers. Writing the OCRnx Registers must be done via the TEMP Register since the compare of all 16 bits is done continuously. The high byte (OCRnxH) has to be written first. When the high byte I/O location is written by the CPU, the TEMP Register will be updated by the value written. Then when the low byte (OCRnxL) is written to the lower eight bits, the high byte will be copied into the upper 8- bits of either the OCRnx buffer or OCRnx Compare Register in the same system clock cycle. For more information of how to access the 16-bit registers refer to “Accessing 16-bit Registers” on page113. 14.6.1 Force Output Compare In non-PWM Waveform Generation modes, the match output of the comparator can be forced by writing a one to the Force Output Compare (FOCnx) bit. Forcing compare match will not set the OCFnx Flag or reload/clear the timer, but the OCnx pin will be updated as if a real compare match had occurred (the COMn1:0 bits settings define whether the OCnx pin is set, cleared or toggled). ATmega16U4/32U4 [DATASHEET] 120 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

14.6.2 Compare Match Blocking by TCNTn Write All CPU writes to the TCNTn Register will block any compare match that occurs in the next timer clock cycle, even when the timer is stopped. This feature allows OCRnx to be initialized to the same value as TCNTn without triggering an interrupt when the Timer/Counter clock is enabled. 14.6.3 Using the Output Compare Unit Since writing TCNTn in any mode of operation will block all compare matches for one timer clock cycle, there are risks involved when changing TCNTn when using any of the Output Compare channels, independent of whether the Timer/Counter is running or not. If the value written to TCNTn equals the OCRnx value, the compare match will be missed, resulting in incorrect waveform generation. Do not write the TCNTn equal to TOP in PWM modes with variable TOP values. The compare match for the TOP will be ignored and the counter will continue to 0xFFFF. Similarly, do not write the TCNTn value equal to BOTTOM when the counter is down- counting. The setup of the OCnx should be performed before setting the Data Direction Register for the port pin to output. The easiest way of setting the OCnx value is to use the Force Output Compare (FOCnx) strobe bits in Normal mode. The OCnx Register keeps its value even when changing between Waveform Generation modes. Be aware that the COMnx1:0 bits are not double buffered together with the compare value. Changing the COMnx1:0 bits will take effect immediately. 14.7 Compare Match Output Unit The Compare Output mode (COMnx1:0) bits have two functions. The Waveform Generator uses the COMnx1:0 bits for defining the Output Compare (OCnx) state at the next compare match. Secondly the COMnx1:0 bits control the OCnx pin output source. Figure 14-5 shows a simplified schematic of the logic affected by the COMnx1:0 bit setting. The I/O Registers, I/O bits, and I/O pins in the figure are shown in bold. Only the parts of the general I/O Port Control Registers (DDR and PORT) that are affected by the COMnx1:0 bits are shown. When referring to the OCnx state, the reference is for the internal OCnx Register, not the OCnx pin. If a system reset occur, the OCnx Register is reset to “0”. Figure 14-5. Compare Match Output Unit, Schematic COMnx1 Waveform COMnx0 D Q Generator FOCnx 1 OCnx OCnx Pin 0 D Q S U B PORT A T A D D Q DDR clk I/O ATmega16U4/32U4 [DATASHEET] 121 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The general I/O port function is overridden by the Output Compare (OCnx) from the Waveform Generator if either of the COMnx1:0 bits are set. However, the OCnx pin direction (input or output) is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction Register bit for the OCnx pin (DDR_OCnx) must be set as output before the OCnx value is visible on the pin. The port override function is generally independent of the Waveform Generation mode, but there are some exceptions. Refer to Table14-1 on page131, Table14-2 on page132, and Table14-3 on page132 for details. The design of the Output Compare pin logic allows initialization of the OCnx state before the output is enabled. Note that some COMnx1:0 bit settings are reserved for certain modes of operation. See “16-bit Timers/Counters (Timer/Counter1 and Timer/Counter3)” on page111. The COMnx1:0 bits have no effect on the Input Capture unit. 14.7.1 Compare Output Mode and Waveform Generation The Waveform Generator uses the COMnx1:0 bits differently in normal, CTC, and PWM modes. For all modes, setting the COMnx1:0 = 0 tells the Waveform Generator that no action on the OCnx Register is to be performed on the next compare match. For compare output actions in the non-PWM modes refer to Table14-1 on page131. For fast PWM mode refer to Table14-2 on page132, and for phase correct and phase and frequency correct PWM refer to Table14-3 on page132. A change of the COMnx1:0 bits state will have effect at the first compare match after the bits are written. For non-PWM modes, the action can be forced to have immediate effect by using the FOCnx strobe bits. 14.8 Modes of Operation The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is defined by the combination of the Waveform Generation mode (WGMn3:0) and Compare Output mode (COMnx1:0) bits. The Compare Output mode bits do not affect the counting sequence, while the Waveform Generation mode bits do. The COMnx1:0 bits control whether the PWM output generated should be inverted or not (inverted or non- inverted PWM). For non-PWM modes the COMnx1:0 bits control whether the output should be set, cleared or toggle at a compare match (See “Compare Match Output Unit” on page121.) For detailed timing information refer to “Timer/Counter Timing Diagrams” on page129. 14.8.1 Normal Mode The simplest mode of operation is the Normal mode (WGMn3:0 = 0). In this mode the counting direction is always up (incrementing), and no counter clear is performed. The counter simply overruns when it passes its maximum 16-bit value (MAX = 0xFFFF) and then restarts from the BOTTOM (0x0000). In normal operation the Timer/Counter Overflow Flag (TOVn) will be set in the same timer clock cycle as the TCNTn becomes zero. The TOVn Flag in this case behaves like a 17th bit, except that it is only set, not cleared. However, combined with the timer overflow interrupt that automatically clears the TOVn Flag, the timer resolution can be increased by software. There are no special cases to consider in the Normal mode, a new counter value can be written anytime. The Input Capture unit is easy to use in Normal mode. However, observe that the maximum interval between the external events must not exceed the resolution of the counter. If the interval between events are too long, the timer overflow interrupt or the prescaler must be used to extend the resolution for the capture unit. The Output Compare units can be used to generate interrupts at some given time. Using the Output Compare to generate waveforms in Normal mode is not recommended, since this will occupy too much of the CPU time. 14.8.2 Clear Timer on Compare Match (CTC) Mode In Clear Timer on Compare or CTC mode (WGMn3:0 = 4 or 12), the OCRnA or ICRn Register are used to manipulate the counter resolution. In CTC mode the counter is cleared to zero when the counter value (TCNTn) ATmega16U4/32U4 [DATASHEET] 122 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

matches either the OCRnA (WGMn3:0 = 4) or the ICRn (WGMn3:0 = 12). The OCRnA or ICRn define the top value for the counter, hence also its resolution. This mode allows greater control of the compare match output frequency. It also simplifies the operation of counting external events. The timing diagram for the CTC mode is shown in Figure 14-6. The counter value (TCNTn) increases until a compare match occurs with either OCRnA or ICRn, and then counter (TCNTn) is cleared. Figure 14-6. CTC Mode, Timing Diagram OCnA Interrupt Flag Set or ICFn Interrupt Flag Set (Interrupt on TOP) TCNTn OCnA (COMnA1:0 = 1) (Toggle) Period 1 2 3 4 An interrupt can be generated at each time the counter value reaches the TOP value by either using the OCFnA or ICFn Flag according to the register used to define the TOP value. If the interrupt is enabled, the interrupt handler routine can be used for updating the TOP value. However, changing the TOP to a value close to BOTTOM when the counter is running with none or a low prescaler value must be done with care since the CTC mode does not have the double buffering feature. If the new value written to OCRnA or ICRn is lower than the current value of TCNTn, the counter will miss the compare match. The counter will then have to count to its maximum value (0xFFFF) and wrap around starting at 0x0000 before the compare match can occur. In many cases this feature is not desirable. An alternative will then be to use the fast PWM mode using OCRnA for defining TOP (WGMn3:0 = 15) since the OCRnA then will be double buffered. For generating a waveform output in CTC mode, the OCnA output can be set to toggle its logical level on each compare match by setting the Compare Output mode bits to toggle mode (COMnA1:0 = 1). The OCnA value will not be visible on the port pin unless the data direction for the pin is set to output (DDR_OCnA = 1). The waveform generated will have a maximum frequency of f = f /2 when OCRnA is set to zero (0x0000). OCnA clk_I/O The waveform frequency is defined by the following equation: f clk_I/O f = --------------------------------------------------- OCnA 2N1+OCRnA The N variable represents the prescaler factor (1, 8, 64, 256, or 1024). As for the Normal mode of operation, the TOVn Flag is set in the same timer clock cycle that the counter counts from MAX to 0x0000. 14.8.3 Fast PWM Mode The fast Pulse Width Modulation or fast PWM mode (WGMn3:0 = 5, 6, 7, 14, or 15) provides a high frequency PWM waveform generation option. The fast PWM differs from the other PWM options by its single-slope operation. The counter counts from BOTTOM to TOP then restarts from BOTTOM. In non-inverting Compare Output mode, the Output Compare (OCnx) is set on the compare match between TCNTn and OCRnx, and cleared at TOP. In inverting Compare Output mode output is cleared on compare match and set at TOP. Due to the single-slope operation, the operating frequency of the fast PWM mode can be twice as high as the phase correct and phase and frequency correct PWM modes that use dual-slope operation. This high frequency makes the fast PWM mode well suited for power regulation, rectification, and DAC applications. High frequency allows physically small sized external components (coils, capacitors), hence reduces total system cost. ATmega16U4/32U4 [DATASHEET] 123 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The PWM resolution for fast PWM can be fixed to 8-, 9-, or 10-bit, or defined by either ICRn or OCRnA. The minimum resolution allowed is 2-bit (ICRn or OCRnA set to 0x0003), and the maximum resolution is 16-bit (ICRn or OCRnA set to MAX). The PWM resolution in bits can be calculated by using the following equation: logTOP+1 R = ----------------------------------- FPWM log2 In fast PWM mode the counter is incremented until the counter value matches either one of the fixed values 0x00FF, 0x01FF, or 0x03FF (WGMn3:0 = 5, 6, or 7), the value in ICRn (WGMn3:0 = 14), or the value in OCRnA (WGMn3:0 = 15). The counter is then cleared at the following timer clock cycle. The timing diagram for the fast PWM mode is shown in Figure 14-7. The figure shows fast PWM mode when OCRnA or ICRn is used to define TOP. The TCNTn value is in the timing diagram shown as a histogram for illustrating the single-slope operation. The diagram includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNTn slopes represent compare matches between OCRnx and TCNTn. The OCnx Interrupt Flag will be set when a compare match occurs. Figure 14-7. Fast PWM Mode, Timing Diagram OCRnx / TOP Update and TOVn Interrupt Flag Set and OCnA Interrupt Flag Set or ICFn Interrupt Flag Set (Interrupt on TOP) TCNTn OCnx (COMnx1:0 = 2) OCnx (COMnx1:0 = 3) Period 1 2 3 4 5 6 7 8 The Timer/Counter Overflow Flag (TOVn) is set each time the counter reaches TOP. In addition the OCnA or ICFn Flag is set at the same timer clock cycle as TOVn is set when either OCRnA or ICRn is used for defining the TOP value. If one of the interrupts are enabled, the interrupt handler routine can be used for updating the TOP and compare values. When changing the TOP value the program must ensure that the new TOP value is higher or equal to the value of all of the Compare Registers. If the TOP value is lower than any of the Compare Registers, a compare match will never occur between the TCNTn and the OCRnx. Note that when using fixed TOP values the unused bits are masked to zero when any of the OCRnx Registers are written. The procedure for updating ICRn differs from updating OCRnA when used for defining the TOP value. The ICRn Register is not double buffered. This means that if ICRn is changed to a low value when the counter is running with none or a low prescaler value, there is a risk that the new ICRn value written is lower than the current value of TCNTn. The result will then be that the counter will miss the compare match at the TOP value. The counter will then have to count to the MAX value (0xFFFF) and wrap around starting at 0x0000 before the compare match can occur. The OCRnA Register however, is double buffered. This feature allows the OCRnA I/O location to be written anytime. When the OCRnA I/O location is written the value written will be put into the OCRnA Buffer Register. The OCRnA Compare Register will then be updated with the value in the Buffer Register at the next timer clock cycle the TCNTn matches TOP. The update is done at the same timer clock cycle as the TCNTn is cleared and the TOVn Flag is set. Using the ICRn Register for defining TOP works well when using fixed TOP values. By using ICRn, the OCRnA Register is free to be used for generating a PWM output on OCnA. However, if the base PWM frequency is ATmega16U4/32U4 [DATASHEET] 124 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

actively changed (by changing the TOP value), using the OCRnA as TOP is clearly a better choice due to its double buffer feature. In fast PWM mode, the compare units allow generation of PWM waveforms on the OCnx pins. Setting the COMnx1:0 bits to two will produce a non-inverted PWM and an inverted PWM output can be generated by setting the COMnx1:0 to three. Refer to Table14-1 on page131, Table14-2 on page132, and Table14-3 on page132. The actual OCnx value will only be visible on the port pin if the data direction for the port pin is set as output (DDR_OCnx). The PWM waveform is generated by setting (or clearing) the OCnx Register at the compare match between OCRnx and TCNTn, and clearing (or setting) the OCnx Register at the timer clock cycle the counter is cleared (changes from TOP to BOTTOM). The PWM frequency for the output can be calculated by the following equation: f clk_I/O f = ----------------------------------- OCnxPWM N1+TOP The N variable represents the prescaler divider (1, 8, 64, 256, or 1024). The extreme values for the OCRnx Register represents special cases when generating a PWM waveform output in the fast PWM mode. If the OCRnx is set equal to BOTTOM (0x0000) the output will be a narrow spike for each TOP+1 timer clock cycle. Setting the OCRnx equal to TOP will result in a constant high or low output (depending on the polarity of the output set by the COMnx1:0 bits). A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting OCnA to toggle its logical level on each compare match (COMnA1:0 = 1). This applies only if OCR1A is used to define the TOP value (WGM13:0 = 15). The waveform generated will have a maximum frequency of f = f /2 OCnA clk_I/O when OCRnA is set to zero (0x0000). This feature is similar to the OCnA toggle in CTC mode, except the double buffer feature of the Output Compare unit is enabled in the fast PWM mode. 14.8.4 Phase Correct PWM Mode The phase correct Pulse Width Modulation or phase correct PWM mode (WGMn3:0=1, 2, 3, 10, or 11) provides a high resolution phase correct PWM waveform generation option. The phase correct PWM mode is, like the phase and frequency correct PWM mode, based on a dual-slope operation. The counter counts repeatedly from BOTTOM (0x0000) to TOP and then from TOP to BOTTOM. In non-inverting Compare Output mode, the Output Compare (OCnx) is cleared on the compare match between TCNTn and OCRnx while upcounting, and set on the compare match while downcounting. In inverting Output Compare mode, the operation is inverted. The dual-slope operation has lower maximum operation frequency than single slope operation. However, due to the symmetric feature of the dual-slope PWM modes, these modes are preferred for motor control applications. The PWM resolution for the phase correct PWM mode can be fixed to 8-, 9-, or 10-bit, or defined by either ICRn or OCRnA. The minimum resolution allowed is 2-bit (ICRn or OCRnA set to 0x0003), and the maximum resolution is 16-bit (ICRn or OCRnA set to MAX). The PWM resolution in bits can be calculated by using the following equation: logTOP+1 R = ----------------------------------- PCPWM log2 In phase correct PWM mode the counter is incremented until the counter value matches either one of the fixed values 0x00FF, 0x01FF, or 0x03FF (WGMn3:0 = 1, 2, or 3), the value in ICRn (WGMn3:0 = 10), or the value in OCRnA (WGMn3:0 = 11). The counter has then reached the TOP and changes the count direction. The TCNTn value will be equal to TOP for one timer clock cycle. The timing diagram for the phase correct PWM mode is shown on Figure 14-8 on page 126. The figure shows phase correct PWM mode when OCRnA or ICRn is used to define TOP. The TCNTn value is in the timing diagram shown as a histogram for illustrating the dual-slope ATmega16U4/32U4 [DATASHEET] 125 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

operation. The diagram includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNTn slopes represent compare matches between OCRnx and TCNTn. The OCnx Interrupt Flag will be set when a compare match occurs. Figure 14-8. Phase Correct PWM Mode, Timing Diagram OCRnx/TOP Update and OCnA Interrupt Flag Set or ICFn Interrupt Flag Set (Interrupt on TOP) TOVn Interrupt Flag Set (Interrupt on Bottom) TCNTn OCnx (COMnx1:0 = 2) OCnx (COMnx1:0 = 3) Period 1 2 3 4 The Timer/Counter Overflow Flag (TOVn) is set each time the counter reaches BOTTOM. When either OCRnA or ICRn is used for defining the TOP value, the OCnA or ICFn Flag is set accordingly at the same timer clock cycle as the OCRnx Registers are updated with the double buffer value (at TOP). The Interrupt Flags can be used to generate an interrupt each time the counter reaches the TOP or BOTTOM value. When changing the TOP value the program must ensure that the new TOP value is higher or equal to the value of all of the Compare Registers. If the TOP value is lower than any of the Compare Registers, a compare match will never occur between the TCNTn and the OCRnx. Note that when using fixed TOP values, the unused bits are masked to zero when any of the OCRnx Registers are written. As the third period shown in Figure 14-8 on page 126 illustrates, changing the TOP actively while the Timer/Counter is running in the phase correct mode can result in an unsymmetrical output. The reason for this can be found in the time of update of the OCRnx Register. Since the OCRnx update occurs at TOP, the PWM period starts and ends at TOP. This implies that the length of the falling slope is determined by the previous TOP value, while the length of the rising slope is determined by the new TOP value. When these two values differ the two slopes of the period will differ in length. The difference in length gives the unsymmetrical result on the output. It is recommended to use the phase and frequency correct mode instead of the phase correct mode when changing the TOP value while the Timer/Counter is running. When using a static TOP value there are practically no differences between the two modes of operation. In phase correct PWM mode, the compare units allow generation of PWM waveforms on the OCnx pins. Setting the COMnx1:0 bits to two will produce a non-inverted PWM and an inverted PWM output can be generated by setting the COMnx1:0 to three. Refer to Table14-1 on page131, Table14-2 on page132, and Table14-3 on page132 The actual OCnx value will only be visible on the port pin if the data direction for the port pin is set as output (DDR_OCnx). The PWM waveform is generated by setting (or clearing) the OCnx Register at the compare match between OCRnx and TCNTn when the counter increments, and clearing (or setting) the OCnx Register at ATmega16U4/32U4 [DATASHEET] 126 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

compare match between OCRnx and TCNTn when the counter decrements. The PWM frequency for the output when using phase correct PWM can be calculated by the following equation: f clk_I/O f = ---------------------------- OCnxPCPWM 2NTOP The N variable represents the prescaler divider (1, 8, 64, 256, or 1024). The extreme values for the OCRnx Register represent special cases when generating a PWM waveform output in the phase correct PWM mode. If the OCRnx is set equal to BOTTOM the output will be continuously low and if set equal to TOP the output will be continuously high for non-inverted PWM mode. For inverted PWM the output will have the opposite logic values. If OCR1A is used to define the TOP value (WGM13:0 = 11) and COM1A1:0 = 1, the OC1A output will toggle with a 50% duty cycle. 14.8.5 Phase and Frequency Correct PWM Mode The Phase and Frequency Correct PWM Mode (PWM4x = 1 and WGM40 = 1) provides a high resolution Phase and Frequency Correct PWM waveform generation option. The Phase and Frequency Correct PWM mode is based on a dual-slope operation. The counter counts repeatedly from BOTTOM to TOP (defined as OCR4C) and then from TOP to BOTTOM. In noninverting Compare Output Mode, and in complimentary Compare Output Mode, the Waveform Output (OCW4x) is cleared on the Compare Match between TCNT4 and OCR4x while upcounting, and set on the Compare Match while down-counting. In inverting Output Compare mode, the operation is inverted. The dual-slope operation has lower maximum operation frequency than single slope operation. However, due to the symmetric feature of the dual-slope PWM modes, these modes are preferred for motor control applications. The main difference between the phase correct, and the phase and frequency correct PWM mode is the time the OCRnx Register is updated by the OCRnx Buffer Register, (see Figure 14-8 on page 126 and Figure 14-9 on page 128). The PWM resolution for the phase and frequency correct PWM mode can be defined by either ICRn or OCRnA. The minimum resolution allowed is 2-bit (ICRn or OCRnA set to 0x0003), and the maximum resolution is 16-bit (ICRn or OCRnA set to MAX). The PWM resolution in bits can be calculated using the following equation: logTOP+1 R = ----------------------------------- PFCPWM log2 In phase and frequency correct PWM mode the counter is incremented until the counter value matches either the value in ICRn (WGMn3:0 = 8), or the value in OCRnA (WGMn3:0 = 9). The counter has then reached the TOP and changes the count direction. The TCNTn value will be equal to TOP for one timer clock cycle. The timing diagram for the phase correct and frequency correct PWM mode is shown on Figure 14-9. The figure shows phase and frequency correct PWM mode when OCRnA or ICRn is used to define TOP. The TCNTn value is in the timing diagram shown as a histogram for illustrating the dual-slope operation. The diagram includes non-inverted and inverted PWM outputs. The small horizontal line marks on the TCNTn slopes represent compare matches between OCRnx and TCNTn. The OCnx Interrupt Flag will be set when a compare match occurs. ATmega16U4/32U4 [DATASHEET] 127 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 14-9. Phase and Frequency Correct PWM Mode, Timing Diagram OCnA Interrupt Flag Set or ICFn Interrupt Flag Set (Interrupt on TOP) OCRnx/TOP Updateand TOVn Interrupt Flag Set (Interrupt on Bottom) TCNTn OCnx (COMnx1:0 = 2) OCnx (COMnx1:0 = 3) Period 1 2 3 4 The Timer/Counter Overflow Flag (TOVn) is set at the same timer clock cycle as the OCRnx Registers are updated with the double buffer value (at BOTTOM). When either OCRnA or ICRn is used for defining the TOP value, the OCnA or ICFn Flag set when TCNTn has reached TOP. The Interrupt Flags can then be used to generate an interrupt each time the counter reaches the TOP or BOTTOM value. When changing the TOP value the program must ensure that the new TOP value is higher or equal to the value of all of the Compare Registers. If the TOP value is lower than any of the Compare Registers, a compare match will never occur between the TCNTn and the OCRnx. As Figure 14-9 on page 128 shows the output generated is, in contrast to the phase correct mode, symmetrical in all periods. Since the OCRnx Registers are updated at BOTTOM, the length of the rising and the falling slopes will always be equal. This gives symmetrical output pulses and is therefore frequency correct. Using the ICRn Register for defining TOP works well when using fixed TOP values. By using ICRn, the OCRnA Register is free to be used for generating a PWM output on OCnA. However, if the base PWM frequency is actively changed by changing the TOP value, using the OCRnA as TOP is clearly a better choice due to its double buffer feature. In phase and frequency correct PWM mode, the compare units allow generation of PWM waveforms on the OCnx pins. Setting the COMnx1:0 bits to two will produce a non-inverted PWM and an inverted PWM output can be generated by setting the COMnx1:0 to three. Refer to Table14-1 on page131, Table14-2 on page132, and Table14-3 on page132. The actual OCnx value will only be visible on the port pin if the data direction for the port pin is set as output (DDR_OCnx). The PWM waveform is generated by setting (or clearing) the OCnx Register at the compare match between OCRnx and TCNTn when the counter increments, and clearing (or setting) the OCnx Register at compare match between OCRnx and TCNTn when the counter decrements. The PWM frequency for the output when using phase and frequency correct PWM can be calculated by the following equation: f clk_I/O f = ---------------------------- OCnxPFCPWM 2NTOP The N variable represents the prescaler divider (1, 8, 64, 256, or 1024). The extreme values for the OCRnx Register represents special cases when generating a PWM waveform output in the phase correct PWM mode. If the OCRnx is set equal to BOTTOM the output will be continuously low and if set equal to TOP the output will be set to high for non-inverted PWM mode. For inverted PWM the ATmega16U4/32U4 [DATASHEET] 128 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

output will have the opposite logic values. If OCR1A is used to define the TOP value (WGM13:0 = 9) and COM1A1:0 = 1, the OC1A output will toggle with a 50% duty cycle. 14.9 Timer/Counter Timing Diagrams The Timer/Counter is a synchronous design and the timer clock (clk ) is therefore shown as a clock enable Tn signal in the following figures. The figures include information on when Interrupt Flags are set, and when the OCRnx Register is updated with the OCRnx buffer value (only for modes utilizing double buffering). Figure 14- 10 shows a timing diagram for the setting of OCFnx. Figure 14-10. Timer/Counter Timing Diagram, Setting of OCFnx, no Prescaling clk I/O clk Tn (clk /1) I/O TCNTn OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2 OCRnx OCRnx Value OCFnx Figure 14-11 shows the same timing data, but with the prescaler enabled. Figure 14-11. Timer/Counter Timing Diagram, Setting of OCFnx, with Prescaler (f /8) clk_I/O clk I/O clk Tn (clk /8) I/O TCNTn OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2 OCRnx OCRnx Value OCFnx Figure 14-12 shows the count sequence close to TOP in various modes. When using phase and frequency correct PWM mode the OCRnx Register is updated at BOTTOM. The timing diagrams will be the same, but TOP should be replaced by BOTTOM, TOP-1 by BOTTOM+1 and so on. The same renaming applies for modes that set the TOVn Flag at BOTTOM. ATmega16U4/32U4 [DATASHEET] 129 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 14-12. Timer/Counter Timing Diagram, no Prescaling clk I/O clk Tn (clk /1) I/O TCNTn TOP - 1 TOP BOTTOM BOTTOM + 1 (CTC and FPWM) TCNTn TOP - 1 TOP TOP - 1 TOP - 2 (PC and PFC PWM) TOVn (FPWM) and ICFn (if used as TOP) OCRnx Old OCRnx Value New OCRnx Value (Update at TOP) Figure 14-13 shows the same timing data, but with the prescaler enabled. Figure 14-13. Timer/Counter Timing Diagram, with Prescaler (f /8) clk_I/O clk I/O clk Tn (clk/8) I/O TCNTn TOP - 1 TOP BOTTOM BOTTOM + 1 (CTC and FPWM) TCNTn TOP - 1 TOP TOP - 1 TOP - 2 (PC and PFC PWM) TOVn (FPWM) and ICFn (if used as TOP) OCRnx Old OCRnx Value New OCRnx Value (Update at TOP) ATmega16U4/32U4 [DATASHEET] 130 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

14.10 16-bit Timer/Counter Register Description 14.10.1Timer/Counter1 Control Register A – TCCR1A Bit 7 6 5 4 3 2 1 0 COM1A1 COM1A0 COM1B1 COM1B0 COM1C1 COM1C0 WGM11 WGM10 TCCR1 A Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 14.10.2Timer/Counter3 Control Register A – TCCR3A Bit 7 6 5 4 3 2 1 0 COM3A1 COM3A0 COM3B1 COM3B0 COM3C1 COM3C0 WGM31 WGM30 TCCR3A Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 7:6 – COMnA1:0: Compare Output Mode for Channel A • Bit 5:4 – COMnB1:0: Compare Output Mode for Channel B • Bit 3:2 – COMnC1:0: Compare Output Mode for Channel C The COMnA1:0, COMnB1:0, and COMnC1:0 control the output compare pins (OCnA, OCnB, and OCnC respectively) behavior. If one or both of the COMnA1:0 bits are written to one, the OCnA output overrides the normal port functionality of the I/O pin it is connected to. If one or both of the COMnB1:0 bits are written to one, the OCnB output overrides the normal port functionality of the I/O pin it is connected to. If one or both of the COMnC1:0 bits are written to one, the OCnC output overrides the normal port functionality of the I/O pin it is connected to. However, note that the Data Direction Register (DDR) bit corresponding to the OCnA, OCnB or OCnC pin must be set in order to enable the output driver. When the OCnA, OCnB or OCnC is connected to the pin, the function of the COMnx1:0 bits is dependent of the WGMn3:0 bits setting. The table shows the COMnx1:0 bit functionality when the WGMn3:0 bits are set to a normal or a CTC mode (non-PWM). Table 14-1. Compare Output Mode, non-PWM COMnA1/COMnB1/COMnC1 COMnA0/COMnB0/COMnC0 Description Normal port operation, OCnA/OCnB/OCnC 0 0 disconnected 0 1 Toggle OCnA/OCnB/OCnC on compare match Clear OCnA/OCnB/OCnC on compare match (set 1 0 output to low level) Set OCnA/OCnB/OCnC on compare match (set output 1 1 to high level) ATmega16U4/32U4 [DATASHEET] 131 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The table shows the COMnx1:0 bit functionality when the WGMn3:0 bits are set to the fast PWM mode. Table 14-2. Compare Output Mode, Fast PWM(1) COMnA1/COMnB1/COMnC0 COMnA0/COMnB0/COMnC0 Description Normal port operation, OCnA/OCnB/OCnC 0 0 disconnected WGM13:0 = 14 or 15: Toggle OC1A on Compare Match, OC1B and OC1C disconnected (normal port 0 1 operation). For all other WGM1 settings, normal port operation, OC1A/OC1B/OC1C disconnected. Clear OCnA/OCnB/OCnC on compare match, set 1 0 OCnA/OCnB/OCnC at TOP Set OCnA/OCnB/OCnC on compare match, clear 1 1 OCnA/OCnB/OCnC at TOP Note: 1. A special case occurs when OCRnA/OCRnB/OCRnC equals TOP and COMnA1/COMnB1/COMnC1 is set. In this case the compare match is ignored, but the set or clear is done at TOP. See “Fast PWM Mode” on page100. for more details. The table shows the COMnx1:0 bit functionality when the WGMn3:0 bits are set to the phase correct and frequency correct PWM mode. Table 14-3. Compare Output Mode, Phase Correct and Phase and Frequency Correct PWM(1) COMnA1/COMnB/COMnC1 COMnA0/COMnB0/COMnC0 Description Normal port operation, OCnA/OCnB/OCnC 0 0 disconnected. WGM13:0 = 8, 9, 10, or 11: Toggle OC1A on Compare Match, OC1B and OC1C disconnected (normal port 0 1 operation). For all other WGM1 settings, normal port operation, OC1A/OC1B/OC1C disconnected. Clear OCnA/OCnB/OCnC on compare match when up- 1 0 counting. Set OCnA/OCnB/OCnC on compare match when down-counting. Set OCnA/OCnB/OCnC on compare match when up- 1 1 counting. Clear OCnA/OCnB/OCnC on compare match when down-counting. Note: 1. A special case occurs when OCRnA/OCRnB/OCRnC equals TOP and COMnA1/COMnB1//COMnC1 is set. See “Phase Correct PWM Mode” on page101. for more details. • Bit 1:0 – WGMn1:0: Waveform Generation Mode Combined with the WGMn3:2 bits found in the TCCRnB Register, these bits control the counting sequence of the counter, the source for maximum (TOP) counter value, and what type of waveform generation to be used, see the table below. Modes of operation supported by the Timer/Counter unit are: Normal mode (counter), Clear Timer on Compare match (CTC) mode, and three types of Pulse Width Modulation (PWM) modes (“Modes of Operation” on page98). ATmega16U4/32U4 [DATASHEET] 132 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 14-4. Waveform Generation Mode Bit Description TOVn WGMn2 WGMn1 WGMn0 Update of Flag Set Mode WGMn3 (CTCn) (PWMn1) (PWMn0) Timer/Counter Mode of Operation TOP OCRnx at on 0 0 0 0 0 Normal 0xFFFF Immediate MAX 1 0 0 0 1 PWM, Phase Correct, 8-bit 0x00FF TOP BOTTOM 2 0 0 1 0 PWM, Phase Correct, 9-bit 0x01FF TOP BOTTOM 3 0 0 1 1 PWM, Phase Correct, 10-bit 0x03FF TOP BOTTOM 4 0 1 0 0 CTC OCRnA Immediate MAX 5 0 1 0 1 Fast PWM, 8-bit 0x00FF TOP TOP 6 0 1 1 0 Fast PWM, 9-bit 0x01FF TOP TOP 7 0 1 1 1 Fast PWM, 10-bit 0x03FF TOP TOP 8 1 0 0 0 PWM, Phase and Frequency Correct ICRn BOTTOM BOTTOM 9 1 0 0 1 PWM, Phase and Frequency Correct OCRnA BOTTOM BOTTOM 10 1 0 1 0 PWM, Phase Correct ICRn TOP BOTTOM 11 1 0 1 1 PWM, Phase Correct OCRnA TOP BOTTOM 12 1 1 0 0 CTC ICRn Immediate MAX 13 1 1 0 1 (Reserved) – – – 14 1 1 1 0 Fast PWM ICRn TOP TOP 15 1 1 1 1 Fast PWM OCRnA TOP TOP Note: 1. The CTCn and PWMn1:0 bit definition names are obsolete. Use the WGMn2:0 definitions. However, the functionality and location of these bits are compatible with previous versions of the timer. 14.10.3Timer/Counter1 Control Register B – TCCR1B Bit 7 6 5 4 3 2 1 0 ICNC1 ICES1 – WGM13 WGM12 CS12 CS11 CS10 TCCR1B Read/Write R/W R/W R R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 14.10.4Timer/Counter3 Control Register B – TCCR3B Bit 7 6 5 4 3 2 1 0 ICNC3 ICES3 – WGM33 WGM32 CS32 CS31 CS30 TCCR3B Read/Write R/W R/W R R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 7 – ICNCn: Input Capture Noise Canceler Setting this bit (to one) activates the Input Capture Noise Canceler. When the Noise Canceler is activated, the input from the Input Capture Pin (ICPn) is filtered. The filter function requires four successive equal valued samples of the ICPn pin for changing its output. The input capture is therefore delayed by four Oscillator cycles when the noise canceler is enabled. ATmega16U4/32U4 [DATASHEET] 133 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 6 – ICESn: Input Capture Edge Select This bit selects which edge on the Input Capture Pin (ICPn) that is used to trigger a capture event. When the ICESn bit is written to zero, a falling (negative) edge is used as trigger, and when the ICESn bit is written to one, a rising (positive) edge will trigger the capture. When a capture is triggered according to the ICESn setting, the counter value is copied into the Input Capture Register (ICRn). The event will also set the Input Capture Flag (ICFn), and this can be used to cause an Input Capture Interrupt, if this interrupt is enabled. When the ICRn is used as TOP value (see description of the WGMn3:0 bits located in the TCCRnA and the TCCRnB Register), the ICPn is disconnected and consequently the input capture function is disabled. • Bit 5 – Reserved Bit This bit is reserved for future use. For ensuring compatibility with future devices, this bit must be written to zero when TCCRnB is written. • Bit 4:3 – WGMn3:2: Waveform Generation Mode See TCCRnA Register description. • Bit 2:0 – CSn2:0: Clock Select The three clock select bits select the clock source to be used by the Timer/Counter, see Figure 13-8 on page 102 and Figure 13-9 on page 103. Table 14-5. Clock Select Bit Description CSn2 CSn1 CSn0 Description 0 0 0 No clock source. (Timer/Counter stopped) 0 0 1 clk /1 (No prescaling I/O 0 1 0 clk /8 (From prescaler) I/O 0 1 1 clk /64 (From prescaler) I/O 1 0 0 clk /256 (From prescaler) I/O 1 0 1 clk /1024 (From prescaler) I/O 1 1 0 External clock source on Tn pin. Clock on falling edge 1 1 1 External clock source on Tn pin. Clock on rising edge If external pin modes are used for the Timer/Countern, transitions on the Tn pin will clock the counter even if the pin is configured as an output. This feature allows software control of the counting. 14.10.5Timer/Counter1 Control Register C – TCCR1C Bit 7 6 5 4 3 2 1 0 FOC1A FOC1B FOC1C – – – – – TCCR1C Read/Write W W W R R R R R Initial Value 0 0 0 0 0 0 0 0 ATmega16U4/32U4 [DATASHEET] 134 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

14.10.6Timer/Counter3 Control Register C – TCCR3C Bit 7 6 5 4 3 2 1 0 FOC3A – – – – – – – TCCR3C Read/Write W R R R R R R R Initial Value 0 0 0 0 0 0 0 0 • Bit 7 – FOCnA: Force Output Compare for Channel A The FOCnA/FOCnB/FOCnC bits are only active when the WGMn3:0 bits specifies a non-PWM mode. When writing a logical one to the FOCnA/FOCnB/FOCnC bit, an immediate compare match is forced on the waveform generation unit. The OCnA/OCnB/OCnC output is changed according to its COMnx1:0 bits setting. Note that the FOCnA/FOCnB/FOCnC bits are implemented as strobes. Therefore it is the value present in the COMnx1:0 bits that determine the effect of the forced compare. A FOCnA/FOCnB/FOCnC strobe will not generate any interrupt nor will it clear the timer in Clear Timer on Compare Match (CTC) mode using OCRnA as TOP. The FOCnA/FOCnB/FOCnB bits are always read as zero. • Bit 4:0 – Reserved Bits These bits are reserved for future use. For ensuring compatibility with future devices, these bits must be written to zero when TCCRnC is written. 14.10.7Timer/Counter1 – TCNT1H and TCNT1L Bit 7 6 5 4 3 2 1 0 TCNT1[15:8] TCNT1H TCNT1[7:0] TCNT1L Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 14.10.8Timer/Counter3 – TCNT3H and TCNT3L Bit 7 6 5 4 3 2 1 0 TCNT3[15:8] TCNT3H TCNT3[7:0] TCNT3L Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 The two Timer/Counter I/O locations (TCNTnH and TCNTnL, combined TCNTn) give direct access, both for read and for write operations, to the Timer/Counter unit 16-bit counter. To ensure that both the high and low bytes are read and written simultaneously when the CPU accesses these registers, the access is performed using an 8-bit temporary High Byte Register (TEMP). This temporary register is shared by all the other 16-bit registers. See “Accessing 16-bit Registers” on page113. Modifying the counter (TCNTn) while the counter is running introduces a risk of missing a compare match between TCNTn and one of the OCRnx Registers. Writing to the TCNTn Register blocks (removes) the compare match on the following timer clock for all compare units. ATmega16U4/32U4 [DATASHEET] 135 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

14.10.9Output Compare Register 1 A – OCR1AH and OCR1AL Bit 7 6 5 4 3 2 1 0 OCR1A[15:8] OCR1AH OCR1A[7:0] OCR1AL Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 14.10.10Output Compare Register 1 B – OCR1BH and OCR1BL Bit 7 6 5 4 3 2 1 0 OCR1B[15:8] OCR1BH OCR1B[7:0] OCR1BL Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 14.10.11Output Compare Register 1 C – OCR1CH and OCR1CL Bit 7 6 5 4 3 2 1 0 OCR1C[15:8] OCR1CH OCR1C[7:0] OCR1CL Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 14.10.12 Output Compare Register 3 A – OCR3AH and OCR3AL Bit 7 6 5 4 3 2 1 0 OCR3A[15:8] OCR3AH OCR3A[7:0] OCR3AL Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 14.10.13Output Compare Register 3 B – OCR3BH and OCR3BL Bit 7 6 5 4 3 2 1 0 OCR3B[15:8] OCR3BH OCR3B[7:0] OCR3BL Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 14.10.14Output Compare Register 3 C – OCR3CH and OCR3CL Bit 7 6 5 4 3 2 1 0 OCR3C[15:8] OCR3CH OCR3C[7:0] OCR3CL Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 The Output Compare Registers contain a 16-bit value that is continuously compared with the counter value (TCNTn). A match can be used to generate an Output Compare interrupt, or to generate a waveform output on the OCnx pin. ATmega16U4/32U4 [DATASHEET] 136 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The Output Compare Registers are 16-bit in size. To ensure that both the high and low bytes are written simultaneously when the CPU writes to these registers, the access is performed using an 8-bit temporary High Byte Register (TEMP). This temporary register is shared by all the other 16-bit registers. See “Accessing 16-bit Registers” on page113. 14.10.15Input Capture Register 1 – ICR1H and ICR1L Bit 7 6 5 4 3 2 1 0 ICR1[15:8] ICR1H ICR1[7:0] ICR1L Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 14.10.16Input Capture Register 3 – ICR3H and ICR3L Bit 7 6 5 4 3 2 1 0 ICR3[15:8] ICR3H ICR3[7:0] ICR3L Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 The Input Capture is updated with the counter (TCNTn) value each time an event occurs on the ICPn pin (or optionally on the Analog Comparator output for Timer/Counter1). The Input Capture can be used for defining the counter TOP value. The Input Capture Register is 16-bit in size. To ensure that both the high and low bytes are read simultaneously when the CPU accesses these registers, the access is performed using an 8-bit temporary High Byte Register (TEMP). This temporary register is shared by all the other 16-bit registers. See “Accessing 16-bit Registers” on page113. 14.10.17Timer/Counter1 Interrupt Mask Register – TIMSK1 Bit 7 6 5 4 3 2 1 0 – – ICIE1 – OCIE1C OCIE1B OCIE1A TOIE1 TIMSK1 Read/Write R R R/W R R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 14.10.18Timer/Counter3 Interrupt Mask Register – TIMSK3 Bit 7 6 5 4 3 2 1 0 – – ICIE3 – OCIE3C OCIE3B OCIE3A TOIE3 TIMSK3 Read/Write R R R/W R R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 5 – ICIEn: Timer/Countern, Input Capture Interrupt Enable When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the Timer/Countern Input Capture interrupt is enabled. The corresponding Interrupt Vector (See “Interrupts” on page63.) is executed when the ICFn Flag, located in TIFRn, is set. ATmega16U4/32U4 [DATASHEET] 137 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 3 – OCIEnC: Timer/Countern, Output Compare C Match Interrupt Enable When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the Timer/Countern Output Compare C Match interrupt is enabled. The corresponding Interrupt Vector (See “Interrupts” on page63.) is executed when the OCFnC Flag, located in TIFRn, is set. • Bit 2 – OCIEnB: Timer/Countern, Output Compare B Match Interrupt Enable When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the Timer/Countern Output Compare B Match interrupt is enabled. The corresponding Interrupt Vector (See “Interrupts” on page63.) is executed when the OCFnB Flag, located in TIFRn, is set. • Bit 1 – OCIEnA: Timer/Countern, Output Compare A Match Interrupt Enable When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the Timer/Countern Output Compare A Match interrupt is enabled. The corresponding Interrupt Vector (See “Interrupts” on page63.) is executed when the OCFnA Flag, located in TIFRn, is set. • Bit 0 – TOIEn: Timer/Countern, Overflow Interrupt Enable When this bit is written to one, and the I-flag in the Status Register is set (interrupts globally enabled), the Timer/Countern Overflow interrupt is enabled. The corresponding Interrupt Vector (See “Interrupts” on page63.) is executed when the TOVn Flag, located in TIFRn, is set. 14.10.19Timer/Counter1 Interrupt Flag Register – TIFR1 Bit 7 6 5 4 3 2 1 0 – – ICF1 – OCF1C OCF1B OCF1A TOV1 TIFR1 Read/Write R R R/W R R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 14.10.20Timer/Counter3 Interrupt Flag Register – TIFR3 Bit 7 6 5 4 3 2 1 0 – – ICF3 – OCF3C OCF3B OCF3A TOV3 TIFR3 Read/Write R R R/W R R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 5 – ICFn: Timer/Countern, Input Capture Flag This flag is set when a capture event occurs on the ICPn pin. When the Input Capture Register (ICRn) is set by the WGMn3:0 to be used as the TOP value, the ICFn Flag is set when the counter reaches the TOP value. ICFn is automatically cleared when the Input Capture Interrupt Vector is executed. Alternatively, ICFn can be cleared by writing a logic one to its bit location. • Bit 3– OCFnC: Timer/Countern, Output Compare C Match Flag This flag is set in the timer clock cycle after the counter (TCNTn) value matches the Output Compare Register C (OCRnC). Note that a Forced Output Compare (FOCnC) strobe will not set the OCFnC Flag. OCFnC is automatically cleared when the Output Compare Match C Interrupt Vector is executed. Alternatively, OCFnC can be cleared by writing a logic one to its bit location. ATmega16U4/32U4 [DATASHEET] 138 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 2 – OCFnB: Timer/Counter1, Output Compare B Match Flag This flag is set in the timer clock cycle after the counter (TCNTn) value matches the Output Compare Register B (OCRnB). Note that a Forced Output Compare (FOCnB) strobe will not set the OCFnB Flag. OCFnB is automatically cleared when the Output Compare Match B Interrupt Vector is executed. Alternatively, OCFnB can be cleared by writing a logic one to its bit location. • Bit 1 – OCF1A: Timer/Counter1, Output Compare A Match Flag This flag is set in the timer clock cycle after the counter (TCNTn value matches the Output Compare Register A (OCRnA). Note that a Forced Output Compare (FOCnA) strobe will not set the OCFnA Flag. OCFnA is automatically cleared when the Output Compare Match A Interrupt Vector is executed. Alternatively, OCFnA can be cleared by writing a logic one to its bit location. • Bit 0 – TOVn: Timer/Countern, Overflow Flag The setting of this flag is dependent of the WGMn3:0 bits setting. In Normal and CTC modes, the TOVn Flag is set when the timer overflows. Refer to Table14-4 on page133 for the TOVn Flag behavior when using another WGMn3:0 bit setting. TOVn is automatically cleared when the Timer/Countern Overflow Interrupt Vector is executed. Alternatively, TOVn can be cleared by writing a logic one to its bit location. ATmega16U4/32U4 [DATASHEET] 139 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

15. 10-bit High Speed Timer/Counter4 15.1 Features • Up to 10-Bit Accuracy • Three Independent Output Compare Units • Clear Timer on Compare Match (Auto Reload) • Glitch Free, Phase and Frequency Correct Pulse Width Modulator (PWM) • Enhanced PWM mode: one optional additional accuracy bit without effect on output frequency • Variable PWM Period • Independent Dead Time Generators for each PWM channels • Synchronous update of PWM registers • Five Independent Interrupt Sources (TOV4, OCF4A, OCF4B, OCF4D, FPF4) • High Speed Asynchronous and Synchronous Clocking Modes • Separate Prescaler Unit 15.2 Overview Timer/Counter4 is a general purpose high speed Timer/Counter module, with three independent Output Compare Units, and with enhanced PWM support. The Timer/Counter4 features a high resolution and a high accuracy usage with the lower prescaling opportunities. It can also support three accurate and high speed Pulse Width Modulators using clock speeds up to 64MHz. In PWM mode Timer/Counter4 and the output compare registers serve as triple stand-alone PWMs with non-overlapping, non-inverted and inverted outputs. The enhanced PWM mode allows to get one more accuracy bit while keeping the frequency identical to normal mode (a PWM 8 bits accuracy in enhanced mode outputs the same frequency that a PWM 7 bits accuracy in normal mode). Similarly, the high prescaling opportunities make this unit useful for lower speed functions or exact timing functions with infrequent actions. A lock feature allows user to update the PWM registers and A simplified block diagram of the Timer/Counter4 is shown in Figure 15-1 on page 141. For actual placement of the I/O pins, refer to “Pinout” on page3. The device-specific I/O register and bit locations are listed in the “Register Description” on page164. ATmega16U4/32U4 [DATASHEET] 140 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 15-1. Timer/Counter4 Block Diagram TOV4 OCF4A OCF4B OCF4D OC4A OC4A OC4B OC4B OC4D OC4D FAULT_PROTECTION DEAD TIME GENERATOR DEAD TIME GENERATOR DEAD TIME GENERATOR OCW4A OCW4B OCW4D TOIE4 OCIE4A OCIE4B OCIE4D TOV4 OCF4A OCF4B OCF4D FPIE4 FPF4 T/C INT. MASK T/C INT. FLAG T/C CONTROL T/C CONTROL T/C CONTROL T/C CONTROL REGISTER (TIMSK4) REGISTER (TIFR4) REGISTER A (TCCR4A) REGISTER B (TCCR4B) REGISTER C (TCCR4C) REGISTER C (TCCR4D) COM4A1COM4A0 COM4B1 COM4B0 FOC4A FOC4BPWM4A PWM4B PSR4 PSR4PSR4 CS43 CS42 CS41 CS40 COM4A1 COM4A0 COM4B1 COM4B0 COM4D1 COM4D0 FOC4D PWM4D FPIE4 FPEN4 FPNC4 FPES4 FPAC4 FPF4 WGM41 WGM40 CLK TIMER/COUNTER4 COUNT TIMER/COUNTER4 CONTROL LOGIC (TCNT4) CLEAR DIRECTION OC4OE5 OC4OE4 OC4OE3 OC4OE2OC4OE1 OC4OE0 10-BIT COMPARATOR 10-BIT COMPARATOR 10-BIT COMPARATOR 10-BIT COMPARATOR TR/CE GCIOSNTTERRO DL (TCCR4E) 10-BIT OUTPUT 10-BIT OUTPUT 10-BIT OUTPUT 10-BIT OUTPUT COMPARE REGISTER A COMPARE REGISTER B COMPARE REGISTER C COMPARE REGISTER D 8-BIT OUTPUT COMPARE 8-BIT OUTPUT COMPARE 8-BIT OUTPUT COMPARE 8-BIT OUTPUT COMPARE 2-BIT HIGH BYTE REGISTER A (OCR4A) REGISTER B (OCR4B) REGISTER C (OCR4C) REGISTER D (OCR4D) REGISTER (TC4H) 8-BIT DATABUS 15.2.1 Speed The maximum speed of the Timer/Counter4 is 64MHz. However, if a supply voltage below 4V is used, it is recommended to decrease the input frequency, because the Timer/Counter4 is not running fast enough on low voltage levels. 15.2.2 Accuracy The Timer/Counter4 is a 10-bit Timer/Counter module that can alternatively be used as an 8-bit Timer/Counter. The Timer/Counter4 registers are basically 8-bit registers, but on top of that there is a 2-bit High Byte Register (TC4H) that can be used as a common temporary buffer to access the two MSBs of the 10-bit Timer/Counter4 registers by the AVR CPU via the 8-bit data bus, if the 10-bit accuracy is used. Whereas, if the two MSBs of the 10-bit registers are written to zero the Timer/Counter4 is working as an 8-bit Timer/Counter. When reading the low byte of any 8-bit register the two MSBs are written to the TC4H register, and when writing the low byte of any 8-bit register the two MSBs are written from the TC4H register. Special procedures must be followed when accessing the 10-bit Timer/Counter4 values via the 8-bit data bus. These procedures are described in the section “Accessing 10-bit Registers” on page160. The Enhanced PWM mode allows to add a resolution bit to each Compare register A/B/D, while the output frequency remains identical to a Normal PWM mode. That means that the TC4H register contains one more bit that will be the MSB in a 11-bits enhanced PWM operation. See the section “Enhanced Compare/PWM mode” on page150 for details about this feature and how to use it. 15.2.3 Registers The Timer/Counter (TCNT4) and Output Compare Registers (OCR4A, OCR4B, OCR4C and OCR4D) are 8-bit registers that are used as a data source to be compared with the TCNT4 contents. The OCR4A, OCR4B and OCR4D registers determine the action on the OC4A, OC4B and OC4D pins and they can also generate the ATmega16U4/32U4 [DATASHEET] 141 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

compare match interrupts. The OCR4C holds the Timer/Counter TOP value, i.e. the clear on compare match value. The Timer/Counter4 High Byte Register (TC4H) is a 2-bit register that is used as a common temporary buffer to access the MSB bits of the Timer/Counter4 registers, if the 10-bit accuracy is used. Interrupt request (overflow TOV4, compare matches OCF4A, OCF4B, OCF4D and fault protection FPF4) signals are visible in the Timer Interrupt Flag Register (TIFR4) and Timer/Counter4 Control Register D (TCCR4D). The interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK4) and the FPIE4 bit in the Timer/Counter4 Control Register D (TCCR4D). Control signals are found in the Timer/Counter Control Registers TCCR4A, TCCR4B, TCCR4C, TCCR4D, and TCCR4E. 15.2.4 Synchronization In asynchronous clocking mode the Timer/Counter4 and the prescaler allow running the CPU from any clock source while the prescaler is operating on the fast peripheral clock (PCK) having frequency up to 64MHz. This is possible because there is a synchronization boundary between the CPU clock domain and the fast peripheral clock domain. Figure 15-2 on page 143 shows Timer/Counter 4 synchronization register block diagram and describes synchronization delays in between registers. Note that all clock gating details are not shown in the figure. The Timer/Counter4 register values go through the internal synchronization registers, which cause the input synchronization delay, before affecting the counter operation. The registers TCCR4A, TCCR4B, TCCR4C, TCCR4D, OCR4A, OCR4B, OCR4C, and OCR4D can be read back right after writing the register. The read back values are delayed for the Timer/Counter4 (TCNT4) register, Timer/Counter4 High Byte Register (TC4H) and flags (OCF4A, OCF4B, OCF4D, and TOV4), because of the input and output synchronization. The system clock frequency must be lower than half of the PCK frequency, because the synchronization mechanism of the asynchronous Timer/Counter4 needs at least two edges of the PCK when the system clock is high. If the frequency of the system clock is too high, it is a risk that data or control values are lost. ATmega16U4/32U4 [DATASHEET] 142 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 15-2. Timer/Counter4 Synchronization Register Block Diagram 8-BIT DATABUS IO-registers Input synchronization Timer/Counter4 Output synchronization registers registers OCR4A OCR4A_SI TCNT4 OCR4B OCR4B_SI TCNT4_SO OCR4C OCR4C_SI TC4H TC4H_SO OCR4D OCR4D_SI TCCR4A TCCR4A_SI TCCR4B TCCR4B_SI OCF4 OCF4A_SO TCCR4C TCCR4C_SI TCNT4 TCCR4D TCCR4D_SI OCF4B TCNT4 TCNT4_SI OCF4B_SO TC4H TC4H_SI OCF4A OCF4A_SI OCF4D OCF4D_SO OCF4B OCF4B_SI OCF4D OCF4D_SI TOV4 TOV4 TOV4_SI TOV4_SO PLLTM1:0 != '00' CK S A S PCK A (clk ) TMR SYNC 1/2 CK Delay 1 CK Delay 1 CK Delay 1/2 CK Delay MODE ASYNC ~1/2 CK Delay 1 PCK Delay 1 PCK Delay ~1 CK Delay MODE 15.2.5 Definitions Many register and bit references in this section are written in general form. A lower case “n” replaces the Timer/Counter number, in this case 0. A lower case “x” replaces the Output Compare Unit, in this case Compare Unit A, B, C or D. However, when using the register or bit defines in a program, the precise form must be used, i.e., TCNT4 for accessing Timer/Counter4 counter value and so on. The definitions in the table are used extensively throughout the document. BOTTOM The counter reaches the BOTTOM when it becomes 0. MAX The counter reaches its MAXimum value when it becomes 0x3FF (decimal 1023). The counter reaches the TOP value (stored in the OCR1C) when it becomes equal to the TOP highest value in the count sequence. The TOP has a value 0x0FF as default after reset. ATmega16U4/32U4 [DATASHEET] 143 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

15.3 Counter Unit The main part of the Timer/Counter4 is the programmable bi-directional counter unit. Figure 15-3 shows a block diagram of the counter and its surroundings. Figure 15-3. Counter Unit Block Diagram DATA BUS TOV4 clk T4 Timer/Counter4 Count Enable count ( From Prescaler ) TCNT4 clear Control Logic PLLTM1:0 PCK direction CK bottom top Signal description (internal signals): count: TCNT4 increment or decrement enable. direction: Select between increment and decrement. clear: Clear TCNT4 (set all bits to zero). clk : Timer/Counter clock, referred to as clk in the following. Tn T4 top: Signalize that TCNT4 has reached maximum value. bottom: Signalize that TCNT4 has reached minimum value (zero). Depending of the mode of operation used, the counter is cleared, incremented, or decremented at each timer clock (clk ). The timer clock is generated from an synchronous system clock or an asynchronous PLL clock T4 using the Clock Select bits (CS4<3:0>) and the PLL Postscaler for High Speed Timer bits (PLLTM1:0). When no clock source is selected (CS4<3:0> = 0) the timer is stopped. However, the TCNT4 value can be accessed by the CPU, regardless of whether clk is present or not. A CPU write overrides (has priority over) all counter clear T1 or count operations. The counting sequence of the Timer/Counter4 is determined by the setting of the WGM10 and PWM4x bits located in the Timer/Counter4 Control Registers (TCCR4A, TCCR4C, and TCCR4D). For more details about advanced counting sequences and waveform generation, see “Modes of Operation” on page151. The Timer/Counter Overflow Flag (TOV4) is set according to the mode of operation selected by the PWM4x and WGM40 bits. The Overflow Flag can be used for generating a CPU interrupt. 15.3.1 Counter Initialization for Asynchronous Mode To change Timer/Counter4 to the asynchronous mode follow the procedure below: 1. Enable PLL. 2. Wait 100µs for PLL to stabilize. 3. Poll the PLOCK bit until it is set. 4. Configure the PLLTM1:0 bits in the PLLFRQ register to enable the asynchronous mode (different from 0:0 value). ATmega16U4/32U4 [DATASHEET] 144 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

15.4 Output Compare Unit The comparator continuously compares TCNT4 with the Output Compare Registers (OCR4A, OCR4B, OCR4C, and OCR4D). Whenever TCNT4 equals to the Output Compare Register, the comparator signals a match. A match will set the Output Compare Flag (OCF4A, OCF4B, or OCF4D) at the next timer clock cycle. If the corresponding interrupt is enabled, the Output Compare Flag generates an Output Compare interrupt. The Output Compare Flag is automatically cleared when the interrupt is executed. Alternatively, the flag can be cleared by software by writing a logical one to its I/O bit location. The Waveform Generator uses the match signal to generate an output according to operating mode set by the PWM4x, WGM40, and Compare Output mode (COM4x1:0) bits. The top and bottom signals are used by the Waveform Generator for handling the special cases of the extreme values in some modes of operation (See “Modes of Operation” on page151.) Figure 15-4 shows a block diagram of the Output Compare unit. Figure 15-4. Output Compare Unit, Block Diagram 8-BIT DATA BUS OCRnx TCnH TCNTn 10-BIT OCRnx 10-BIT TCNTn = (10-bit Comparator ) OCFnx (Int.Req.) TOP PWMnx BOTTOM Waveform Generator WGMn0 FOCn COMnX1:0 OCWnx The OCR4x Registers are double buffered when using any of the Pulse Width Modulation (PWM) modes. For the normal mode of operation, the double buffering is disabled. The double buffering synchronizes the update of the OCR4x Compare Registers to either top or bottom of the counting sequence. The synchronization prevents the occurrence of odd-length, non-symmetrical PWM pulses, thereby making the output glitch-free. See Figure 15-5 on page 146 for an example. During the time between the write and the update operation, a read from OCR4A, OCR4B, OCR4C, or OCR4D will read the contents of the temporary location. This means that the most recently written value always will read out of OCR4A, OCR4B, OCR4C, or OCR4D. ATmega16U4/32U4 [DATASHEET] 145 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 15-5. Effects of Unsynchronized OCR Latching Compare Value changes Counter Value Compare Value Output Compare Synchronized WFnx Latch Waveform OCWnx Compare Value changes Counter Value Compare Value Output Compare Unsynchronized WFnx Latch Glitch Wafeform OCWnx 15.4.1 Force Output Compare In non-PWM waveform generation modes, the match output of the comparator can be forced by writing a one to the Force Output Compare (FOC4x) bit. Forcing Compare Match will not set the OCF4x Flag or reload/clear the timer, but the Waveform Output (OCW4x) will be updated as if a real Compare Match had occurred (the COM4x1:0 bits settings define whether the Waveform Output (OCW4x) is set, cleared or toggled). 15.4.2 Compare Match Blocking by TCNT4 Write All CPU write operations to the TCNT4 Register will block any Compare Match that occur in the next timer clock cycle, even when the timer is stopped. This feature allows OCR4x to be initialized to the same value as TCNT4 without triggering an interrupt when the Timer/Counter clock is enabled. 15.4.3 Using the Output Compare Unit Since writing TCNT4 in any mode of operation will block all Compare Matches for one timer clock cycle, there are risks involved when changing TCNT4 when using the Output Compare Unit, independently of whether the Timer/Counter is running or not. If the value written to TCNT4 equals the OCR4x value, the Compare Match will be missed, resulting in incorrect waveform generation. Similarly, do not write the TCNT4 value equal to BOTTOM when the counter is down-counting. The setup of the Waveform Output (OCW4x) should be performed before setting the Data Direction Register for the port pin to output. The easiest way of setting the OCW4x value is to use the Force Output Compare (FOC4x) strobe bits in Normal mode. The OC4x keeps its value even when changing between Waveform Generation modes. Be aware that the COM4x1:0 bits are not double buffered together with the compare value. Changing the COM4x1:0 bits will take effect immediately. ATmega16U4/32U4 [DATASHEET] 146 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

15.5 Dead Time Generator The Dead Time Generator is provided for the Timer/Counter4 PWM output pairs to allow driving external power control switches safely. The Dead Time Generator is a separate block that can be used to insert dead times (non-overlapping times) for the Timer/Counter4 complementary output pairs OC4x and OC4x when the PWM mode is enabled and the COM4x1:0 bits are set to “01”. The sharing of tasks is as follows: the Waveform Generator generates the Waveform Output (OCW4x) and the Dead Time Generator generates the non- overlapping PWM output pair from the Waveform Output. Three Dead Time Generators are provided, one for each PWM output. The non-overlap time is adjustable and the PWM output and it’s complementary output are adjusted separately, and independently for both PWM outputs. Figure 15-6. Output Compare Unit, Block Diagram top OCnx OCnx pin OCWnx bottom Waveform Generator Dead Time Generator OCnx FOCn OCnx pin CK OR PCK PWMnx WGMn0 COMnx CLOCK DTPSn DTnH DTnL The Dead Time Generation is based on the 4-bit down counters that count the dead time, as shown in Figure 15-7. There is a dedicated prescaler in front of the Dead Time Generator that can divide the Timer/Counter4 clock (PCK or CK) by 1, 2, 4, or 8. This provides for large range of dead times that can be generated. The prescaler is controlled by two control bits DTPS41..40. The block has also a rising and falling edge detector that is used to start the dead time counting period. Depending on the edge, one of the transitions on the rising edges, OC4x or OC4x is delayed until the counter has counted to zero. The comparator is used to compare the counter with zero and stop the dead time insertion when zero has been reached. The counter is loaded with a 4- bit DT4H or DT4L value from DT4 I/O register, depending on the edge of the Waveform Output (OCW4x) when the dead time insertion is started. The Output Compare Output are delayed by one timer clock cycle at minimum from the Waveform Output when the Dead Time is adjusted to zero. The outputs OC4x and OC4x are inverted, if the PWM Inversion Mode bit PWM4X is set. This will also cause both outputs to be high during the dead time. Figure 15-7. Dead Time Generator PWMnX COMPARATOR OCnx CK OR PCK CLOCK DEAD TIME PRE-SCALER CLOCK CONTROL 4-BIT COUNTER OCnx DTPSn DTnH DTnL PWMnX TCCRnB REGISTER DTn I/O REGISTER OCWnx DATA BUS (8-bit) The length of the counting period is user adjustable by selecting the dead time prescaler setting by using the DTPS41:40 control bits, and selecting then the dead time value in I/O register DT4. The DT4 register consists of two 4-bit fields, DT4H and DT4L that control the dead time periods of the PWM output and its' complementary output separately in terms of the number of prescaled dead time generator clock cycles. Thus the rising edge of ATmega16U4/32U4 [DATASHEET] 147 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

OC4x and OC4x can have different dead time periods as the t is adjusted by the 4-bit DT4H non-overlap / rising edge value and the t is adjusted by the 4-bit DT4L value. non-overlap / falling edge Figure 15-8. The Complementary Output Pair, COM4x1:0 = 1 OCWnx OCnx OCnx (COMnx = 1) t t non-overlap / rising edge non-overlap / falling edge 15.6 Compare Match Output Unit The Compare Output Mode (COM4x1:0) bits have two functions. The Waveform Generator uses the COM4x1:0 bits for defining the inverted or non-inverted Waveform Output (OCW4x) at the next Compare Match. Also, the COM4x1:0 bits control the OC4x and OC4x pin output source. Figure 15-9 shows a simplified schematic of the logic affected by the COM4x1:0 bit setting. The I/O Registers, I/O bits, and I/O pins in the figure are shown in bold. Only the parts of the general I/O Port Control Registers (DDR and PORT) that are affected by the COM4x1:0 bits are shown. In Normal Mode (non-PWM) the Dead Time Generator is disabled and it is working like a synchronizer: the Output Compare (OC4x) is delayed from the Waveform Output (OCW4x) by one timer clock cycle. Whereas in Fast PWM Mode and in Phase and Frequency Correct PWM Mode when the COM4x1:0 bits are set to “01” both the non-inverted and the inverted Output Compare output are generated, and an user programmable Dead Time delay is inserted for these complementary output pairs (OC4x and OC4x). The functionality in PWM modes is similar to Normal mode when any other COM4x1:0 bit setup is used. When referring to the OC4x state, the reference is for the Output Compare output (OC4x) from the Dead Time Generator, not the OC4x pin. If a system reset occur, the OC4x is reset to “0”. The general I/O port function is overridden by the Output Compare (OC4x / OC4x) from the Dead Time Generator if either of the COM4x1:0 bits are set. However, the OC4x pin direction (input or output) is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction Register bit for the OC4x and OC4x pins (DDR_OC4x and DDR_OC4x) must be set as output before the OC4x and OC4x values are visible on the pin. The port override function is independent of the Output Compare mode. The design of the Output Compare Pin Configuration logic allows initialization of the OC4x state before the output is enabled. Note that some COM4x1:0 bit settings are reserved for certain modes of operation. For Output Compare Pin Configurations refer to Table15-1 on page152, Table15-2 on page154, Table15-3 on page155, and Table15-4 on page157. ATmega16U4/32U4 [DATASHEET] 148 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 15-9. Compare Match Output Unit, Schematic WGM41 clkI/O OC4OE1:0 Output Compare COM4A1:0 Pin Configuration D Q PORTC6 0 1 1 OC4A D Q 0 PIN DDRC6 OCW4A OC4A D Q Dead Time Q PORTC7 clk Tn Generator A Q OC4A 1 OC4A PIN 0 D Q DDRC7 WGM41 OC4OE3:2 Output Compare COM4B1:0 Pin Configuration D Q PORTB5 2 1 1 BUS D Q 0 0 O PCI4NB A DDRB5 T A D OCW4B OC4B D Q Dead Time Q 1 PORTB6 clk Tn Generator B Q OC4B 0 1 OC4B PIN 0 D Q DDRB6 WGM41 OC4OE5:4 Output Compare COM4D1:0 Pin Configuration D Q PORTD6 2 1 1 0 OC4D D Q 0 PIN DDRD6 OCW4D OC4 D Q Dead Time Q 1 PORTD7 clk Tn Generator D Q OC4D 0 1 OC4D PIN 0 D Q DDRD7 15.6.1 Compare Output Mode and Waveform Generation The Waveform Generator uses the COM4x1:0 bits differently in Normal mode and PWM modes. For all modes, setting the COM4x1:0 = 0 tells the Waveform Generator that no action on the OCW4x Output is to be performed on the next Compare Match. For compare output actions in the non-PWM modes refer to Table15-5 on page164. For fast PWM mode, refer to Table15-6 on page164, and for the Phase and Frequency Correct PWM refer to Table15-7 on page165. A change of the COM4x1:0 bits state will have effect at the first Compare Match after the bits are written. For non-PWM modes, the action can be forced to have immediate effect by using the FOC4x strobe bits. ATmega16U4/32U4 [DATASHEET] 149 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

15.6.2 Enhanced Compare/PWM mode When the bit ENHC4 of TCCR4E register is set, the Enhanced Compare/PWM mode is enabled. This mode allows user to add an accuracy bit to Output Compare Register OCR4A, OCR4B, and OCR4D. Like explained previously, a compare condition appears when one of the three Output Compare Registers (OCR4A/B/D) matches the value of TCNT4 (10-bits resolution). In basic PWM Mode, the corresponding enabled output toggles on the Compare Match. The Enhanced Compare/PWM mode introduces a bit that determines on which internal clock edge the Compare Match condition is actually signalled. That means that the corresponding outputs will toggle on the standard clock edge (like in Normal mode) if the LSB of OCR4A/B/D is ‘0’, or on the opposite (next) edge if the LSB is ‘1’. User will notice that between Normal and Enhanced PWM modes, the output frequency will be identical, while the PWM resolution will be better in second case. Writing to the Output Compare registers OCR4A/B/D or reading them will be identical in both modes. In Enhanced mode, user must just consider that the TC4H register can be up to 3-bits wide (and have the same behavior than during 2-bits operation). That will concern OCR4A, OCR4B and OCR4D registers accesses only. Indeed, the OCR4C register must not include the additional accuracy bit, and remains in the resolution that determines the output signal period. Figure 15-10. How Register Access Works in Enhanced Mode (TC4H) (OCR4A/B/D) 10 9 8 7 6 5 4 3 2 1 0 User Interface Side Timer Logic Side True 9 8 7 6 5 4 3 2 1 0 OCR4A/B/D B) S L ( Output Compare Module A/B/D Waveform Generation TCNT4<9:0> Enhanced OCR4C<9:0> Mode Configuration ENHC4 bits Pin Toggle Figure 15-10 shows that the true OCR4A/B/D value corresponds to the value loaded by the user shifted on the right in order to transfer the least significant bit directly to the Waveform generation module. The maximum available resolution is 11-bits, but any other resolution can be specified. For example, a 8-bits resolution will allow to obtain the same frequency than a Normal PWM mode with 7-bits resolution. Example: ̶ PLL Postcaler output = 64MHz, No Prescaler on Timer/Counter4. ̶ Setting OCR4C = 0x7F determines a full 7-bits theoretical resolution, and so a 500kHz output frequency. ̶ Setting OCR4A = 0x85 (= b’10000101’) signifies that the true value of “Compare A” register is 0x42 (b’01000010’) and that the Enhanced bit is set. That means that the duty cycle obtained (51.95%) will be the intermediate value between duty cycles that can be obtained by 0x42 and 0x43 Compare values (51.56%, 52.34%). ATmega16U4/32U4 [DATASHEET] 150 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

15.7 Synchronous update To avoid unasynchronous and incoherent values in a cycle, if a synchronous update of one of several values is necessary, all values can be updated at the same time at the end of the PWM cycle by the Timer controller. The new set of values is calculated by software and the effective update can be initiated by software. Figure 15-11. Lock Feature and Synchronous update TLOCK4=1 TLOCK4=0 Regulation Loop Request for an Calculation Writing to Timer Update Registers Set j Cycle with Cycle with Cycle with Cycle with Cycle with Set i Set i Set i Set i Set j In normal operation, each write to a Compare register is effective at the end of the current cycle. But some cases require that two or more Compare registers are updated synchronously, and that may not be always possible, mostly at high speed PWM frequencies. That may result in some PWM periods with incoherent values. When using the Lock feature (TLOCK4=1), the values written to the Compare registers are not effective and temporarily buffered. When releasing the TLOCK4 bit, the update is initiated and the new whole set of values will be loaded at the end of the current PWM cycle. Refer to “TCCR4E – Timer/Counter4 Control Register E” on page171. 15.8 Modes of Operation The mode of operation, i.e., the behavior of the Timer/Counter and the Output Compare pins, is defined by the combination of the Waveform Generation mode (bits PWM4x and WGM40) and Compare Output mode (COM4x1:0) bits. The Compare Output mode bits do not affect the counting sequence, while the Waveform Generation mode bits do. The COM4x1:0 bits control whether the PWM output generated should be inverted, non-inverted or complementary. For non-PWM modes the COM4x1:0 bits control whether the output should be set, cleared, or toggled at a Compare Match. 15.8.1 Normal Mode The simplest mode of operation is the Normal mode (PWM4x = 0), the counter counts from BOTTOM to TOP (defined as OCR4C) then restarts from BOTTOM. The OCR4C defines the TOP value for the counter, hence also its resolution, and allows control of the Compare Match output frequency. In toggle Compare Output Mode the Waveform Output (OCW4x) is toggled at Compare Match between TCNT4 and OCR4x. In non-inverting Compare Output Mode the Waveform Output is cleared on the Compare Match. In inverting Compare Output Mode the Waveform Output is set on Compare Match. The timing diagram for the Normal mode is shown in Figure 15-12. The counter value (TCNT4) that is shown as a histogram in the timing diagram is incremented until the counter value matches the TOP value. The counter is then cleared at the following clock cycle The diagram includes the Waveform Output (OCW4x) in toggle Compare Mode. The small horizontal line marks on the TCNT4 slopes represent Compare Matches between OCR4x and TCNT4. ATmega16U4/32U4 [DATASHEET] 151 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 15-12. Normal Mode, Timing Diagram TOVn Interrupt Flag Set OCnx Interrupt Flag Set TCNTn OCWnx (COMnx=1) Period 1 2 3 4 The Timer/Counter Overflow Flag (TOV4) is set in the same clock cycle as the TCNT4 becomes zero. The TOV4 Flag in this case behaves like a 11th bit, except that it is only set, not cleared. However, combined with the timer overflow interrupt, that automatically clears the TOV4 Flag, the timer resolution can be increased by software. There are no special cases to consider in the Normal mode, a new counter value can be written anytime. The Output Compare Unit can be used to generate interrupts at some given time. Using the Output Compare to generate waveforms in Normal mode is not recommended, since this will occupy too much of the CPU time. For generating a waveform, the OCW4x output can be set to toggle its logical level on each Compare Match by setting the Compare Output mode bits to toggle mode (COM4x1:0 = 1). The OC4x value will not be visible on the port pin unless the data direction for the pin is set to output. The waveform generated will have a maximum frequency of f = f /4 when OCR4C is set to zero. The waveform frequency is defined by the following OC4x clkT4 equation: f clkT4 f = ------------------------------------------- OC4x 21+OCR4C Resolution shows how many bit is required to express the value in the OCR4C register. It is calculated by following equation: Resolution = log (OCR4C + 1). PWM 2 The Output Compare Pin configurations in Normal Mode are described in the table below. Table 15-1. Output Compare Pin Configurations in Normal Mode COM4x1 COM4x0 OC4x Pin OC4x Pin 0 0 Disconnected Disconnected 0 1 Disconnected OC4x 1 0 Disconnected OC4x 1 1 Disconnected OC4x 15.8.2 Fast PWM Mode The fast Pulse Width Modulation or fast PWM mode (PWM4x = 1 and WGM40 = 0) provides a high frequency PWM waveform generation option. The fast PWM differs from the other PWM option by its single-slope ATmega16U4/32U4 [DATASHEET] 152 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

operation. The counter counts from BOTTOM to TOP (defined as OCR4C) then restarts from BOTTOM. In non- inverting Compare Output mode the Waveform Output (OCW4x) is cleared on the Compare Match between TCNT4 and OCR4x and set at BOTTOM. In inverting Compare Output mode, the Waveform Output is set on Compare Match and cleared at BOTTOM. In complementary Compare Output mode the Waveform Output is cleared on the Compare Match and set at BOTTOM. Due to the single-slope operation, the operating frequency of the fast PWM mode can be twice as high as the Phase and Frequency Correct PWM mode that use dual-slope operation. This high frequency makes the fast PWM mode well suited for power regulation, rectification, and DAC applications. High frequency allows physically small sized external components (coils, capacitors), and therefore reduces total system cost. The timing diagram for the fast PWM mode is shown in Figure 15-13. The counter is incremented until the counter value matches the TOP value. The counter is then cleared at the following timer clock cycle. The TCNT4 value is in the timing diagram shown as a histogram for illustrating the single-slope operation. The diagram includes the Waveform Output in non-inverted and inverted Compare Output modes. The small horizontal line marks on the TCNT4 slopes represent Compare Matches between OCR4x and TCNT4. Figure 15-13. Fast PWM Mode, Timing Diagram OCRnx Interrupt Flag Set OCRnx Update and TOVn Interrupt Flag Set TCNTn OCWnx (COMnx1:0 = 2) OCWnx (COMnx1:0 = 3) Period 1 2 3 4 5 6 7 The Timer/Counter Overflow Flag (TOV4) is set each time the counter reaches TOP. If the interrupt is enabled, the interrupt handler routine can be used for updating the compare value. In fast PWM mode, the compare unit allows generation of PWM waveforms on the OC4x pins. Setting the COM4x1:0 bits to two will produce a non- inverted PWM and setting the COM4x1:0 to three will produce an inverted PWM output. Setting the COM4x1:0 bits to one will enable complementary Compare Output mode and produce both the non-inverted (OC4x) and inverted output (OC4x). The actual value will only be visible on the port pin if the data direction for the port pin is set as output. The PWM waveform is generated by setting (or clearing) the Waveform Output (OCW4x) at the Compare Match between OCR4x and TCNT4, and clearing (or setting) the Waveform Output at the timer clock cycle the counter is cleared (changes from TOP to BOTTOM). The PWM frequency for the output can be calculated by the following equation: f clkT4 f = ------------- OCnxPWM N The N variable represents the number of steps in single-slope operation. The value of N equals either to the TOP value. The extreme values for the OCR4C Register represents special cases when generating a PWM waveform output in the fast PWM mode. If the OCR4C is set equal to BOTTOM, the output will be a narrow spike for each MAX+1 timer clock cycle. Setting the OCR4C equal to MAX will result in a constantly high or low output (depending on the polarity of the output set by the COM4x1:0 bits.) ATmega16U4/32U4 [DATASHEET] 153 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

A frequency (with 50% duty cycle) waveform output in fast PWM mode can be achieved by setting the Waveform Output (OCW4x) to toggle its logical level on each Compare Match (COM4x1:0 = 1). The waveform generated will have a maximum frequency of f = f /4 when OCR4C is set to three. OC4 clkT4 The general I/O port function is overridden by the Output Compare value (OC4x / OC4x) from the Dead Time Generator, if either of the COM4x1:0 bits are set and the Data Direction Register bits for the OC4X and OC4X pins are set as an output. If the COM4x1:0 bits are cleared, the actual value from the port register will be visible on the port pin. The Output Compare Pin configurations are described in the table below. Table 15-2. Output Compare Pin Configurations in Fast PWM Mode COM4x1 COM4x0 OC4x Pin OC4x Pin 0 0 Disconnected Disconnected 0 1 OC4x OC4x 1 0 Disconnected OC4x 1 1 Disconnected OC4x 15.8.3 Phase and Frequency Correct PWM Mode The Phase and Frequency Correct PWM Mode (PWM4x = 1 and WGM40 = 1) provides a high resolution Phase and Frequency Correct PWM waveform generation option. The Phase and Frequency Correct PWM mode is based on a dual-slope operation. The counter counts repeatedly from BOTTOM to TOP (defined as OCR4C) and then from TOP to BOTTOM. In non-inverting Compare Output Mode, and in complimentary Compare Output Mode, the Waveform Output (OCW4x) is cleared on the Compare Match between TCNT4 and OCR4x while upcounting, and set on the Compare Match while down-counting. In inverting Output Compare mode, the operation is inverted. The dual-slope operation has lower maximum operation frequency than single slope operation. However, due to the symmetric feature of the dual-slope PWM modes, these modes are preferred for motor control applications. The timing diagram for the Phase and Frequency Correct PWM mode is shown on Figure 15-14 in which the TCNT4 value is shown as a histogram for illustrating the dual-slope operation. The counter is incremented until the counter value matches TOP. When the counter reaches TOP, it changes the count direction. The TCNT4 value will be equal to TOP for one timer clock cycle. The diagram includes the Waveform Output (OCW4x) in non-inverted and inverted Compare Output Mode. The small horizontal line marks on the TCNT4 slopes represent Compare Matches between OCR4x and TCNT4. ATmega16U4/32U4 [DATASHEET] 154 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 15-14. Phase and Frequency Correct PWM Mode, Timing Diagram OCnx Interrupt Flag Set OCRnx Update TOVn Interrupt Flag Set TCNTn OCWnx (COMnx = 2) OCWnx (COMnx = 3) Period 1 2 3 The Timer/Counter Overflow Flag (TOV4) is set each time the counter reaches BOTTOM. The Interrupt Flag can be used to generate an interrupt each time the counter reaches the BOTTOM value. In the Phase and Frequency Correct PWM mode, the compare unit allows generation of PWM waveforms on the OC4x pins. Setting the COM4x1:0 bits to two will produce a non-inverted PWM and setting the COM4x1:0 to three will produce an inverted PWM output. Setting the COM4A1:0 bits to one will enable complementary Compare Output mode and produce both the non-inverted (OC4x) and inverted output (OC4x). The actual values will only be visible on the port pin if the data direction for the port pin is set as output. The PWM waveform is generated by clearing (or setting) the Waveform Output (OCW4x) at the Compare Match between OCR4x and TCNT4 when the counter increments, and setting (or clearing) the Waveform Output at Compare Match when the counter decrements. The PWM frequency for the output when using the Phase and Frequency Correct PWM can be calculated by the following equation: f clkT4 f = ------------- OCnxPCPWM N The N variable represents the number of steps in dual-slope operation. The value of N equals to the TOP value. The extreme values for the OCR4C Register represent special cases when generating a PWM waveform output in the Phase and Frequency Correct PWM mode. If the OCR4C is set equal to BOTTOM, the output will be continuously low and if set equal to MAX the output will be continuously high for non-inverted PWM mode. For inverted PWM the output will have the opposite logic values. The general I/O port function is overridden by the Output Compare value (OC4x / OC4x) from the Dead Time Generator, if either of the COM4x1:0 bits are set and the Data Direction Register bits for the OC4X and OC4X pins are set as an output. If the COM4x1:0 bits are cleared, the actual value from the port register will be visible on the port pin. The configurations of the Output Compare Pins are described in the table below. Table 15-3. Output Compare pin configurations in Phase and Frequency Correct PWM Mode COM4x1 COM4x0 OC4x Pin OC4x Pin 0 0 Disconnected Disconnected 0 1 OC4x OC4x 1 0 Disconnected OC4x 1 1 Disconnected OC4x ATmega16U4/32U4 [DATASHEET] 155 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

15.8.4 PWM6 Mode The PWM6 Mode (PWM4A = 1, WGM41 = 1, and WGM40 = x) provide PWM waveform generation option e.g. for controlling Brushless DC (BLDC) motors. In the PWM6 Mode the OCR4A Register controls all six Output Compare waveforms as the same Waveform Output (OCW4A) from the Waveform Generator is used for generating all waveforms. The PWM6 Mode also provides an Output Compare Override Enable Register (OC4OE) that can be used with an instant response for disabling or enabling the Output Compare pins. If the Output Compare Override Enable bit is cleared, the actual value from the port register will be visible on the port pin. The PWM6 Mode provides two counter operation modes, a single-slope operation and a dual-slope operation. If the single-slope operation is selected (the WGM40 bit is set to 0), the counter counts from BOTTOM to TOP (defined as OCR4C) then restart from BOTTOM like in Fast PWM Mode. The PWM waveform is generated by setting (or clearing) the Waveform Output (OCW4A) at the Compare Match between OCR4A and TCNT4, and clearing (or setting) the Waveform Output at the timer clock cycle the counter is cleared (changes from TOP to BOTTOM). The Timer/Counter Overflow Flag (TOV4) is set each time the counter reaches the TOP and, if the interrupt is enabled, the interrupt handler routine can be used for updating the compare value. Whereas, if the dual-slope operation is selected (the WGM40 bit is set to 1), the counter counts repeatedly from BOTTOM to TOP (defined as OCR4C) and then from TOP to BOTTOM like in Phase and Frequency Correct PWM Mode. The PWM waveform is generated by setting (or clearing) the Waveform Output (OCW4A) at the Compare Match between OCR4A and TCNT4 when the counter increments, and clearing (or setting) the Waveform Output at the he Compare Match between OCR4A and TCNT4 when the counter decrements. The Timer/Counter Overflow Flag (TOV4) is set each time the counter reaches the BOTTOM and, if the interrupt is enabled, the interrupt handler routine can be used for updating the compare value. The timing diagram for the PWM6 Mode in single-slope operation (WGM41 = 0) when the COM4A1:0 bits are set to “10” is shown in Figure 15-15 on page 157. The counter is incremented until the counter value matches the TOP value. The counter is then cleared at the following timer clock cycle. The TCNT4 value is in the timing diagram shown as a histogram for illustrating the single-slope operation. The timing diagram includes Output Compare pins OC4A and OC4A, and the corresponding Output Compare Override Enable bits (OC4OE1..OC4OE0). ATmega16U4/32U4 [DATASHEET] 156 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 15-15. PWM6 Mode, Single-slope Operation, Timing Diagram TCNT4 OCW4A OC4OE0 OC4A Pin OC4OE1 OC4A Pin OC4OE2 OC4B Pin OC4OE3 OC4B Pin OC4OE4 OC4D Pin OC4OE5 OC4D Pin The general I/O port function is overridden by the Output Compare value (OC4x / OC4x) from the Dead Time Generator if either of the COM4x1:0 bits are set. The Output Compare pins can also be overridden by the Output Compare Override Enable bits OC4OE5..OC4OE0. If an Override Enable bit is cleared, the actual value from the port register will be visible on the port pin and, if the Override Enable bit is set, the Output Compare pin is allowed to be connected on the port pin. The Output Compare Pin configurations are described in the table. Table 15-4. Output Compare Pin configurations in PWM6 Mode COM4A1 COM4A0 OC4A Pin (PC6) OC4A Pin (PC7) 0 0 Disconnected Disconnected 0 1 OC4A • OC4OE0 OC4A • OC4OE1 1 0 OC4A • OC4OE0 OC4A • OC4OE1 1 1 OC4A • OC4OE0 OC4A • OC4OE1 COM4B1 COM4B0 OC4B Pin (PB5) OC4B Pin (PB6) 0 0 Disconnected Disconnected 0 1 OC4A • OC4OE2 OC4A • OC4OE3 1 0 OC4A • OC4OE2 OC4A • OC4OE3 1 1 OC4A • OC4OE2 OC4A • OC4OE3 COM4D1 COM4D0 OC4D Pin (PD6) OC4D Pin (PD7) ATmega16U4/32U4 [DATASHEET] 157 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 15-4. Output Compare Pin configurations in PWM6 Mode COM4A1 COM4A0 OC4A Pin (PC6) OC4A Pin (PC7) 0 0 Disconnected Disconnected 0 1 OC4A • OC4OE4 OC4A • OC4OE5 1 0 OC4A • OC4OE4 OC4A • OC4OE5 1 1 OC4A • OC4OE4 OC4A • OC4OE5 15.9 Timer/Counter Timing Diagrams The Timer/Counter is a synchronous design and the timer clock (clk ) is therefore shown as a clock enable T4 signal in the following figures. The figures include information on when Interrupt Flags are set. Figure 15-16 contains timing data for basic Timer/Counter operation. The figure shows the count sequence close to the MAX value in all modes other than Phase and Frequency Correct PWM Mode. Figure 15-17 shows the same timing data, but with the prescaler enabled, in all modes other than Phase and Frequency Correct PWM Mode. Figure 15-18 on page 159 shows the setting of OCF4A, OCF4B, and OCF4D in all modes, and Figure 15-19 on page 159 shows the setting of TOV4 in Phase and Frequency Correct PWM Mode. Figure 15-16. Timer/Counter Timing Diagram, no Prescaling clk PCK clk Tn (clk /1) PCK TCNTn TOP - 1 TOP BOTTOM BOTTOM + 1 TOVn Figure 15-17. Timer/Counter Timing Diagram, with Prescaler (f /8) clkT4 clk PCK clk Tn (clk /8) PCK TCNTn TOP - 1 TOP BOTTOM BOTTOM + 1 TOVn ATmega16U4/32U4 [DATASHEET] 158 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 15-18. Timer/Counter Timing Diagram, Setting of OCF1x, with Prescaler (f /8) clkT4 clk PCK clk Tn (clk /8) PCK TCNTn OCRnx - 1 OCRnx OCRnx + 1 OCRnx + 2 OCRnx OCRnx Value OCFnx Figure 15-19. Timer/Counter Timing Diagram, with Prescaler (f /8) clkT4 clk PCK clk Tn (clk /8) PCK TCNTn BOTTOM + 1 BOTTOM + 1 BOTTOM BOTTOM + 1 TOVn 15.10 Fault Protection Unit The Timer/Counter4 incorporates a Fault Protection unit that can disable the PWM output pins, if an external event is triggered. The external signal indicating an event can be applied via the external interrupt INT0 pin or alternatively, via the analog-comparator unit. The Fault Protection unit is illustrated by the block diagram shown in Figure 15-20. The elements of the block diagram that are not directly a part of the Fault Protection unit are gray shaded. Figure 15-20. Fault Protection Unit Block Diagram FAULT_PROTECTION (Int. Req.) ACO* FPAC4 FPNC4 FPES4 FPEN4 Analog Comparator Noise Edge Timer/Counter4 Canceler Detector INT0 When the Fault Protection mode is enabled by the Fault Protection Enable (FPEN4) bit and a change of the logic level (an event) occurs on the external interrupt pin (INT0), alternatively on the Analog Comparator output (ACO), and this change confirms to the setting of the edge detector, a Fault Protection mode will be triggered. When a Fault Protection is triggered, the COM4x bits are cleared, Output Comparators are disconnected from the PWM output pins and the PORTB register bits are connected on the PWM output pins. The Fault Protection Enable (FPEN4) is automatically cleared at the same system clock as the COM4nx bits are cleared. If the Fault Protection Interrupt Enable bit (FPIE4) is set, a Fault Protection interrupt is generated and the FPEN4 bit is cleared. Alternatively the FPEN4 bit can be polled by software to figure out when the Timer/Counter has entered to Fault Protection mode. ATmega16U4/32U4 [DATASHEET] 159 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

15.10.1Fault Protection Trigger Source The main trigger source for the Fault Protection unit is the external interrupt pin (INT0). Alternatively the Analog Comparator output can be used as trigger source for the Fault Protection unit. The Analog Comparator is selected as trigger source by setting the Fault Protection Analog Comparator (FPAC4) bit in the Timer/Counter4 Control Register (TCCR4D). Be aware that changing trigger source can trigger a Fault Protection mode. Therefore it is recommended to clear the FPF4 flag after changing trigger source, setting edge detector or enabling the Fault Protection. Both the external interrupt pin (INT0) and the Analog Comparator output (ACO) inputs are sampled using the same technique as for the T0 pin (Figure 12-1 on page 92). The edge detector is also identical. However, when the noise canceler is enabled, additional logic is inserted before the edge detector, which increases the delay by four system clock cycles. An Input Capture can also be triggered by software by controlling the port of the INT0 pin. 15.10.2Noise Canceler The noise canceler improves noise immunity by using a simple digital filtering scheme. The noise canceler input is monitored over four samples, and all four must be equal for changing the output that in turn is used by the edge detector. The noise canceler is enabled by setting the Fault Protection Noise Canceler (FPNC4) bit in Timer/Counter4 Control Register D (TCCR4D). When enabled the noise canceler introduces additional four system clock cycles of delay from a change applied to the input. The noise canceler uses the system clock and is therefore not affected by the prescaler. 15.11 Accessing 10-bit Registers If 10-bit values are written to the TCNTn and OCRnA/B/C/D registers, the 10-bit registers can be byte accessed by the AVR CPU via the 8-bit data bus using two read or write operations. The 10-bit registers have a common 2-bit Timer/Counter4 High Byte Register (TC4H) that is used for temporary storing of the two MSBs of the 10-bit access. The same TC4H register is shared between all 10-bit registers. Accessing the low byte triggers the 10- bit read or write operation. When the low byte of a 10-bit register is written by the CPU, the high byte stored in the TC4H register, and the low byte written are both copied into the 10-bit register in the same clock cycle. When the low byte of a 10-bit register is read by the CPU, the high byte of the 10-bit register is copied into the TC4H register in the same clock cycle as the low byte is read. To do a 10-bit write, the high byte must be written to the TC4H register before the low byte is written. For a 10- bit read, the low byte must be read before the high byte. ATmega16U4/32U4 [DATASHEET] 160 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The following code examples show how to access the 10-bit timer registers assuming that no interrupts updates the TC4H register. The same principle can be used directly for accessing the OCRnA/B/C/C/D registers. Assembly Code Example ... ; Set TCNTn to 0x01FF ldi r17,0x01 ldi r16,0xFF out TCnH,r17 out TCNTn,r16 ; Read TCNTn into r17:r16 in r16,TCNTn in r17,TCnH ... C Code Example unsigned int i; ... /* Set TCNTn to 0x01FF */ TCnH = 0x01; TCNTn = 0xFF; /* Read TCNTn into i */ i = TCNTn; i |= ((unsigned int)TCnH << 8); ... Note: 1. The example code assumes that the part specific header file is included. For I/O registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must be replaced with instructions that allow access to extended I/O. Typically “LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”. The assembly code example returns the TCNTn value in the r17:r16 register pair. It is important to notice that accessing 10-bit registers are atomic operations. If an interrupt occurs between the two instructions accessing the 10-bit register, and the interrupt code updates the TC4H register by accessing the same or any other of the 10-bit timer registers, then the result of the access outside the interrupt will be corrupted. Therefore, when both the main code and the interrupt code update the TC4H register, the main code must disable the interrupts during the 16-bit access. ATmega16U4/32U4 [DATASHEET] 161 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The following code examples show how to do an atomic read of the TCNTn register contents. Reading any of the OCRnA/B/C/D registers can be done by using the same principle. Assembly Code Example TIM1_ReadTCNTn: ; Save global interrupt flag in r18,SREG ; Disable interrupts cli ; Read TCNTn into r17:r16 in r16,TCNTn in r17,TCnH ; Restore global interrupt flag out SREG,r18 ret C Code Example unsigned int TIM1_ReadTCNTn( void ) { unsigned char sreg; unsigned int i; /* Save global interrupt flag */ sreg = SREG; /* Disable interrupts */ _CLI(); /* Read TCNTn into i */ i = TCNTn; i |= ((unsigned int)TCnH << 8); /* Restore global interrupt flag SREG = sreg; return i; } Note: 1. The example code assumes that the part specific header file is included. For I/O registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must be replaced with instructions that allow access to extended I/O. Typically “LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”. The assembly code example returns the TCNTn value in the r17:r16 register pair. ATmega16U4/32U4 [DATASHEET] 162 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The following code examples show how to do an atomic write of the TCNTn register contents. Writing any of the OCRnA/B/C/D registers can be done by using the same principle. Assembly Code Example TIM1_WriteTCNTn: ; Save global interrupt flag in r18,SREG ; Disable interrupts cli ; Set TCNTn to r17:r16 out TCnH,r17 out TCNTn,r16 ; Restore global interrupt flag out SREG,r18 ret C Code Example void TIM1_WriteTCNTn( unsigned int i ) { unsigned char sreg; unsigned int i; /* Save global interrupt flag */ sreg = SREG; /* Disable interrupts */ _CLI(); /* Set TCNTn to i */ TCnH = (i >> 8); TCNTn = (unsigned char)i; /* Restore global interrupt flag */ SREG = sreg; } Note: 1. The example code assumes that the part specific header file is included. For I/O registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must be replaced with instructions that allow access to extended I/O. Typically “LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”. The assembly code example requires that the r17:r16 register pair contains the value to be written to TCNTn. 15.11.1 Reusing the Temporary High Byte Register If writing to more than one 10-bit register where the high byte is the same for all registers written, then the high byte only needs to be written once. However, note that the same rule of atomic operation described previously also applies in this case. ATmega16U4/32U4 [DATASHEET] 163 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

15.12 Register Description 15.12.1TCCR4A – Timer/Counter4 Control Register A Bit 7 6 5 4 3 2 1 0 COM4A1 COM4A0 COM4B1 COM4B0 FOC4A FOC4B PWM4A PWM4B TCCR4A Read/Write R/W R/W R/W R/W W W R/W R/W Initial value 0 0 0 0 0 0 0 0 • Bits 7, 6 - COM4A1, COM4A0: Comparator A Output Mode, Bits 1 and 0 These bits control the behavior of the Waveform Output (OCW4A) and the connection of the Output Compare pin (OC4A). If one or both of the COM4A1:0 bits are set, the OC4A output overrides the normal port functionality of the I/O pin it is connected to. The complementary OC4B output is connected only in PWM modes when the COM4A1:0 bits are set to “01”. Note that the Data Direction Register (DDR) bit corresponding to the OC4A and OC4A pins must be set in order to enable the output driver. The function of the COM4A1:0 bits depends on the PWM4A, WGM40 and WGM41 bit settings. The table shows the COM4A1:0 bit functionality when the PWM4A bit is set to Normal Mode (non-PWM). Table 15-5. Compare Output Mode, Normal Mode (non-PWM) COM4A1..0 OCW4A Behavior OC4A Pin OC4A Pin 00 Normal port operation Disconnected Disconnected 01 Toggle on Compare Match Connected Disconnected 10 Clear on Compare Match Connected Disconnected 11 Set on Compare Match Connected Disconnected The table shows the COM4A1:0 bit functionality when the PWM4A, WGM40 and WGM41 bits are set to fast PWM mode. Table 15-6. Compare Output Mode, Fast PWM Mode COM4A1..0 OCW4A Behavior OC4A OC4A 00 Normal port operation Disconnected Disconnected Cleared on Compare Match. 01 Connected Connected Set when TCNT4 = 0x000. Cleared on Compare Match. 10 Connected Disconnected Set when TCNT4 = 0x000. Set on Compare Match. 11 Connected Disconnected Cleared when TCNT4 = 0x000. The table shows the COM4A1:0 bit functionality when the PWM4A, WGM40, and WGM41 bits are set to Phase and Frequency Correct PWM Mode. ATmega16U4/32U4 [DATASHEET] 164 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 15-7. Compare Output Mode, Phase and Frequency Correct PWM Mode COM1A1..0 OCW1A Behavior OC4A Pin OC4A Pin 00 Normal port operation. Disconnected Disconnected Cleared on Compare Match when up-counting. 01 Connected Connected Set on Compare Match when down-counting. Cleared on Compare Match when up-counting. 10 Connected Disconnected Set on Compare Match when down-counting. Set on Compare Match when up-counting. 11 Connected Disconnected Cleared on Compare Match when down-counting. The table shows the COM4A1:0 bit functionality when the PWM4A, WGM40, and WGM41 bits are set to single- slope PWM6 Mode. In the PWM6 Mode the same Waveform Output (OCW4A) is used for generating all waveforms and the Output Compare values OC4A and OC4A are connected on OC4x and OC4x pins as described below. Table 15-8. Compare Output Mode, Single-Slope PWM6 Mode COM4A1..0 OCW4A Behavior OC4x Pin OC4x Pin 00 Normal port operation Disconnected Disconnected Cleared on Compare Match. 01 OC4A OC4A Set when TCNT4 = 0x000. Cleared on Compare Match. 10 OC4A OC4A Set when TCNT4 = 0x000. Set on Compare Match. 11 OC4A OC4A Cleared when TCNT4 = 0x000. The table shows the COM4A1:0 bit functionality when the PWM4A, WGM40, and WGM41 bits are set to dual- slope PWM6 Mode. Table 15-9. Compare Output Mode, Dual-Slope PWM6 Mode COM4A1..0 OCW4A Behavior OC4x Pin OC4x Pin 00 Normal port operation Disconnected Disconnected Cleared on Compare Match when up-counting. 01 OC4A OC4A Set on Compare Match when down-counting. Cleared on Compare Match when up-counting. 10 OC4A OC4A Set on Compare Match when down-counting. Set on Compare Match when up-counting. 11 OC4A OC4A Cleared on Compare Match when down-counting. • Bits 5,4 - COM4B1, COM4B0: Comparator B Output Mode, Bits 1 and 0 These bits control the behavior of the Waveform Output (OCW4B) and the connection of the Output Compare pin (OC4B). If one or both of the COM4B1:0 bits are set, the OC4B output overrides the normal port functionality ATmega16U4/32U4 [DATASHEET] 165 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

of the I/O pin it is connected to. The complementary OC4B output is connected only in PWM modes when the COM4B1:0 bits are set to “01”. Note that the Data Direction Register (DDR) bit corresponding to the OC4B pin must be set in order to enable the output driver. The function of the COM4B1:0 bits depends on the PWM4B and WGM40 bit settings. The table shows the COM4B1:0 bit functionality when the PWM4B bit is set to Normal Mode (non-PWM). Table 15-10. Compare Output Mode, Normal Mode (non-PWM) COM4B1..0 OCW4B Behavior OC4B Pin OC4B Pin 00 Normal port operation Disconnected Disconnected 01 Toggle on Compare Match Connected Disconnected 10 Clear on Compare Match Connected Disconnected 11 Set on Compare Match Connected Disconnected The table shows the COM4B1:0 bit functionality when the PWM4B and WGM40 bits are set to Fast PWM Mode. Table 15-11. Compare Output Mode, Fast PWM Mode COM4B1..0 OCW4B Behavior OC4B Pin OC4B Pin 00 Normal port operation Disconnected Disconnected Cleared on Compare Match. 01 Connected Connected Set when TCNT4 = 0x000. Cleared on Compare Match. 10 Connected Disconnected Set when TCNT4 = 0x000. Set on Compare Match. 11 Connected Disconnected Cleared when TCNT4 = 0x000. The table shows the COM4B1:0 bit functionality when the PWM4B and WGM40 bits are set to Phase and Frequency Correct PWM Mode. Table 15-12. Compare Output Mode, Phase and Frequency Correct PWM Mode COM4B1..0 OCW4B Behavior OC4B Pin OC4B Pin 00 Normal port operation Disconnected Disconnected Cleared on Compare Match when up-counting. 01 Connected Connected Set on Compare Match when down-counting. Cleared on Compare Match when up-counting. 10 Connected Disconnected Set on Compare Match when down-counting. Set on Compare Match when up-counting. 11 Connected Disconnected Cleared on Compare Match when down-counting. • Bit 3 - FOC4A: Force Output Compare Match 4A The FOC4A bit is only active when the PWM4A bit specify a non-PWM mode. ATmega16U4/32U4 [DATASHEET] 166 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Writing a logical one to this bit forces a change in the Waveform Output (OCW4A) and the Output Compare pin (OC4A) according to the values already set in COM4A1 and COM4A0. If COM4A1 and COM4A0 written in the same cycle as FOC4A, the new settings will be used. The Force Output Compare bit can be used to change the output pin value regardless of the timer value. The automatic action programmed in COM4A1 and COM4A0 takes place as if a compare match had occurred, but no interrupt is generated. The FOC4A bit is always read as zero. • Bit 2 - FOC4B: Force Output Compare Match 4B The FOC4B bit is only active when the PWM4B bit specify a non-PWM mode. Writing a logical one to this bit forces a change in the Waveform Output (OCW4B) and the Output Compare pin (OC4B) according to the values already set in COM4B1 and COM4B0. If COM4B1 and COM4B0 written in the same cycle as FOC4B, the new settings will be used. The Force Output Compare bit can be used to change the output pin value regardless of the timer value. The automatic action programmed in COM4B1 and COM4B0 takes place as if a compare match had occurred, but no interrupt is generated. The FOC4B bit is always read as zero. • Bit 1 - PWM4A: Pulse Width Modulator A Enable When set (one) this bit enables PWM mode based on comparator OCR4A. • Bit 0 - PWM4B: Pulse Width Modulator B Enable When set (one) this bit enables PWM mode based on comparator OCR4B. 15.12.2TCCR4B – Timer/Counter4 Control Register B Bit 7 6 5 4 3 2 1 0 PWM4X PSR4 DTPS41 DTPS40 CS43 CS42 CS41 CS40 TCCR4B Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial value 0 0 0 0 0 0 0 0 • Bit 7 - PWM4X: PWM Inversion Mode When this bit is set (one), the PWM Inversion Mode is selected and the Dead Time Generator outputs, OC4x and OC4x are inverted. • Bit 6 - PSR4: Prescaler Reset Timer/Counter4 When this bit is set (one), the Timer/Counter4 prescaler (TCNT4 is unaffected) will be reset. The bit will be cleared by hardware after the operation is performed. Writing a zero to this bit will have no effect. This bit will always read as zero. • Bits 5,4 - DTPS41, DTPS40: Dead Time Prescaler Bits The Timer/Counter4 Control Register B is a 8-bit read/write register. The dedicated Dead Time prescaler in front of the Dead Time Generator can divide the Timer/Counter4 clock (PCK or CK) by 1, 2, 4, or 8 providing a large range of dead times that can be generated. The Dead Time prescaler is controlled by two bits DTPS41 and DTPS40 from the Dead Time Prescaler register. These bits define the division factor of the Dead Time prescaler. The division factors are given in the table below. ATmega16U4/32U4 [DATASHEET] 167 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 15-13. Division factors of the Dead Time prescaler DTPS41 DTPS40 Prescaler divides the T/C4 clock by 0 0 1x (no division) 0 1 2x 1 0 4x 1 1 8x • Bits 3..0 - CS43, CS42, CS41, CS40: Clock Select Bits 3, 2, 1, and 0 The Clock Select bits 3, 2, 1, and 0 define the prescaling source of Timer/Counter4. Table 15-14. Timer/Counter4 Prescaler Select CS43 CS42 CS41 CS40 Asynchronous Clocking Mode Synchronous Clocking Mode 0 0 0 0 T/C4 stopped T/C4 stopped 0 0 0 1 PCK CK 0 0 1 0 PCK/2 CK/2 0 0 1 1 PCK/4 CK/4 0 1 0 0 PCK/8 CK/8 0 1 0 1 PCK/16 CK/16 0 1 1 0 PCK/32 CK/32 0 1 1 1 PCK/64 CK/64 1 0 0 0 PCK/128 CK/128 1 0 0 1 PCK/256 CK/256 1 0 1 0 PCK/512 CK/512 1 0 1 1 PCK/1024 CK/1024 1 1 0 0 PCK/2048 CK/2048 1 1 0 1 PCK/4096 CK/4096 1 1 1 0 PCK/8192 CK/8192 1 1 1 1 PCK/16384 CK/16384 The Stop condition provides a Timer Enable/Disable function. ATmega16U4/32U4 [DATASHEET] 168 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

15.12.3TCCR4C – Timer/Counter4 Control Register C Bit 7 6 5 4 3 2 1 0 COM4A1S COM4A0S COM4B1S COMAB0S COM4D1 COM4D0 FOC4D PWM4D TCCR4C Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial value 0 0 0 0 0 0 0 0 • Bits 7,6 - COM4A1S, COM4A0S: Comparator A Output Mode, Bits 1 and 0 These bits are the shadow bits of the COM4A1 and COM4A0 bits that are described in the section “TCCR4A – Timer/Counter4 Control Register A” on page164. • Bits 5,4 - COM4B1S, COM4B0S: Comparator B Output Mode, Bits 1 and 0 These bits are the shadow bits of the COM4A1 and COM4A0 bits that are described in the section “TCCR4A – Timer/Counter4 Control Register A” on page164. • Bits 3,2 - COM4D1, COM4D0: Comparator D Output Mode, Bits 1 and 0 These bits control the behavior of the Waveform Output (OCW4D) and the connection of the Output Compare pin (OC4D). If one or both of the COM4D1:0 bits are set, the OC4D output overrides the normal port functionality of the I/O pin it is connected to. The complementary OC4D output is connected only in PWM modes when the COM4D1:0 bits are set to “01”. Note that the Data Direction Register (DDR) bit corresponding to the OC4D pin must be set in order to enable the output driver. The function of the COM4D1:0 bits depends on the PWM4D and WGM40 bit settings. The table shows the COM4D1:0 bit functionality when the PWM4D bit is set to a Normal Mode (non-PWM). Table 15-15. Compare Output Mode, Normal Mode (non-PWM) COM4D1..0 OCW4D Behavior OC4D Pin OC4D Pin 00 Normal port operation Disconnected Disconnected 01 Toggle on Compare Match Connected Disconnected 10 Clear on Compare Match Connected Disconnected 11 Set on Compare Match Connected Disconnected The table shows the COM4D1:0 bit functionality when the PWM4D and WGM40 bits are set to Fast PWM Mode. Table 15-16. Compare Output Mode, Fast PWM Mode COM4D1..0 OCW4D Behavior OC4D Pin OC4D Pin 00 Normal port operation Disconnected Disconnected Cleared on Compare Match 01 Connected Connected Set when TCNT4 = 0x000 Cleared on Compare Match 10 Connected Disconnected Set when TCNT4 = 0x000 Set on Compare Match 11 Connected Disconnected Clear when TCNT4 = 0x000 ATmega16U4/32U4 [DATASHEET] 169 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The table shows the COM4D1:0 bit functionality when the PWM4D and WGM40 bits are set to Phase and Frequency Correct PWM Mode Table 15-17. Compare Output Mode, Phase and Frequency Correct PWM Mode COM4D1..0 OCW4D Behavior OC4D Pin OC4D Pin 00 Normal port operation Disconnected Disconnected Cleared on Compare Match when up-counting 01 Connected Connected Set on Compare Match when down-counting Cleared on Compare Match when up-counting 10 Connected Disconnected Set on Compare Match when down-counting Set on Compare Match when up-counting 11 Connected Disconnected Cleared on Compare Match when down-counting • Bit 1 - FOC4D: Force Output Compare Match 4D The FOC4D bit is only active when the PWM4D bit specify a non-PWM mode. Writing a logical one to this bit forces a change in the Waveform Output (OCW4D) and the Output Compare pin (OC4D) according to the values already set in COM4D1 and COM4D0. If COM4D1 and COM4D0 written in the same cycle as FOC4D, the new settings will be used. The Force Output Compare bit can be used to change the output pin value regardless of the timer value. The automatic action programmed in COM4D1 and COM4D0 takes place as if a compare match had occurred, but no interrupt is generated. The FOC4D bit is always read as zero. • Bit 0 - PWM4D: Pulse Width Modulator D Enable When set (one) this bit enables PWM mode based on comparator OCR4D. 15.12.4TCCR4D – Timer/Counter4 Control Register D Bit 7 6 5 4 3 2 1 0 FPIE4 FPEN4 FPNC4 FPES4 FPAC4 FPF4 WGM41 WGM40 TCCR4D Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial value 0 0 0 0 0 0 0 0 • Bit 7 - FPIE4: Fault Protection Interrupt Enable Setting this bit (to one) enables the Fault Protection Interrupt. • Bit 6– FPEN4: Fault Protection Mode Enable Setting this bit (to one) activates the Fault Protection Mode. • Bit 5 – FPNC4: Fault Protection Noise Canceler Setting this bit activates the Fault Protection Noise Canceler. When the noise canceler is activated, the input from the Fault Protection Pin (INT0) is filtered. The filter function requires four successive equal valued samples of the INT0 pin for changing its output. The Fault Protection is therefore delayed by four Oscillator cycles when the noise canceler is enabled. • Bit 4 – FPES4: Fault Protection Edge Select This bit selects which edge on the Fault Protection pin (INT0) is used to trigger a fault event. When the FPES4 bit is written to zero, a falling (negative) edge is used as trigger, and when the FPES4 bit is written to one, a rising (positive) edge will trigger the fault. ATmega16U4/32U4 [DATASHEET] 170 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 3 - FPAC4: Fault Protection Analog Comparator Enable When written logic one, this bit enables the Fault Protection function in Timer/Counter4 to be triggered by the Analog Comparator. The comparator output is in this case directly connected to the Fault Protection front-end logic, making the comparator utilize the noise canceler and edge select features of the Timer/Counter4 Fault Protection interrupt. When written logic zero, no connection between the Analog Comparator and the Fault Protection function exists. To make the comparator trigger the Timer/Counter4 Fault Protection interrupt, the FPIE4 bit in the Timer/Counter4 Control Register D (TCCR4D) must be set. • Bit 2- FPF4: Fault Protection Interrupt Flag When the FPIE4 bit is set (one), the Fault Protection Interrupt is enabled. Activity on the pin will cause an interrupt request even, if the Fault Protection pin is configured as an output. The corresponding interrupt of Fault Protection Interrupt Request is executed from the Fault Protection Interrupt Vector. The bit FPF4 is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, FPF4 is cleared after a synchronization clock cycle by writing a logical one to the flag. When the SREG I-bit, FPIE4 and FPF4 are set, the Fault Interrupt is executed. • Bits 1:0 - WGM41, WGM40: Waveform Generation Mode Bits This bit associated with the PWM4x bits control the counting sequence of the counter, the source for type of waveform generation to be used, see the table below. The Modes of operation supported by the Timer/Counter4 are: Normal mode (counter), Fast PWM Mode, Phase and Frequency Correct PWM and PWM6 Modes. Table 15-18. Waveform Generation Mode Bit Description Update of TOV4 Flag PWM4x WGM41..40 Timer/Counter Mode of Operation TOP OCR4x at Set on 0 xx Normal OCR4C Immediate TOP 1 00 Fast PWM OCR4C TOP TOP 1 01 Phase and Frequency Correct PWM OCR4C BOTTOM BOTTOM 1 10 PWM6 / Single-slope OCR4C TOP TOP 1 11 PWM6 / Dual-slope OCR4C BOTTOM BOTTOM 15.12.5TCCR4E – Timer/Counter4 Control Register E Bit 7 6 5 4 3 2 1 0 TLOCK4 ENHC4 OC4OE5 OC4OE4 OC4OE3 OC4OE2 OC4OE1 OC4OE0 TCCR4E Read/Write R R R/W R/W R/W R/W R/W R/W Initial value 0 0 0 0 0 0 0 0 • Bit 7 - TLOCK4: Register Update Lock This bit controls the Compare registers update. When this bit is set, writing to the Compare registers will not affect the output, however the values are stored and will be updated to the Compare registers when the TLOCK4 bit will be cleared. Refer to “Synchronous update” on page151 for more details. ATmega16U4/32U4 [DATASHEET] 171 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 6- ENHC4: Enhanced Compare/PWM Mode When this bit is set, the Waveform Generation Module works in enhanced mode: the compare registers OCR4A/B/D can welcome one more accuracy bit, while the LSB determines on which clock edge the Compare condition is signalled and the output pin level is updated. • Bits 5:0 – OC4OE5:OC4OE0: Output Compare Override Enable Bits These bits are the Output Compare Override Enable bits that are used to connect or disconnect the Output Compare Pins in PWM6 Modes with an instant response on the corresponding Output Compare Pins. The actual value from the port register will be visible on the port pin, when the Output Compare Override Enable Bit is cleared. The table shows the Output Compare Override Enable Bits and their corresponding Output Compare pins. OC4OE0 OC4OE1 OC4OE2 OC4OE3 OC4OE4 OC4OE5 OC4A (PC6) OC4A (PC7) OC4B (PB5) OC4B (PB6) OC4D (PD6) OC4D (PD7) 15.12.6TCNT4 – Timer/Counter4 Bit 7 6 5 4 3 2 1 0 4 MSB LSB TCNT4 Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial value 0 0 0 0 0 0 0 0 This 8-bit register contains the value of Timer/Counter4. The Timer/Counter4 is realized as a 10-bit up/down counter with read and write access. Due to synchronization of the CPU, Timer/Counter4 data written into Timer/Counter4 is delayed by one and half CPU clock cycles in synchronous mode and at most one CPU clock cycles for asynchronous mode. When a 10-bit accuracy is preferred, special procedures must be followed for accessing the 10-bit TCNT4 register via the 8-bit AVR data bus. These procedures are described in section “Accessing 10-bit Registers” on page160. Alternatively the Timer/Counter4 can be used as an 8-bit Timer/Counter. Note that the Timer/Counter4 always starts counting up after writing the TCNT4 register. 15.12.7TC4H – Timer/Counter4 High Byte Bit 7 6 5 4 3 2 1 0 - - - - - TC410 TC49 TC48 TC4H Read/Write R R R R R R R/W R/W Initial value 0 0 0 0 0 0 0 0 The temporary Timer/Counter4 register is an 2-bit read/write register. • Bits 7:3- Res: Reserved Bits These bits are reserved bits and always reads as zero. • Bits 2- TC410: Additional MSB bits for 11-bit accesses in Enhanced PWM mode If 10-bit accuracy is used, the Timer/Counter4 High Byte Register (TC4H) is used for temporary storing the MSB bits (TC49, TC48) of the 10-bit accesses. The same TC4H register is shared between all 10-bit registers within the Timer/Counter4. Note that special procedures must be followed when accessing the 10-bit TCNT4 register via the 8-bit AVR data bus. These procedures are described in section “Accessing 10-bit Registers” on page160. ATmega16U4/32U4 [DATASHEET] 172 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bits 1:0 - TC49, TC48: Two MSB bits of the 10-bit accesses If 10-bit accuracy is used, the Timer/Counter4 High Byte Register (TC4H) is used for temporary storing the MSB bits (TC49, TC48) of the 10-bit accesses. The same TC4H register is shared between all 10-bit registers within the Timer/Counter4. Note that special procedures must be followed when accessing the 10-bit TCNT4 register via the 8-bit AVR data bus. These procedures are described in section “Accessing 10-bit Registers” on page160. 15.12.8OCR4A – Timer/Counter4 Output Compare Register A Bit 7 6 5 4 3 2 1 0 MSB LSB OCR4A Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial value 0 0 0 0 0 0 0 0 The output compare register A is an 8-bit read/write register. The Timer/Counter Output Compare Register A contains data to be continuously compared with Timer/Counter4. Actions on compare matches are specified in TCCR4A. A compare match does only occur if Timer/Counter4 counts to the OCR4A value. A software write that sets TCNT4 and OCR4A to the same value does not generate a compare match. A compare match will set the compare interrupt flag OCF4A after a synchronization delay following the compare event. Note that, if 10-bit accuracy is used special procedures must be followed when accessing the internal 10-bit Output Compare Registers via the 8-bit AVR data bus. These procedures are described in section “Accessing 10-bit Registers” on page160. 15.12.9OCR4B – Timer/Counter4 Output Compare Register B Bit 7 6 5 4 3 2 1 0 MSB LSB OCR4B Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial value 0 0 0 0 0 0 0 0 The output compare register B is an 8-bit read/write register. The Timer/Counter Output Compare Register B contains data to be continuously compared with Timer/Counter4. Actions on compare matches are specified in TCCR4. A compare match does only occur if Timer/Counter4 counts to the OCR4B value. A software write that sets TCNT4 and OCR4B to the same value does not generate a compare match. A compare match will set the compare interrupt flag OCF4B after a synchronization delay following the compare event. Note that, if 10-bit accuracy is used special procedures must be followed when accessing the internal 10-bit Output Compare Registers via the 8-bit AVR data bus. These procedures are described in section “Accessing 10-bit Registers” on page160. 15.12.10OCR4C – Timer/Counter4 Output Compare Register C Bit 7 6 5 4 3 2 1 0 MSB LSB OCR44C Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial value 1 1 1 1 1 1 1 1 The output compare register C is an 8-bit read/write register. ATmega16U4/32U4 [DATASHEET] 173 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The Timer/Counter Output Compare Register C contains data to be continuously compared with Timer/Counter4, and a compare match will clear TCNT4. This register has the same function in Normal mode and PWM modes. Note that, if a smaller value than three is written to the Output Compare Register C, the value is automatically replaced by three as it is a minimum value allowed to be written to this register. Note that, if 10-bit accuracy is used special procedures must be followed when accessing the internal 10-bit Output Compare Registers via the 8-bit AVR data bus. These procedures are described in section “Accessing 10-bit Registers” on page160. 15.12.11OCR4D – Timer/Counter4 Output Compare Register D The output compare register D is an 8-bit read/write register. Bit 7 6 5 4 3 2 1 0 MSB LSB OCR4D Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial value 0 0 0 0 0 0 0 0 The Timer/Counter Output Compare Register D contains data to be continuously compared with Timer/Counter4. Actions on compare matches are specified in TCCR4A. A compare match does only occur if Timer/Counter4 counts to the OCR4D value. A software write that sets TCNT4 and OCR4D to the same value does not generate a compare match. A compare match will set the compare interrupt flag OCF4D after a synchronization delay following the compare event. Note that, if 10-bit accuracy is used special procedures must be followed when accessing the internal 10-bit Output Compare Registers via the 8-bit AVR data bus. These procedures are described in section “Accessing 10-bit Registers” on page160. 15.12.12TIMSK4 – Timer/Counter4 Interrupt Mask Register Bit 7 6 5 4 3 2 1 0 OCIE4D OCIE4A OCIE4B TOIE4 TIMSK4 Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial value 0 0 0 0 0 0 0 0 • Bit 7- OCIE4D: Timer/Counter4 Output Compare Interrupt Enable When the OCIE4D bit is set (one) and the I-bit in the Status Register is set (one), the Timer/Counter4 Compare Match D interrupt is enabled. The corresponding interrupt at vector $010 is executed if a compare match D occurs. The Compare Flag in Timer/Counter4 is set (one) in the Timer/Counter Interrupt Flag Register. • Bit 6 - OCIE4A: Timer/Counter4 Output Compare Interrupt Enable When the OCIE4A bit is set (one) and the I-bit in the Status Register is set (one), the Timer/Counter4 Compare Match A interrupt is enabled. The corresponding interrupt at vector $003 is executed if a compare match A occurs. The Compare Flag in Timer/Counter4 is set (one) in the Timer/Counter Interrupt Flag Register. • Bit 5 - OCIE4B: Timer/Counter4 Output Compare Interrupt Enable When the OCIE4B bit is set (one) and the I-bit in the Status Register is set (one), the Timer/Counter4 Compare Match B interrupt is enabled. The corresponding interrupt at vector $009 is executed if a compare match B occurs. The Compare Flag in Timer/Counter4 is set (one) in the Timer/Counter Interrupt Flag Register. ATmega16U4/32U4 [DATASHEET] 174 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 2 - TOIE4: Timer/Counter4 Overflow Interrupt Enable When the TOIE4 bit is set (one) and the I-bit in the Status Register is set (one), the Timer/Counter4 Overflow interrupt is enabled. The corresponding interrupt (at vector $004) is executed if an overflow in Timer/Counter4 occurs. The Overflow Flag (Timer4) is set (one) in the Timer/Counter Interrupt Flag Register - TIFR4. 15.12.13TIFR4 – Timer/Counter4 Interrupt Flag Register Bit 7 6 5 4 3 2 1 0 OCF4D OCF4A OCF4B TOV4 TIFR4 Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial value 0 0 0 0 0 0 0 0 • Bit 7- OCF4D: Output Compare Flag 4D The OCF4D bit is set (one) when compare match occurs between Timer/Counter4 and the data value in OCR4D - Output Compare Register 4D. OCF4D is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, OCF4D is cleared, after synchronization clock cycle, by writing a logic one to the flag. When the I-bit in SREG, OCIE4D, and OCF4D are set (one), the Timer/Counter4 D compare match interrupt is executed. • Bit 6 - OCF4A: Output Compare Flag 4A The OCF4A bit is set (one) when compare match occurs between Timer/Counter4 and the data value in OCR4A - Output Compare Register 4A. OCF4A is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, OCF4A is cleared, after synchronization clock cycle, by writing a logic one to the flag. When the I-bit in SREG, OCIE4A, and OCF4A are set (one), the Timer/Counter4 A compare match interrupt is executed. • Bit 5 - OCF4B: Output Compare Flag 4B The OCF4B bit is set (one) when compare match occurs between Timer/Counter4 and the data value in OCR4B - Output Compare Register 4B. OCF4B is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, OCF4B is cleared, after synchronization clock cycle, by writing a logic one to the flag. When the I-bit in SREG, OCIE4B, and OCF4B are set (one), the Timer/Counter4 B compare match interrupt is executed. • Bit 2 - TOV4: Timer/Counter4 Overflow Flag In Normal Mode and Fast PWM Mode the TOV4 bit is set (one) each time the counter reaches TOP at the same clock cycle when the counter is reset to BOTTOM. In Phase and Frequency Correct PWM Mode the TOV4 bit is set (one) each time the counter reaches BOTTOM at the same clock cycle when zero is clocked to the counter. The bit TOV4 is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, TOV4 is cleared, after synchronization clock cycle, by writing a logical one to the flag. When the SREG I-bit, and TOIE4 (Timer/Counter4 Overflow Interrupt Enable), and TOV4 are set (one), the Timer/Counter4 Overflow interrupt is executed. 15.12.14DT4 – Timer/Counter4 Dead Time Value Bit 7 6 5 4 3 2 1 0 DT4H3 DT4H2 DT4H1 DT4H0 DT4L3 DT4L2 DT4L1 DT4L0 DT4 Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial value 0 0 0 0 0 0 0 0 The dead time value register is an 8-bit read/write register. ATmega16U4/32U4 [DATASHEET] 175 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The dead time delay of all Timer/Counter4 channels are adjusted by the dead time value register, DT4. The register consists of two fields, DT4H3..0 and DT4L3..0, one for each complementary output. Therefore a different dead time delay can be adjusted for the rising edge of OC4x and the rising edge of OC4x. • Bits 7:4- DT4H3:DT4H0: Dead Time Value for OC4x Output The dead time value for the OC1x output. The dead time delay is set as a number of the prescaled timer/counter clocks. The minimum dead time is zero and the maximum dead time is the prescaled time/counter clock period multiplied by 15. • Bits 3:0- DT4L3:DT4L0: Dead Time Value for OC4x Output The dead time value for the OC4x output. The dead time delay is set as a number of the prescaled timer/counter clocks. The minimum dead time is zero and the maximum dead time is the prescaled time/counter clock period multiplied by 15. ATmega16U4/32U4 [DATASHEET] 176 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

16. Output Compare Modulator (OCM1C0A) 16.1 Overview The Output Compare Modulator (OCM) allows generation of waveforms modulated with a carrier frequency. The modulator uses the outputs from the Output Compare Unit C of the 16-bit Timer/Counter1 and the Output Compare Unit of the 8-bit Timer/Counter0. For more details about these Timer/Counters see “Timer/Counter0, Timer/Counter1, and Timer/Counter3 Prescalers” on page92. Figure 16-1. Output Compare Modulator, Block Diagram Timer/Counter 1 OC1C Pin OC1C / Timer/Counter 0 OC0A OC0A / PB7 When the modulator is enabled, the two output compare channels are modulated together as shown in the block diagram (Figure 16-1). 16.2 Description The Output Compare unit 1C and Output Compare unit 2 shares the PB7 port pin for output. The outputs of the Output Compare units (OC1C and OC0A) overrides the normal PORTB7 Register when one of them is enabled (i.e., when COMnx1:0 is not equal to zero). When both OC1C and OC0A are enabled at the same time, the modulator is automatically enabled. The functional equivalent schematic of the modulator is shown on Figure 16-2. The schematic includes part of the Timer/Counter units and the port B pin 7 output driver circuit. Figure 16-2. Output Compare Modulator, Schematic COMA01 Vcc COMA00 COM1C1 Modulator COM1C0 0 1 ( From Waveform Generator ) D Q 1 OC1C Pin 0 OC1C / ( From Waveform Generator ) D Q OC0A/ PB7 OC0A D Q D Q PORTB7 DDRB7 DATABUS When the modulator is enabled the type of modulation (logical AND or OR) can be selected by the PORTB7 Register. When PORTB7=0, logical AND will be performed and when PORTB7=1, logical OR will be performed (see Figure 16-3 on page 178). Note that the DDRB7 controls the direction of the port independent of the COMnx1:0 bit setting. ATmega16U4/32U4 [DATASHEET] 177 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

16.2.1 Timing Example Figure 16-3 illustrates the modulator in action. In this example the Timer/Counter1 is set to operate in fast PWM mode (non-inverted) and Timer/Counter0 uses CTC waveform mode with toggle Compare Output mode (COMnx1:0 = 1). Figure 16-3. Output Compare Modulator, Timing Diagram clk I/O OC1C (FPWM Mode) OC0A (CTC Mode) PB7 (PORTB7 = 0) PB7 (PORTB7 = 1) 1 2 3 (Period) In this example, Timer/Counter0 provides the carrier, while the modulating signal is generated by the Output Compare unit C of the Timer/Counter1. The resolution of the PWM signal (OC1C) is reduced by the modulation. The reduction factor is equal to the number of system clock cycles of one period of the carrier (OC0A). In this example the resolution is reduced by a factor of two. The reason for the reduction is illustrated in Figure 16-3 at the second and third period of the PB7 output when PORTB7 equals zero. The period 2 high time is one cycle longer than the period 3 high time, but the result on the PB7 output is equal in both periods. ATmega16U4/32U4 [DATASHEET] 178 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

17. Serial Peripheral Interface – SPI The Serial Peripheral Interface (SPI) allows high-speed synchronous data transfer between the ATmega16U4/ATmega32U4 and peripheral devices or between several AVR devices. The SPI includes the following features: • Full-duplex, Three-wire Synchronous Data Transfer • Master or Slave Operation • LSB First or MSB First Data Transfer • Seven Programmable Bit Rates • End of Transmission Interrupt Flag • Write Collision Flag Protection • Wake-up from Idle Mode • Double Speed (CK/2) Master SPI Mode USART can also be used in Master SPI mode, see “USART in SPI Mode” on page 214. The Power Reduction SPI bit, PRSPI, in “Power Reduction Register 0 - PRR0” on page47 must be written to zero to enable SPI module. Figure 17-1. SPI Block Diagram(1) DIVIDER /2/4/8/16/32/64/128 X 2 PI S X 2 PI S Note: 1. Refer to “Pinout” on page3, and Table10-3 on page74 for SPI pin placement. The interconnection between Master and Slave CPUs with SPI is shown in Figure 17-2 on page 180. The system consists of two shift Registers, and a Master clock generator. The SPI Master initiates the communication cycle when pulling low the Slave Select SS pin of the desired Slave. Master and Slave prepare the data to be sent in their respective shift Registers, and the Master generates the required clock pulses on the ATmega16U4/32U4 [DATASHEET] 179 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

SCK line to interchange data. Data is always shifted from Master to Slave on the Master Out – Slave In, MOSI, line, and from Slave to Master on the Master In – Slave Out, MISO, line. After each data packet, the Master will synchronize the Slave by pulling high the Slave Select, SS, line. When configured as a Master, the SPI interface has no automatic control of the SS line. This must be handled by user software before communication can start. When this is done, writing a byte to the SPI Data Register starts the SPI clock generator, and the hardware shifts the eight bits into the Slave. After shifting one byte, the SPI clock generator stops, setting the end of Transmission Flag (SPIF). If the SPI Interrupt Enable bit (SPIE) in the SPCR Register is set, an interrupt is requested. The Master may continue to shift the next byte by writing it into SPDR, or signal the end of packet by pulling high the Slave Select, SS line. The last incoming byte will be kept in the Buffer Register for later use. When configured as a Slave, the SPI interface will remain sleeping with MISO tri-stated as long as the SS pin is driven high. In this state, software may update the contents of the SPI Data Register, SPDR, but the data will not be shifted out by incoming clock pulses on the SCK pin until the SS pin is driven low. As one byte has been completely shifted, the end of Transmission Flag, SPIF is set. If the SPI Interrupt Enable bit, SPIE, in the SPCR Register is set, an interrupt is requested. The Slave may continue to place new data to be sent into SPDR before reading the incoming data. The last incoming byte will be kept in the Buffer Register for later use. Figure 17-2. SPI Master-slave Interconnection SHIFT ENABLE The system is single buffered in the transmit direction and double buffered in the receive direction. This means that bytes to be transmitted cannot be written to the SPI Data Register before the entire shift cycle is completed. When receiving data, however, a received character must be read from the SPI Data Register before the next character has been completely shifted in. Otherwise, the first byte is lost. In SPI Slave mode, the control logic will sample the incoming signal of the SCK pin. To ensure correct sampling of the clock signal, the frequency of the SPI clock should never exceed f /4. osc When the SPI is enabled, the data direction of the MOSI, MISO, SCK, and SS pins is overridden according to the table below. For more details on automatic port overrides, refer to “Alternate Port Functions” on page72. Table 17-1. SPI Pin Overrides(1) Pin Direction, Master SPI Direction, Slave SPI MOSI User Defined Input MISO Input User Defined SCK User Defined Input SS User Defined Input Note: 1. See “Alternate Functions of Port B” on page74 for a detailed description of how to define the direction of the user defined SPI pins. ATmega16U4/32U4 [DATASHEET] 180 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The following code examples show how to initialize the SPI as a Master and how to perform a simple transmission. DDR_SPI in the examples must be replaced by the actual Data Direction Register controlling the SPI pins. DD_MOSI, DD_MISO and DD_SCK must be replaced by the actual data direction bits for these pins. E.g. if MOSI is placed on pin PB5, replace DD_MOSI with DDB5 and DDR_SPI with DDRB. Assembly Code Example(1) SPI_MasterInit: ; Set MOSI and SCK output, all others input ldi r17,(1<<DD_MOSI)|(1<<DD_SCK) out DDR_SPI,r17 ; Enable SPI, Master, set clock rate fck/16 ldi r17,(1<<SPE)|(1<<MSTR)|(1<<SPR0) out SPCR,r17 ret SPI_MasterTransmit: ; Start transmission of data (r16) out SPDR,r16 Wait_Transmit: ; Wait for transmission complete sbis SPSR,SPIF rjmp Wait_Transmit ret C Code Example(1) void SPI_MasterInit(void) { /* Set MOSI and SCK output, all others input */ DDR_SPI = (1<<DD_MOSI)|(1<<DD_SCK); /* Enable SPI, Master, set clock rate fck/16 */ SPCR = (1<<SPE)|(1<<MSTR)|(1<<SPR0); } void SPI_MasterTransmit(char cData) { /* Start transmission */ SPDR = cData; /* Wait for transmission complete */ while(!(SPSR & (1<<SPIF))) ; } Note: 1. See “Code Examples” on page 8. ATmega16U4/32U4 [DATASHEET] 181 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The following code examples show how to initialize the SPI as a Slave and how to perform a simple reception. Assembly Code Example(1) SPI_SlaveInit: ; Set MISO output, all others input ldi r17,(1<<DD_MISO) out DDR_SPI,r17 ; Enable SPI ldi r17,(1<<SPE) out SPCR,r17 ret SPI_SlaveReceive: ; Wait for reception complete sbis SPSR,SPIF rjmp SPI_SlaveReceive ; Read received data and return in r16,SPDR ret C Code Example(1) void SPI_SlaveInit(void) { /* Set MISO output, all others input */ DDR_SPI = (1<<DD_MISO); /* Enable SPI */ SPCR = (1<<SPE); } char SPI_SlaveReceive(void) { /* Wait for reception complete */ while(!(SPSR & (1<<SPIF))) ; /* Return Data Register */ return SPDR; } Note: 1. See “Code Examples” on page 8. 17.1 SS Pin Functionality 17.1.1 Slave Mode When the SPI is configured as a Slave, the Slave Select (SS) pin is always input. When SS is held low, the SPI is activated, and MISO becomes an output if configured so by the user. All other pins are inputs. When SS is driven high, all pins are inputs, and the SPI is passive, which means that it will not receive incoming data. Note that the SPI logic will be reset once the SS pin is driven high. ATmega16U4/32U4 [DATASHEET] 182 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The SS pin is useful for packet/byte synchronization to keep the slave bit counter synchronous with the master clock generator. When the SS pin is driven high, the SPI slave will immediately reset the send and receive logic, and drop any partially received data in the Shift Register. 17.1.2 Master Mode When the SPI is configured as a Master (MSTR in SPCR is set), the user can determine the direction of the SS pin. If SS is configured as an output, the pin is a general output pin which does not affect the SPI system. Typically, the pin will be driving the SS pin of the SPI Slave. If SS is configured as an input, it must be held high to ensure Master SPI operation. If the SS pin is driven low by peripheral circuitry when the SPI is configured as a Master with the SS pin defined as an input, the SPI system interprets this as another master selecting the SPI as a slave and starting to send data to it. To avoid bus contention, the SPI system takes the following actions: 1. The MSTR bit in SPCR is cleared and the SPI system becomes a Slave. As a result of the SPI becom- ing a Slave, the MOSI and SCK pins become inputs. 2. The SPIF Flag in SPSR is set, and if the SPI interrupt is enabled, and the I-bit in SREG is set, the interrupt routine will be executed. Thus, when interrupt-driven SPI transmission is used in Master mode, and there exists a possibility that SS is driven low, the interrupt should always check that the MSTR bit is still set. If the MSTR bit has been cleared by a slave select, it must be set by the user to re-enable SPI Master mode. 17.1.3 Data Modes There are four combinations of SCK phase and polarity with respect to serial data, which are determined by control bits CPHA and CPOL. The SPI data transfer formats are shown in Figure 17-3 and Figure 17-4 on page 184. Data bits are shifted out and latched in on opposite edges of the SCK signal, ensuring sufficient time for data signals to stabilize. This is clearly seen by summarizing Table 17-3 and Table17-4 on page185, as done below: Table 17-2. CPOL and CPHA Functionality Leading Edge Trailing eDge SPI Mode CPOL=0, CPHA=0 Sample (Rising) Setup (Falling) 0 CPOL=0, CPHA=1 Setup (Rising) Sample (Falling) 1 CPOL=1, CPHA=0 Sample (Falling) Setup (Rising) 2 CPOL=1, CPHA=1 Setup (Falling) Sample (Rising) 3 ATmega16U4/32U4 [DATASHEET] 183 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 17-3. SPI Transfer Format with CPHA = 0 SCK (CPOL = 0) mode 0 SCK (CPOL = 1) mode 2 SAMPLE I MOSI/MISO CHANGE 0 MOSI PIN CHANGE 0 MISO PIN SS MSB first (DORD = 0) MSB Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 LSB LSB first (DORD = 1) LSB Bit 1 Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 MSB Figure 17-4. SPI Transfer Format with CPHA = 1 SCK (CPOL = 0) mode 1 SCK (CPOL = 1) mode 3 SAMPLE I MOSI/MISO CHANGE 0 MOSI PIN CHANGE 0 MISO PIN SS MSB first (DORD = 0) MSB Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 LSB LSB first (DORD = 1) LSB Bit 1 Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 MSB ATmega16U4/32U4 [DATASHEET] 184 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

17.2 Register Description 17.2.1 SPI Control Register – SPCR Bit 7 6 5 4 3 2 1 0 SPIE SPE DORD MSTR CPOL CPHA SPR1 SPR0 SPCR Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 7 – SPIE: SPI Interrupt Enable This bit causes the SPI interrupt to be executed if SPIF bit in the SPSR Register is set and the if the Global Interrupt Enable bit in SREG is set. • Bit 6 – SPE: SPI Enable When the SPE bit is written to one, the SPI is enabled. This bit must be set to enable any SPI operations. • Bit 5 – DORD: Data Order When the DORD bit is written to one, the LSB of the data word is transmitted first. When the DORD bit is written to zero, the MSB of the data word is transmitted first. • Bit 4 – MSTR: Master/Slave Select This bit selects Master SPI mode when written to one, and Slave SPI mode when written logic zero. If SS is configured as an input and is driven low while MSTR is set, MSTR will be cleared, and SPIF in SPSR will become set. The user will then have to set MSTR to re-enable SPI Master mode. • Bit 3 – CPOL: Clock Polarity When this bit is written to one, SCK is high when idle. When CPOL is written to zero, SCK is low when idle. Refer to Figure 17-3 and Figure 17-4 for an example. The CPOL functionality is summarized below: Table 17-3. CPOL Functionality CPOL Leading Edge Trailing Edge 0 Rising Falling 1 Falling Rising • Bit 2 – CPHA: Clock Phase The settings of the Clock Phase bit (CPHA) determine if data is sampled on the leading (first) or trailing (last) edge of SCK. Refer to Figure 17-3 on page 184 and Figure 17-4 on page 184 for an example. The CPOL functionality is summarized below: Table 17-4. CPHA Functionality CPHA Leading Edge Trailing Edge 0 Sample Setup 1 Setup Sample • Bits 1, 0 – SPR1, SPR0: SPI Clock Rate Select 1 and 0 These two bits control the SCK rate of the device configured as a Master. SPR1 and SPR0 have no effect on the Slave. The relationship between SCK and the Oscillator Clock frequency f is shown below. osc ATmega16U4/32U4 [DATASHEET] 185 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 17-5. Relationship Between SCK and the Oscillator Frequency SPI2X SPR1 SPR0 SCK Frequency 0 0 0 f /4 osc 0 0 1 f /16 osc 0 1 0 f /64 osc 0 1 1 f /128 osc 1 0 0 f /2 osc 1 0 1 f /8 osc 1 1 0 f /32 osc 1 1 1 f /64 osc 17.2.2 SPI Status Register – SPSR Bit 7 6 5 4 3 2 1 0 SPIF WCOL – – – – – SPI2X SPSR Read/Write R R R R R R R R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 7 – SPIF: SPI Interrupt Flag When a serial transfer is complete, the SPIF Flag is set. An interrupt is generated if SPIE in SPCR is set and global interrupts are enabled. If SS is an input and is driven low when the SPI is in Master mode, this will also set the SPIF Flag. SPIF is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, the SPIF bit is cleared by first reading the SPI Status Register with SPIF set, then accessing the SPI Data Register (SPDR). • Bit 6 – WCOL: Write COLlision Flag The WCOL bit is set if the SPI Data Register (SPDR) is written during a data transfer. The WCOL bit (and the SPIF bit) are cleared by first reading the SPI Status Register with WCOL set, and then accessing the SPI Data Register. • Bit 5..1 – Res: Reserved Bits These bits are reserved and will always read as zero. • Bit 0 – SPI2X: Double SPI Speed Bit When this bit is written logic one the SPI speed (SCK Frequency) will be doubled when the SPI is in Master mode (see Table17-2 on page183). This means that the minimum SCK period will be two CPU clock periods. When the SPI is configured as Slave, the SPI is only guaranteed to work at f /4 or lower. osc The SPI interface on the device is also used for program memory and EEPROM downloading or uploading. See page367 for serial programming and verification. 17.2.3 SPI Data Register – SPDR Bit 7 6 5 4 3 2 1 0 MSB LSB SPDR Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value X X X X X X X X Undefined ATmega16U4/32U4 [DATASHEET] 186 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The SPI Data Register is a read/write register used for data transfer between the Register File and the SPI Shift Register. Writing to the register initiates data transmission. Reading the register causes the Shift Register Receive buffer to be read. ATmega16U4/32U4 [DATASHEET] 187 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

18. USART The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) is a highly flexible serial communication device. The main features are: • Full Duplex Operation (Independent Serial Receive and Transmit Registers) • Asynchronous or Synchronous Operation • Flow control CTS/RTS signals hardware management • Master or Slave Clocked Synchronous Operation • High Resolution Baud Rate Generator • Supports Serial Frames with 5, 6, 7, 8, or 9 Data Bits and 1 or 2 Stop Bits • Odd or Even Parity Generation and Parity Check Supported by Hardware • Data OverRun Detection • Framing Error Detection • Noise Filtering Includes False Start Bit Detection and Digital Low Pass Filter • Three Separate Interrupts on TX Complete, TX Data Register Empty, and RX Complete • Multi-processor Communication Mode • Double Speed Asynchronous Communication Mode . 18.1 Overview A simplified block diagram of the USART Transmitter is shown in Figure 18-1 on page 189. CPU accessible I/O Registers and I/O pins are shown in bold. ATmega16U4/32U4 [DATASHEET] 188 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 18-1. USART Block Diagram(1) Clock Generator UBRR[H:L] OSC BAUD RATE GENERATOR SYNC LOGIC PIN XCK CONTROL Transmitter TX UDR (Transmit) CONTROL PARITY S GENERATOR U PIN B TRANSMIT SHIFT REGISTER CONTROL TxD A T A D Receiver CLOCK RX RECOVERY CONTROL DATA PIN RECEIVE SHIFT REGISTER RxD RECOVERY CONTROL PARITY UDR (Receive) CHECKER UCSRA UCSRB UCSRC Note: 1. See “Pinout” on page3, Table10-8 on page78 and for USART pin placement. The dashed boxes in the block diagram separate the three main parts of the USART (listed from the top): Clock Generator, Transmitter and Receiver. Control Registers are shared by all units. The Clock Generation logic consists of synchronization logic for external clock input used by synchronous slave operation, and the baud rate generator. The XCKn (Transfer Clock) pin is only used by synchronous transfer mode. The Transmitter consists of a single write buffer, a serial Shift Register, Parity Generator, and Control logic for handling different serial frame formats. The write buffer allows a continuous transfer of data without any delay between frames. The Receiver is the most complex part of the USART module due to its clock and data recovery units. The recovery units are used for asynchronous data reception. In addition to the recovery units, the Receiver includes a Parity Checker, Control logic, a Shift Register and a two level receive buffer (UDRn). The Receiver supports the same frame formats as the Transmitter, and can detect Frame Error, Data OverRun and Parity Errors. 18.2 Clock Generation The Clock Generation logic generates the base clock for the Transmitter and Receiver. The USARTn supports four modes of clock operation: Normal asynchronous, Double Speed asynchronous, Master synchronous and Slave synchronous mode. The UMSELn bit in USART Control and Status Register C (UCSRnC) selects between asynchronous and synchronous operation. Double Speed (asynchronous mode only) is controlled by the U2Xn found in the UCSRnA Register. When using synchronous mode (UMSELn = 1), the Data Direction Register for the XCKn pin (DDR_XCKn) controls whether the clock source is internal (Master mode) or external (Slave mode). The XCKn pin is only active when using synchronous mode. Figure 18-2 shows a block diagram of the clock generation logic. ATmega16U4/32U4 [DATASHEET] 189 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 18-2. Clock Generation Logic, Block Diagram UBRR U2X fosc Prescaling UBRR+1 /2 /4 /2 Down-Counter 0 1 0 OSC txclk 1 DDR_XCK Sync Edge xcki Register Detector 0 XCK UMSEL xcko 1 Pin DDR_XCK UCPOL 1 rxclk 0 Signal description: txclk Transmitter clock (Internal Signal) rxclk Receiver base clock (Internal Signal) xcki Input from XCK pin (internal Signal). Used for synchronous slave operation xcko Clock output to XCK pin (Internal Signal). Used for synchronous master operation f XTAL pin frequency (System Clock) OSC 18.2.1 Internal Clock Generation – The Baud Rate Generator Internal clock generation is used for the asynchronous and the synchronous master modes of operation. The description in this section refers to Figure 18-2 on page 190. The USART Baud Rate Register (UBRRn) and the down-counter connected to it function as a programmable prescaler or baud rate generator. The down-counter, running at system clock (f ), is loaded with the UBRRn osc value each time the counter has counted down to zero or when the UBRRLn Register is written. A clock is generated each time the counter reaches zero. This clock is the baud rate generator clock output (=f /(UBRRn+1)). The Transmitter divides the baud rate generator clock output by 2, 8, or 16 depending on osc mode. The baud rate generator output is used directly by the Receiver’s clock and data recovery units. However, the recovery units use a state machine that uses 2, 8, or 16 states depending on mode set by the state of the UMSELn, U2Xn, and DDR_XCKn bits. The following table contains equations for calculating the baud rate (in bits per second) and for calculating the UBRRn value for each mode of operation using an internally generated clock source. ATmega16U4/32U4 [DATASHEET] 190 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 18-1. Equations for Calculating Baud Rate Register Setting Operating Mode Equation for Calculating Baud Rate(1) Equation for Calculating UBRR Value f f OSC Asynchronous Normal OSC UBRRn = ------------------------–1 BAUD = ------------------------------------------ 16BAUD mode (U2Xn = 0) 16UBRRn+1 Asynchronous Double BAUD = --------------f--O----S---C---------------- UBRRn = -----f--O----S--C-------–1 Speed mode (U2Xn = 1) 8UBRRn+1 8BAUD f f OSC Synchronous Master OSC UBRRn = --------------------–1 BAUD = --------------------------------------- 2BAUD mode 2UBRRn+1 Note: 1. The baud rate is defined to be the transfer rate in bit per second (bps) BAUD Baud rate (in bits per second, bps) f System Oscillator clock frequency OSC UBRRn Contents of the UBRRHn and UBRRLn Registers, (0-4095) Some examples of UBRRn values for some system clock frequencies are found in Table18-6 on page208. 18.2.2 Double Speed Operation (U2Xn) The transfer rate can be doubled by setting the U2Xn bit in UCSRnA. Setting this bit only has effect for the asynchronous operation. Set this bit to zero when using synchronous operation. Setting this bit will reduce the divisor of the baud rate divider from 16 to 8, effectively doubling the transfer rate for asynchronous communication. Note however that the Receiver will in this case only use half the number of samples (reduced from 16 to 8) for data sampling and clock recovery, and therefore a more accurate baud rate setting and system clock are required when this mode is used. For the Transmitter, there are no downsides. 18.2.3 External Clock External clocking is used by the synchronous slave modes of operation. The description in this section refers to Figure 18-2 on page 190 for details. External clock input from the XCKn pin is sampled by a synchronization register to minimize the chance of meta- stability. The output from the synchronization register must then pass through an edge detector before it can be used by the Transmitter and Receiver. This process introduces a two CPU clock period delay and therefore the maximum external XCKn clock frequency is limited by the following equation: f OSC f ----------- XCK 4 Note that f depends on the stability of the system clock source. It is therefore recommended to add some osc margin to avoid possible loss of data due to frequency variations. 18.2.4 Synchronous Clock Operation When synchronous mode is used (UMSELn = 1), the XCKn pin will be used as either clock input (Slave) or clock output (Master). The dependency between the clock edges and data sampling or data change is the same. The ATmega16U4/32U4 [DATASHEET] 191 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

basic principle is that data input (on RxDn) is sampled at the opposite XCKn clock edge of the edge the data output (TxDn) is changed. Figure 18-3. Synchronous Mode XCKn Timing UCPOL = 1 XCK RxD / TxD Sample UCPOL = 0 XCK RxD / TxD Sample The UCPOLn bit UCRSC selects which XCKn clock edge is used for data sampling and which is used for data change. As the above figure shows, when UCPOLn is zero the data will be changed at rising XCKn edge and sampled at falling XCKn edge. If UCPOLn is set, the data will be changed at falling XCKn edge and sampled at rising XCKn edge. 18.3 Frame Formats A serial frame is defined to be one character of data bits with synchronization bits (start and stop bits), and optionally a parity bit for error checking. The USART accepts all 30 combinations of the following as valid frame formats:  1 start bit  5, 6, 7, 8, or 9 data bits  no, even or odd parity bit  1 or 2 stop bits A frame starts with the start bit followed by the least significant data bit. Then the next data bits, up to a total of nine, are succeeding, ending with the most significant bit. If enabled, the parity bit is inserted after the data bits, before the stop bits. When a complete frame is transmitted, it can be directly followed by a new frame, or the communication line can be set to an idle (high) state. The following figure illustrates the possible combinations of the frame formats. Bits inside brackets are optional. Figure 18-4. Frame Formats FRAME (IDLE) St 0 1 2 3 4 [5] [6] [7] [8] [P] Sp1 [Sp2] (St / IDLE) St Start bit, always low (n) Data bits (0 to 8) P Parity bit. Can be odd or even Sp Stop bit, always high IDLE No transfers on the communication line (RxDn or TxDn). An IDLE line must be high ATmega16U4/32U4 [DATASHEET] 192 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The frame format used by the USART is set by the UCSZn2:0, UPMn1:0, and USBSn bits in UCSRnB and UCSRnC. The Receiver and Transmitter use the same setting. Note that changing the setting of any of these bits will corrupt all ongoing communication for both the Receiver and Transmitter. The USART Character SiZe (UCSZn2:0) bits select the number of data bits in the frame. The USART Parity mode (UPMn1:0) bits enable and set the type of parity bit. The selection between one or two stop bits is done by the USART Stop Bit Select (USBSn) bit. The Receiver ignores the second stop bit. An FE (Frame Error) will therefore only be detected in the cases where the first stop bit is zero. 18.3.1 Parity Bit Calculation The parity bit is calculated by doing an exclusive-or of all the data bits. If odd parity is used, the result of the exclusive or is inverted. The relation between the parity bit and data bits is as follows: P = d d d d d 0 even n–1 3 2 1 0 P = d d d d d 1 odd n–1 3 2 1 0 P Start bit, always low even P Parity bit using odd parity odd d Data bit n of the character n If used, the parity bit is located between the last data bit and first stop bit of a serial frame. 18.4 USART Initialization The USART has to be initialized before any communication can take place. The initialization process normally consists of setting the baud rate, setting frame format and enabling the Transmitter or the Receiver depending on the usage. For interrupt driven USART operation, the Global Interrupt Flag should be cleared (and interrupts globally disabled) when doing the initialization. Before doing a re-initialization with changed baud rate or frame format, be sure that there are no ongoing transmissions during the period the registers are changed. The TXCn Flag can be used to check that the Transmitter has completed all transfers, and the RXC Flag can be used to check that there are no unread data in the receive buffer. Note that the TXCn Flag must be cleared before each transmission (before UDRn is written) if it is used for this purpose. The following simple USART initialization code examples show one assembly and one C function that are equal in functionality. The examples assume asynchronous operation using polling (no interrupts enabled) and a fixed ATmega16U4/32U4 [DATASHEET] 193 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

frame format. The baud rate is given as a function parameter. For the assembly code, the baud rate parameter is assumed to be stored in the r17:r16 Registers. Assembly Code Example(1) USART_Init: ; Set baud rate out UBRRHn, r17 out UBRRLn, r16 ; Enable receiver and transmitter ldi r16, (1<<RXENn)|(1<<TXENn) out UCSRnB,r16 ; Set frame format: 8data, 2stop bit ldi r16, (1<<USBSn)|(3<<UCSZn0) out UCSRnC,r16 ret C Code Example(1) void USART_Init( unsigned int baud ) { /* Set baud rate */ UBRRHn = (unsigned char)(baud>>8); UBRRLn = (unsigned char)baud; /* Enable receiver and transmitter */ UCSRnB = (1<<RXENn)|(1<<TXENn); /* Set frame format: 8data, 2stop bit */ UCSRnC = (1<<USBSn)|(3<<UCSZn0); } Note: 1. See “Code Examples” on page 8. More advanced initialization routines can be made that include frame format as parameters, disable interrupts and so on. However, many applications use a fixed setting of the baud and control registers, and for these types of applications the initialization code can be placed directly in the main routine, or be combined with initialization code for other I/O modules. 18.5 Data Transmission – The USART Transmitter The USART Transmitter is enabled by setting the Transmit Enable (TXEN) bit in the UCSRnB Register. When the Transmitter is enabled, the normal port operation of the TxDn pin is overridden by the USART and given the function as the Transmitter’s serial output. The baud rate, mode of operation and frame format must be set up once before doing any transmissions. If synchronous operation is used, the clock on the XCKn pin will be overridden and used as transmission clock. 18.5.1 Sending Frames with 5 to 8 Data Bit A data transmission is initiated by loading the transmit buffer with the data to be transmitted. The CPU can load the transmit buffer by writing to the UDRn I/O location. The buffered data in the transmit buffer will be moved to the Shift Register when the Shift Register is ready to send a new frame. The Shift Register is loaded with new data if it is in idle state (no ongoing transmission) or immediately after the last stop bit of the previous frame is transmitted. When the Shift Register is loaded with new data, it will transfer one complete frame at the rate given by the Baud Register, U2Xn bit or by XCKn depending on mode of operation. ATmega16U4/32U4 [DATASHEET] 194 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The following code examples show a simple USART transmit function based on polling of the Data Register Empty (UDREn) Flag. When using frames with less than eight bits, the most significant bits written to the UDRn are ignored. The USART has to be initialized before the function can be used. For the assembly code, the data to be sent is assumed to be stored in Register R16. Assembly Code Example(1) USART_Transmit: ; Wait for empty transmit buffer sbis UCSRnA,UDREn rjmp USART_Transmit ; Put data (r16) into buffer, sends the data out UDRn,r16 ret C Code Example(1) void USART_Transmit( unsigned char data ) { /* Wait for empty transmit buffer */ while ( !( UCSRnA & (1<<UDREn)) ) ; /* Put data into buffer, sends the data */ UDRn = data; } Note: 1. See “Code Examples” on page 8. The function simply waits for the transmit buffer to be empty by checking the UDREn Flag, before loading it with new data to be transmitted. If the Data Register Empty interrupt is utilized, the interrupt routine writes the data into the buffer. 18.5.2 Sending Frames with 9 Data Bit If 9-bit characters are used (UCSZn = 7), the ninth bit must be written to the TXB8 bit in UCSRnB before the low byte of the character is written to UDRn. The following code examples show a transmit function that handles 9- bit characters. For the assembly code, the data to be sent is assumed to be stored in registers R17:R16. ATmega16U4/32U4 [DATASHEET] 195 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Assembly Code Example(1)(2) USART_Transmit: ; Wait for empty transmit buffer sbis UCSRnA,UDREn rjmp USART_Transmit ; Copy 9th bit from r17 to TXB8 cbi UCSRnB,TXB8 sbrc r17,0 sbi UCSRnB,TXB8 ; Put LSB data (r16) into buffer, sends the data out UDRn,r16 ret C Code Example(1)(2) void USART_Transmit( unsigned int data ) { /* Wait for empty transmit buffer */ while ( !( UCSRnA & (1<<UDREn))) ) ; /* Copy 9th bit to TXB8 */ UCSRnB &= ~(1<<TXB8); if ( data & 0x0100 ) UCSRnB |= (1<<TXB8); /* Put data into buffer, sends the data */ UDRn = data; } Notes: 1. These transmit functions are written to be general functions. They can be optimized if the contents of the UCSRnB is static. For example, only the TXB8 bit of the UCSRnB Register is used after initialization. 2. See “Code Examples” on page 8. The 9th bit can be used for indicating an address frame when using multi processor communication mode or for other protocol handling as for example synchronization. 18.5.3 Transmitter Flags and Interrupts The USART Transmitter has two flags that indicate its state: USART Data Register Empty (UDREn) and Transmit Complete (TXCn). Both flags can be used for generating interrupts. The Data Register Empty (UDREn) Flag indicates whether the transmit buffer is ready to receive new data. This bit is set when the transmit buffer is empty, and cleared when the transmit buffer contains data to be transmitted that has not yet been moved into the Shift Register. For compatibility with future devices, always write this bit to zero when writing the UCSRnA Register. When the Data Register Empty Interrupt Enable (UDRIEn) bit in UCSRnB is written to one, the USART Data Register Empty Interrupt will be executed as long as UDREn is set (provided that global interrupts are enabled). UDREn is cleared by writing UDRn. When interrupt-driven data transmission is used, the Data Register Empty interrupt routine must either write new data to UDRn in order to clear UDREn or disable the Data Register Empty interrupt, otherwise a new interrupt will occur once the interrupt routine terminates. The Transmit Complete (TXCn) Flag bit is set one when the entire frame in the Transmit Shift Register has been shifted out and there are no new data currently present in the transmit buffer. The TXCn Flag bit is automatically ATmega16U4/32U4 [DATASHEET] 196 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

cleared when a transmit complete interrupt is executed, or it can be cleared by writing a one to its bit location. The TXCn Flag is useful in half-duplex communication interfaces (like the RS-485 standard), where a transmitting application must enter receive mode and free the communication bus immediately after completing the transmission. When the Transmit Compete Interrupt Enable (TXCIEn) bit in UCSRnB is set, the USART Transmit Complete Interrupt will be executed when the TXCn Flag becomes set (provided that global interrupts are enabled). When the transmit complete interrupt is used, the interrupt handling routine does not have to clear the TXCn Flag, this is done automatically when the interrupt is executed. 18.5.4 Parity Generator The Parity Generator calculates the parity bit for the serial frame data. When parity bit is enabled (UPMn1 = 1), the transmitter control logic inserts the parity bit between the last data bit and the first stop bit of the frame that is sent. 18.5.5 Disabling the Transmitter The disabling of the Transmitter (setting the TXEN to zero) will not become effective until ongoing and pending transmissions are completed, i.e., when the Transmit Shift Register and Transmit Buffer Register do not contain data to be transmitted. When disabled, the Transmitter will no longer override the TxDn pin. 18.6 Data Reception – The USART Receiver The USART Receiver is enabled by writing the Receive Enable (RXENn) bit in the UCSRnB Register to one. When the Receiver is enabled, the normal pin operation of the RxDn pin is overridden by the USART and given the function as the Receiver’s serial input. The baud rate, mode of operation and frame format must be set up once before any serial reception can be done. If synchronous operation is used, the clock on the XCKn pin will be used as transfer clock. 18.6.1 Receiving Frames with 5 to 8 Data Bits The Receiver starts data reception when it detects a valid start bit. Each bit that follows the start bit will be sampled at the baud rate or XCKn clock, and shifted into the Receive Shift Register until the first stop bit of a frame is received. A second stop bit will be ignored by the Receiver. When the first stop bit is received, i.e., a complete serial frame is present in the Receive Shift Register, the contents of the Shift Register will be moved into the receive buffer. The receive buffer can then be read by reading the UDRn I/O location. The following code example shows a simple USART receive function based on polling of the Receive Complete (RXCn) Flag. When using frames with less than eight bits the most significant bits of the data read from the UDRn will be masked to zero. The USART has to be initialized before the function can be used. ATmega16U4/32U4 [DATASHEET] 197 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Assembly Code Example(1) USART_Receive: ; Wait for data to be received sbis UCSRnA, RXCn rjmp USART_Receive ; Get and return received data from buffer in r16, UDRn ret C Code Example(1) unsigned char USART_Receive( void ) { /* Wait for data to be received */ while ( !(UCSRnA & (1<<RXCn)) ) ; /* Get and return received data from buffer */ return UDRn; } Note: 1. See “Code Examples” on page 8. The function simply waits for data to be present in the receive buffer by checking the RXCn Flag, before reading the buffer and returning the value. 18.6.2 Receiving Frames with 9 Data Bits If 9-bit characters are used (UCSZn=7) the ninth bit must be read from the RXB8n bit in UCSRnB before reading the low bits from the UDRn. This rule applies to the FEn, DORn, and UPEn Status Flags as well. Read status from UCSRnA, then data from UDRn. Reading the UDRn I/O location will change the state of the receive buffer FIFO and consequently the TXB8n, FEn, DORn, and UPEn bits, which all are stored in the FIFO, will change. The following code example shows a simple USART receive function that handles both nine bit characters and the status bits. ATmega16U4/32U4 [DATASHEET] 198 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Assembly Code Example(1) USART_Receive: ; Wait for data to be received sbis UCSRnA, RXCn rjmp USART_Receive ; Get status and 9th bit, then data from buffer in r18, UCSRnA in r17, UCSRnB in r16, UDRn ; If error, return -1 andi r18,(1<<FEn)|(1<<DORn)|(1<<UPEn) breq USART_ReceiveNoError ldi r17, HIGH(-1) ldi r16, LOW(-1) USART_ReceiveNoError: ; Filter the 9th bit, then return lsr r17 andi r17, 0x01 ret C Code Example(1) unsigned int USART_Receive( void ) { unsigned char status, resh, resl; /* Wait for data to be received */ while ( !(UCSRnA & (1<<RXCn)) ) ; /* Get status and 9th bit, then data */ /* from buffer */ status = UCSRnA; resh = UCSRnB; resl = UDRn; /* If error, return -1 */ if ( status & (1<<FEn)|(1<<DORn)|(1<<UPEn) ) return -1; /* Filter the 9th bit, then return */ resh = (resh >> 1) & 0x01; return ((resh << 8) | resl); } Note: 1. See “Code Examples” on page 8. The receive function example reads all the I/O Registers into the Register File before any computation is done. This gives an optimal receive buffer utilization since the buffer location read will be free to accept new data as early as possible. 18.6.3 Receive Compete Flag and Interrupt The USART Receiver has one flag that indicates the Receiver state. The Receive Complete (RXCn) Flag indicates if there are unread data present in the receive buffer. This flag is one when unread data exist in the receive buffer, and zero when the receive buffer is empty (i.e., does not ATmega16U4/32U4 [DATASHEET] 199 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

contain any unread data). If the Receiver is disabled (RXENn = 0), the receive buffer will be flushed and consequently the RXCn bit will become zero. When the Receive Complete Interrupt Enable (RXCIEn) in UCSRnB is set, the USART Receive Complete interrupt will be executed as long as the RXCn Flag is set (provided that global interrupts are enabled). When interrupt-driven data reception is used, the receive complete routine must read the received data from UDRn in order to clear the RXCn Flag, otherwise a new interrupt will occur once the interrupt routine terminates. 18.6.4 Receiver Error Flags The USART Receiver has three Error Flags: Frame Error (FEn), Data OverRun (DORn), and Parity Error (UPEn). All can be accessed by reading UCSRnA. Common for the Error Flags is that they are located in the receive buffer together with the frame for which they indicate the error status. Due to the buffering of the Error Flags, the UCSRnA must be read before the receive buffer (UDRn), since reading the UDRn I/O location changes the buffer read location. Another equality for the Error Flags is that they can not be altered by software doing a write to the flag location. However, all flags must be set to zero when the UCSRnA is written for upward compatibility of future USART implementations. None of the Error Flags can generate interrupts. The Frame Error (FEn) Flag indicates the state of the first stop bit of the next readable frame stored in the receive buffer. The FEn Flag is zero when the stop bit was correctly read (as one), and the FEn Flag will be one when the stop bit was incorrect (zero). This flag can be used for detecting out-of-sync conditions, detecting break conditions and protocol handling. The FEn Flag is not affected by the setting of the USBSn bit in UCSRnC since the Receiver ignores all, except for the first, stop bits. For compatibility with future devices, always set this bit to zero when writing to UCSRnA. The Data OverRun (DORn) Flag indicates data loss due to a receiver buffer full condition. A Data OverRun occurs when the receive buffer is full (two characters), it is a new character waiting in the Receive Shift Register, and a new start bit is detected. If the DORn Flag is set there was one or more serial frame lost between the frame last read from UDRn, and the next frame read from UDRn. For compatibility with future devices, always write this bit to zero when writing to UCSRnA. The DORn Flag is cleared when the frame received was successfully moved from the Shift Register to the receive buffer. The Parity Error (UPEn) Flag indicates that the next frame in the receive buffer had a Parity Error when received. If Parity Check is not enabled the UPEn bit will always be read zero. For compatibility with future devices, always set this bit to zero when writing to UCSRnA. For more details see “Parity Bit Calculation” on page193 and “Parity Checker” on page200. 18.6.5 Parity Checker The Parity Checker is active when the high USART Parity mode (UPMn1) bit is set. Type of Parity Check to be performed (odd or even) is selected by the UPMn0 bit. When enabled, the Parity Checker calculates the parity of the data bits in incoming frames and compares the result with the parity bit from the serial frame. The result of the check is stored in the receive buffer together with the received data and stop bits. The Parity Error (UPEn) Flag can then be read by software to check if the frame had a Parity Error. The UPEn bit is set if the next character that can be read from the receive buffer had a Parity Error when received and the Parity Checking was enabled at that point (UPMn1 = 1). This bit is valid until the receive buffer (UDRn) is read. 18.6.6 Disabling the Receiver In contrast to the Transmitter, disabling of the Receiver will be immediate. Data from ongoing receptions will therefore be lost. When disabled (i.e., the RXENn is set to zero) the Receiver will no longer override the normal function of the RxDn port pin. The Receiver buffer FIFO will be flushed when the Receiver is disabled. Remaining data in the buffer will be lost ATmega16U4/32U4 [DATASHEET] 200 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

18.6.7 Flushing the Receive Buffer The receiver buffer FIFO will be flushed when the Receiver is disabled, i.e., the buffer will be emptied of its contents. Unread data will be lost. If the buffer has to be flushed during normal operation, due to for instance an error condition, read the UDRn I/O location until the RXCn Flag is cleared. The following code example shows how to flush the receive buffer. Assembly Code Example(1) USART_Flush: sbis UCSRnA, RXCn ret in r16, UDRn rjmp USART_Flush C Code Example(1) void USART_Flush( void ) { unsigned char dummy; while ( UCSRnA & (1<<RXCn) ) dummy = UDRn; } Note: 1. See “Code Examples” on page 8. 18.7 Asynchronous Data Reception The USART includes a clock recovery and a data recovery unit for handling asynchronous data reception. The clock recovery logic is used for synchronizing the internally generated baud rate clock to the incoming asynchronous serial frames at the RxDn pin. The data recovery logic samples and low pass filters each incoming bit, thereby improving the noise immunity of the Receiver. The asynchronous reception operational range depends on the accuracy of the internal baud rate clock, the rate of the incoming frames, and the frame size in number of bits. 18.7.1 Asynchronous Clock Recovery The clock recovery logic synchronizes internal clock to the incoming serial frames. Figure 18-5 on page 201 illustrates the sampling process of the start bit of an incoming frame. The sample rate is 16 times the baud rate for Normal mode, and eight times the baud rate for Double Speed mode. The horizontal arrows illustrate the synchronization variation due to the sampling process. Note the larger time variation when using the Double Speed mode (U2Xn = 1) of operation. Samples denoted zero are samples done when the RxDn line is idle (i.e., no communication activity). Figure 18-5. Start Bit Sampling RxD IDLE START BIT 0 Sample (U2X = 0) 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 2 3 Sample (U2X = 1) 0 1 2 3 4 5 6 7 8 1 2 When the clock recovery logic detects a high (idle) to low (start) transition on the RxDn line, the start bit detection sequence is initiated. Let sample 1 denote the first zero-sample as shown in Figure 18-5. The clock ATmega16U4/32U4 [DATASHEET] 201 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

recovery logic then uses samples 8, 9, and 10 for Normal mode, and samples 4, 5, and 6 for Double Speed mode (indicated with sample numbers inside boxes on the figure), to decide if a valid start bit is received. If two or more of these three samples have logical high levels (the majority wins), the start bit is rejected as a noise spike and the Receiver starts looking for the next high to low-transition. If however, a valid start bit is detected, the clock recovery logic is synchronized and the data recovery can begin. The synchronization process is repeated for each start bit. 18.7.2 Asynchronous Data Recovery When the receiver clock is synchronized to the start bit, the data recovery can begin. The data recovery unit uses a state machine that has 16 states for each bit in Normal mode and eight states for each bit in Double Speed mode. Figure 18-6 shows the sampling of the data bits and the parity bit. Each of the samples is given a number that is equal to the state of the recovery unit. Figure 18-6. Sampling of Data and Parity Bit RxD BIT n Sample (U2X = 0) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 Sample (U2X = 1) 1 2 3 4 5 6 7 8 1 The decision of the logic level of the received bit is taken by doing a majority voting of the logic value to the three samples in the center of the received bit. The center samples are emphasized on the figure by having the sample number inside boxes. The majority voting process is done as follows: If two or all three samples have high levels, the received bit is registered to be a logic 1. If two or all three samples have low levels, the received bit is registered to be a logic 0. This majority voting process acts as a low pass filter for the incoming signal on the RxDn pin. The recovery process is then repeated until a complete frame is received. Including the first stop bit. Note that the Receiver only uses the first stop bit of a frame. Figure 18-7 on page 202 shows the sampling of the stop bit and the earliest possible beginning of the start bit of the next frame. Figure 18-7. Stop Bit Sampling and Next Start Bit Sampling RxD STOP 1 (A) (B) (C) Sample (U2X = 0) 1 2 3 4 5 6 7 8 9 10 0/1 0/1 0/1 Sample (U2X = 1) 1 2 3 4 5 6 0/1 The same majority voting is done to the stop bit as done for the other bits in the frame. If the stop bit is registered to have a logic 0 value, the Frame Error (FEn) Flag will be set. A new high to low transition indicating the start bit of a new frame can come right after the last of the bits used for majority voting. For Normal Speed mode, the first low level sample can be at point marked (A) in Figure 18- 7. For Double Speed mode the first low level must be delayed to (B). (C) marks a stop bit of full length. The early start bit detection influences the operational range of the Receiver. ATmega16U4/32U4 [DATASHEET] 202 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

18.7.3 Asynchronous Operational Range The operational range of the Receiver is dependent on the mismatch between the received bit rate and the internally generated baud rate. If the Transmitter is sending frames at too fast or too slow bit rates, or the internally generated baud rate of the Receiver does not have a similar (see Table18-6 on page208) base frequency, the Receiver will not be able to synchronize the frames to the start bit. The following equations can be used to calculate the ratio of the incoming data rate and internal receiver baud rate. D+1S D+2S R = ------------------------------------------- R = ----------------------------------- slow S–1+DS+S fast D+1S+S F M D Sum of character size and parity size (D = 5 to 10 bit) S Samples per bit. S = 16 for Normal Speed mode and S = 8 for Double Speed mode S First sample number used for majority voting. S = 8 for normal speed and S = 4 for Double Speed mode F F F S Middle sample number used for majority voting. S = 9 for normal speed and S = 5 for Double Speed mode M M M R is the ratio of the slowest incoming data rate that can be accepted in relation to the receiver baud rate slow R is the ratio of the fastest incoming data rate that can be accepted in relation to the receiver baud rate fast The following tables list the maximum receiver baud rate error that can be tolerated. Note that Normal Speed mode has higher toleration of baud rate variations. Table 18-2. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode (U2Xn = 0) D Recommended max. receiver # (Data+Parity Bit) R [%] R [%] Max. total error [%] error [%] slow fast 5 93.20 106.67 +6.67/-6.8 ±3.0 6 94.12 105.79 +5.79/-5.88 ±2.5 7 94.81 105.11 +5.11/-5.19 ±2.0 8 95.36 104.58 +4.58/-4.54 ±2.0 9 95.81 104.14 +4.14/-4.19 ±1.5 10 96.17 103.78 +3.78/-3.83 ±1.5 Table 18-3. Recommended Maximum Receiver Baud Rate Error for Double Speed Mode (U2Xn = 1) D Recommended max. receiver # (Data+Parity Bit) R [%] R [%] Max. total error [%] error [%] slow fast 5 94.12 105.66 +5.66/-5.88 ±2.5 6 94.92 104.92 +4.92/-5.08 ±2.0 7 95.52 104,35 +4.35/-4.48 ±1.5 8 96.00 103.90 +3.90/-4.00 ±1.5 9 96.39 103.53 +3.53/-3.61 ±1.5 10 96.70 103.23 +3.23/-3.30 ±1.0 The recommendations of the maximum receiver baud rate error was made under the assumption that the Receiver and Transmitter equally divides the maximum total error. ATmega16U4/32U4 [DATASHEET] 203 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

There are two possible sources for the receivers baud rate error. The Receiver’s system clock (XTAL) will always have some minor instability over the supply voltage range and the temperature range. When using a crystal to generate the system clock, this is rarely a problem, but for a resonator the system clock may differ more than 2% depending of the resonators tolerance. The second source for the error is more controllable. The baud rate generator can not always do an exact division of the system frequency to get the baud rate wanted. In this case an UBRR value that gives an acceptable low error can be used if possible. 18.8 Multi-processor Communication Mode Setting the Multi-processor Communication mode (MPCMn) bit in UCSRnA enables a filtering function of incoming frames received by the USART Receiver. Frames that do not contain address information will be ignored and not put into the receive buffer. This effectively reduces the number of incoming frames that has to be handled by the CPU, in a system with multiple MCUs that communicate via the same serial bus. The Transmitter is unaffected by the MPCMn setting, but has to be used differently when it is a part of a system utilizing the Multi-processor Communication mode. If the Receiver is set up to receive frames that contain 5 to 8 data bits, then the first stop bit indicates if the frame contains data or address information. If the Receiver is set up for frames with nine data bits, then the ninth bit (RXB8n) is used for identifying address and data frames. When the frame type bit (the first stop or the ninth bit) is one, the frame contains an address. When the frame type bit is zero the frame is a data frame. The Multi-processor Communication mode enables several slave MCUs to receive data from a master MCU. This is done by first decoding an address frame to find out which MCU has been addressed. If a particular slave MCU has been addressed, it will receive the following data frames as normal, while the other slave MCUs will ignore the received frames until another address frame is received. 18.8.1 Using MPCMn For an MCU to act as a master MCU, it can use a 9-bit character frame format (UCSZn = 7). The ninth bit (TXB8n) must be set when an address frame (TXB8n = 1) or cleared when a data frame (TXB = 0) is being transmitted. The slave MCUs must in this case be set to use a 9-bit character frame format. The following procedure should be used to exchange data in Multi-processor Communication mode: 1. All Slave MCUs are in Multi-processor Communication mode (MPCMn in UCSRnA is set). 2. The Master MCU sends an address frame, and all slaves receive and read this frame. In the Slave MCUs, the RXCn Flag in UCSRnA will be set as normal. 3. Each Slave MCU reads the UDRn Register and determines if it has been selected. If so, it clears the MPCMn bit in UCSRnA, otherwise it waits for the next address byte and keeps the MPCMn setting. 4. The addressed MCU will receive all data frames until a new address frame is received. The other Slave MCUs, which still have the MPCMn bit set, will ignore the data frames. 5. When the last data frame is received by the addressed MCU, the addressed MCU sets the MPCMn bit and waits for a new address frame from master. The process then repeats from 2. Using any of the 5- to 8-bit character frame formats is possible, but impractical since the Receiver must change between using n and n+1 character frame formats. This makes full-duplex operation difficult since the Transmitter and Receiver uses the same character size setting. If 5- to 8-bit character frames are used, the Transmitter must be set to use two stop bit (USBSn = 1) since the first stop bit is used for indicating the frame type. Do not use Read-Modify-Write instructions (SBI and CBI) to set or clear the MPCMn bit. The MPCMn bit shares the same I/O location as the TXCn Flag and this might accidentally be cleared when using SBI or CBI instructions. ATmega16U4/32U4 [DATASHEET] 204 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

18.9 Hardware Flow Control The hardware flow control can be enabled by software. CTS: (Clear to Send) RTS: (Request to Send) HOST ATmega16U4/ATm TXD TXD RXD RXD CTS CTS RTS RTS 18.9.1 Receiver Flow Control The reception flow can be controlled by hardware using the RTS pin. The aim of the flow control is to inform the external transmitter when the internal receive Fifo is full. Thus the transmitter can stop sending characters. RTS usage and so associated flow control is enabled using RTSEN bit in UCSRnD. Figure 18-8. shows a reception example. Figure 18-8. Reception Flow Control Waveform Example FIFO 0 1 2 1 0 1 Index CPU Read RXD C1 C2 C3 RTS Figure 18-9. RTS behavior RXD Start Byte0 Stop Start Byte1 Stop Start Byte2 1 additional byte may be sent if the transmitter misses the RTS trig RTS Read from CPU RTS will rise at 2/3 of the last received stop bit if the receive fifo is full. To ensure reliable transmissions, even after a RTS rise, an extra-data can still be received and stored in the Receive Shift Register. 18.9.2 Transmission Flow Control The transmission flow can be controlled by hardware using the CTS pin controlled by the external receiver. The aim of the flow control is to stop transmission when the receiver is full of data (CTS = 1). CTS usage and so associated flow control is enabled using CTSEN bit in UCSRnD. The CTS pin is sampled at each CPU write and at the middle of the last stop bit that is currently being sent. ATmega16U4/32U4 [DATASHEET] 205 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 18-10. CTS behavior Write from CPU TXD Start Byte0 Stop Start Byte1 Stop Start Byte2 sample sample sample CTS 18.10 Examples of Baud Rate Setting For standard crystal and resonator frequencies, the most commonly used baud rates for asynchronous operation can be generated by using the UBRR settings in Table18-4 on page206 to Table18-11 on page212. UBRR values which yield an actual baud rate differing less than 0.5% from the target baud rate, are bold in the table. Higher error ratings are acceptable, but the Receiver will have less noise resistance when the error ratings are high, especially for large serial frames (see “Asynchronous Operational Range” on page203). The error values are calculated using the following equation: BaudRate  Closest Match  Error[%] = --------------------------------------------------------–1 100%  BaudRate  Table 18-4. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies f = 1.0000MHz f = 1.8432MHz f = 2.0000MHz osc osc osc Baud U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 Rate [bps] UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error 2400 25 0.2% 51 0.2% 47 0.0% 95 0.0% 51 0.2% 103 0.2% 4800 12 0.2% 25 0.2% 23 0.0% 47 0.0% 25 0.2% 51 0.2% 9600 6 -7.0% 12 0.2% 11 0.0% 23 0.0% 12 0.2% 25 0.2% 14.4k 3 8.5% 8 -3.5% 7 0.0% 15 0.0% 8 -3.5% 16 2.1% 19.2k 2 8.5% 6 -7.0% 5 0.0% 11 0.0% 6 -7.0% 12 0.2% 28.8k 1 8.5% 3 8.5% 3 0.0% 7 0.0% 3 8.5% 8 -3.5% 38.4k 1 -18.6% 2 8.5% 2 0.0% 5 0.0% 2 8.5% 6 -7.0% 57.6k 0 8.5% 1 8.5% 1 0.0% 3 0.0% 1 8.5% 3 8.5% 76.8k – – 1 -18.6% 1 -25.0% 2 0.0% 1 -18.6% 2 8.5% 115.2k – – 0 8.5% 0 0.0% 1 0.0% 0 8.5% 1 8.5% 230.4k – – – – – – 0 0.0% – – – – 250k – – – – – – – – – – 0 0.0% Max.(1) 62.5kbps 125kbps 115.2kbps 230.4kbps 125kbps 250kbps Note: 1. UBRR = 0, Error = 0.0% ATmega16U4/32U4 [DATASHEET] 206 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 18-5. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies f = 3.6864MHz f = 4.0000MHz f = 7.3728MHz osc osc osc Baud U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 Rate [bps] UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error 2400 95 0.0% 191 0.0% 103 0.2% 207 0.2% 191 0.0% 383 0.0% 4800 47 0.0% 95 0.0% 51 0.2% 103 0.2% 95 0.0% 191 0.0% 9600 23 0.0% 47 0.0% 25 0.2% 51 0.2% 47 0.0% 95 0.0% 14.4k 15 0.0% 31 0.0% 16 2.1% 34 -0.8% 31 0.0% 63 0.0% 19.2k 11 0.0% 23 0.0% 12 0.2% 25 0.2% 23 0.0% 47 0.0% 28.8k 7 0.0% 15 0.0% 8 -3.5% 16 2.1% 15 0.0% 31 0.0% 38.4k 5 0.0% 11 0.0% 6 -7.0% 12 0.2% 11 0.0% 23 0.0% 57.6k 3 0.0% 7 0.0% 3 8.5% 8 -3.5% 7 0.0% 15 0.0% 76.8k 2 0.0% 5 0.0% 2 8.5% 6 -7.0% 5 0.0% 11 0.0% 115.2k 1 0.0% 3 0.0% 1 8.5% 3 8.5% 3 0.0% 7 0.0% 230.4k 0 0.0% 1 0.0% 0 8.5% 1 8.5% 1 0.0% 3 0.0% 250k 0 -7.8% 1 -7.8% 0 0.0% 1 0.0% 1 -7.8% 3 -7.8% 0.5M – – 0 -7.8% – – 0 0.0% 0 -7.8% 1 -7.8% 1M – – – – – – – – – – 0 -7.8% Max.(1) 230.4kbps 460.8kbps 250kbps 0.5Mbps 460.8kbps 921.6kbps Note: 1. UBRR = 0, Error = 0.0% ATmega16U4/32U4 [DATASHEET] 207 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 18-6. Examples of UBRRn Settings for Commonly Used Oscillator Frequencies f = 8.0000MHz f = 11.0592MHz f = 14.7456MHz osc osc osc Baud U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 U2Xn = 0 U2Xn = 1 Rate [bps] UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error 2400 207 0.2% 416 -0.1% 287 0.0% 575 0.0% 383 0.0% 767 0.0% 4800 103 0.2% 207 0.2% 143 0.0% 287 0.0% 191 0.0% 383 0.0% 9600 51 0.2% 103 0.2% 71 0.0% 143 0.0% 95 0.0% 191 0.0% 14.4k 34 -0.8% 68 0.6% 47 0.0% 95 0.0% 63 0.0% 127 0.0% 19.2k 25 0.2% 51 0.2% 35 0.0% 71 0.0% 47 0.0% 95 0.0% 28.8k 16 2.1% 34 -0.8% 23 0.0% 47 0.0% 31 0.0% 63 0.0% 38.4k 12 0.2% 25 0.2% 17 0.0% 35 0.0% 23 0.0% 47 0.0% 57.6k 8 -3.5% 16 2.1% 11 0.0% 23 0.0% 15 0.0% 31 0.0% 76.8k 6 -7.0% 12 0.2% 8 0.0% 17 0.0% 11 0.0% 23 0.0% 115.2k 3 8.5% 8 -3.5% 5 0.0% 11 0.0% 7 0.0% 15 0.0% 230.4k 1 8.5% 3 8.5% 2 0.0% 5 0.0% 3 0.0% 7 0.0% 250k 1 0.0% 3 0.0% 2 -7.8% 5 -7.8% 3 -7.8% 6 5.3% 0.5M 0 0.0% 1 0.0% – – 2 -7.8% 1 -7.8% 3 -7.8% 1M – – 0 0.0% – – – – 0 -7.8% 1 -7.8% Max.(1) 0.5Mbps 1Mbps 691.2kbps 1.3824Mbps 921.6kbps 1.8432Mbps Note: 1. UBRR = 0, Error = 0.0% ATmega16U4/32U4 [DATASHEET] 208 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

18.11 USART Register Description 18.11.1 USART I/O Data Register n– UDRn Bit 7 6 5 4 3 2 1 0 RXB[7:0] UDRn (Read) TXB[7:0] UDRn (Write) Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 The USART Transmit Data Buffer Register and USART Receive Data Buffer Registers share the same I/O address referred to as USART Data Register or UDRn. The Transmit Data Buffer Register (TXB) will be the destination for data written to the UDRn Register location. Reading the UDRn Register location will return the contents of the Receive Data Buffer Register (RXB). For 5-, 6-, or 7-bit characters the upper unused bits will be ignored by the Transmitter and set to zero by the Receiver. The transmit buffer can only be written when the UDREn Flag in the UCSRnA Register is set. Data written to UDRn when the UDREn Flag is not set, will be ignored by the USART Transmitter. When data is written to the transmit buffer, and the Transmitter is enabled, the Transmitter will load the data into the Transmit Shift Register when the Shift Register is empty. Then the data will be serially transmitted on the TxDn pin. The receive buffer consists of a two level FIFO. The FIFO will change its state whenever the receive buffer is accessed. Due to this behavior of the receive buffer, do not use Read-Modify-Write instructions (SBI and CBI) on this location. Be careful when using bit test instructions (SBIC and SBIS), since these also will change the state of the FIFO. 18.11.2 USART Control and Status Register A – UCSRnA Bit 7 6 5 4 3 2 1 0 RXCn TXCn UDREn FEn DORn UPEn U2Xn MPCMn UCSRnA Read/Write R R/W R R R R R/W R/W Initial Value 0 0 1 0 0 0 0 0 • Bit 7 – RXCn: USART Receive Complete This flag bit is set when there are unread data in the receive buffer and cleared when the receive buffer is empty (i.e., does not contain any unread data). If the Receiver is disabled, the receive buffer will be flushed and consequently the RXCn bit will become zero. The RXCn Flag can be used to generate a Receive Complete interrupt (see description of the RXCIEn bit). • Bit 6 – TXCn: USART Transmit Complete This flag bit is set when the entire frame in the Transmit Shift Register has been shifted out and there are no new data currently present in the transmit buffer (UDRn). The TXCn Flag bit is automatically cleared when a transmit complete interrupt is executed, or it can be cleared by writing a one to its bit location. The TXCn Flag can generate a Transmit Complete interrupt (see description of the TXCIEn bit). • Bit 5 – UDREn: USART Data Register Empty The UDREn Flag indicates if the transmit buffer (UDRn) is ready to receive new data. If UDREn is one, the buffer is empty, and therefore ready to be written. The UDREn Flag can generate a Data Register Empty interrupt (see description of the UDRIEn bit). UDREn is set after a reset to indicate that the Transmitter is ready. ATmega16U4/32U4 [DATASHEET] 209 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 4 – FEn: Frame Error This bit is set if the next character in the receive buffer had a Frame Error when received. I.e., when the first stop bit of the next character in the receive buffer is zero. This bit is valid until the receive buffer (UDRn) is read. The FEn bit is zero when the stop bit of received data is one. Always set this bit to zero when writing to UCSRnA. • Bit 3 – DORn: Data OverRun This bit is set if a Data OverRun condition is detected. A Data OverRun occurs when the receive buffer is full (two characters), it is a new character waiting in the Receive Shift Register, and a new start bit is detected. This bit is valid until the receive buffer (UDRn) is read. Always set this bit to zero when writing to UCSRnA. • Bit 2 – UPEn: USART Parity Error This bit is set if the next character in the receive buffer had a Parity Error when received and the Parity Checking was enabled at that point (UPMn1 = 1). This bit is valid until the receive buffer (UDRn) is read. Always set this bit to zero when writing to UCSRnA. • Bit 1 – U2Xn: Double the USART Transmission Speed This bit only has effect for the asynchronous operation. Write this bit to zero when using synchronous operation. Writing this bit to one will reduce the divisor of the baud rate divider from 16 to 8 effectively doubling the transfer rate for asynchronous communication. • Bit 0 – MPCMn: Multi-processor Communication Mode This bit enables the Multi-processor Communication mode. When the MPCMn bit is written to one, all the incoming frames received by the USART Receiver that do not contain address information will be ignored. The Transmitter is unaffected by the MPCMn setting. For more detailed information see “Multi-processor Communication Mode” on page204. 18.11.3 USART Control and Status Register n B – UCSRnB Bit 7 6 5 4 3 2 1 0 RXCIEn TXCIEn UDRIEn RXENn TXENn UCSZn2 RXB8n TXB8n UCSRnB Read/Write R/W R/W R/W R/W R/W R/W R R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 7 – RXCIEn: RX Complete Interrupt Enable n Writing this bit to one enables interrupt on the RXCn Flag. A USART Receive Complete interrupt will be generated only if the RXCIEn bit is written to one, the Global Interrupt Flag in SREG is written to one and the RXCn bit in UCSRnA is set. • Bit 6 – TXCIEn: TX Complete Interrupt Enable n Writing this bit to one enables interrupt on the TXCn Flag. A USART Transmit Complete interrupt will be generated only if the TXCIEn bit is written to one, the Global Interrupt Flag in SREG is written to one and the TXCn bit in UCSRnA is set. • Bit 5 – UDRIEn: USART Data Register Empty Interrupt Enable n Writing this bit to one enables interrupt on the UDREn Flag. A Data Register Empty interrupt will be generated only if the UDRIEn bit is written to one, the Global Interrupt Flag in SREG is written to one and the UDREn bit in UCSRnA is set. ATmega16U4/32U4 [DATASHEET] 210 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 4 – RXENn: Receiver Enable n Writing this bit to one enables the USART Receiver. The Receiver will override normal port operation for the RxDn pin when enabled. Disabling the Receiver will flush the receive buffer invalidating the FEn, DORn, and UPEn Flags. • Bit 3 – TXENn: Transmitter Enable n Writing this bit to one enables the USART Transmitter. The Transmitter will override normal port operation for the TxDn pin when enabled. The disabling of the Transmitter (writing TXENn to zero) will not become effective until ongoing and pending transmissions are completed, i.e., when the Transmit Shift Register and Transmit Buffer Register do not contain data to be transmitted. When disabled, the Transmitter will no longer override the TxDn port. • Bit 2 – UCSZn2: Character Size n The UCSZn2 bits combined with the UCSZn1:0 bit in UCSRnC sets the number of data bits (Character SiZe) in a frame the Receiver and Transmitter use. • Bit 1 – RXB8n: Receive Data Bit 8 n RXB8n is the ninth data bit of the received character when operating with serial frames with nine data bits. Must be read before reading the low bits from UDRn. • Bit 0 – TXB8n: Transmit Data Bit 8 n TXB8n is the ninth data bit in the character to be transmitted when operating with serial frames with nine data bits. Must be written before writing the low bits to UDRn. 18.11.4 USART Control and Status Register n C – UCSRnC Bit 7 6 5 4 3 2 1 0 UMSELn1 UMSELn0 UPMn1 UPMn0 USBSn UCSZn1 UCSZn0 UCPOLn UCSRnC Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Val- 0 0 0 0 0 1 1 0 ue • Bits 7:6 – UMSELn1:0 USART Mode Select These bits select the mode of operation of the USARTn as shown below. Table 18-7. UMSELn Bit Settings UMSELn1 UMSELn0 Mode 0 0 Asynchronous USART 0 1 Synchronous USART 1 0 (Reserved) 1 1 Master SPI (MSPIM)(1) Note: 1. See “USART in SPI Mode” on page214 for full description of the Master SPI Mode (MSPIM) operation • Bits 5:4 – UPMn1:0: Parity Mode These bits enable and set type of parity generation and check. If enabled, the Transmitter will automatically generate and send the parity of the transmitted data bits within each frame. The Receiver will generate a parity value for the incoming data and compare it to the UPMn setting. If a mismatch is detected, the UPEn Flag in UCSRnA will be set. ATmega16U4/32U4 [DATASHEET] 211 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 18-8. UPMn Bit Settings UPMn1 UPMn0 Parity Mode 0 0 Disabled 0 1 Reserved 1 0 Enabled, Even Parity 1 1 Enabled, Odd Parity • Bit 3 – USBSn: Stop Bit Select This bit selects the number of stop bits to be inserted by the Transmitter. The Receiver ignores this setting. Table 18-9. USBS Bit Settings USBSn Stop Bit(s) 0 1-bit 1 2-bit • Bit 2:1 – UCSZn1:0: Character Size The UCSZn1:0 bits combined with the UCSZn2 bit in UCSRnB sets the number of data bits (Character SiZe) in a frame the Receiver and Transmitter use. Table 18-10. UCSZn Bit Settings UCSZn2 UCSZn1 UCSZn0 Character Size 0 0 0 5-bit 0 0 1 6-bit 0 1 0 7-bit 0 1 1 8-bit 1 0 0 Reserved 1 0 1 Reserved 1 1 0 Reserved 1 1 1 9-bit • Bit 0 – UCPOLn: Clock Polarity This bit is used for synchronous mode only. Write this bit to zero when asynchronous mode is used. The UCPOLn bit sets the relationship between data output change and data input sample, and the synchronous clock (XCKn). Table 18-11. UCPOLn Bit Settings UCPOLn Transmitted Data Changed (Output of TxDn Pin) Received Data Sampled (Input on RxDn Pin) 0 Rising XCKn Edge Falling XCKn Edge 1 Falling XCKn Edge Rising XCKn Edge ATmega16U4/32U4 [DATASHEET] 212 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

18.11.5 USART Control and Status Register n D– UCSRnD Bit 7 6 5 4 3 2 1 0 – – – – – – CTSEN RTSEN UCSRnD Read/Write R R R R R R R/W R/W Initial Val- 0 0 0 0 0 0 0 0 ue • Bits 7:2 – Reserved bits These bits are reserved and will be read as ‘0’. Do not set these bits. • Bits 1 – CTSEN: UART CTS Signal Enable Set this bit by firmware to enable the transmission flow control signal (CTS). Transmission will be enabled only if CTS input = 0. Clear this bit to disable the transmission flow control signal. Transmission will occur without hardware condition. Data Direction Register bit must be correctly clear to enable the pin as an input. • Bits 0 – RTSEN: UART RTS Signal Enable Set this bit by firmware to enable the reception flow control signal (RTS). In this case the RTS line will automatically rise when the FIFO is full. Clear this bit to disable the reception flow control signal. Data Direction Register bit must be correctly set to enable the pin as an output. 18.11.6 USART Baud Rate Registers – UBRRLn and UBRRHn Bit 15 14 13 12 11 10 9 8 – – – – UBRR[11:8] UBRRHn UBRR[7:0] UBRRLn 7 6 5 4 3 2 1 0 Read/Write R R R R R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 • Bit 15:12 – Reserved Bits These bits are reserved for future use. For compatibility with future devices, these bit must be written to zero when UBRRH is written. • Bit 11:0 – UBRR11:0: USART Baud Rate Register This is a 12-bit register which contains the USART baud rate. The UBRRH contains the four most significant bits, and the UBRRL contains the eight least significant bits of the USART baud rate. Ongoing transmissions by the Transmitter and Receiver will be corrupted if the baud rate is changed. Writing UBRRL will trigger an immediate update of the baud rate prescaler. ATmega16U4/32U4 [DATASHEET] 213 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

19. USART in SPI Mode The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) can be set to a master SPI compliant mode of operation. The Master SPI Mode (MSPIM) has the following features: • Full Duplex, Three-wire Synchronous Data Transfer • Master Operation • Supports all four SPI Modes of Operation (Mode 0, 1, 2, and 3) • LSB First or MSB First Data Transfer (Configurable Data Order) • Queued Operation (Double Buffered) • High Resolution Baud Rate Generator • High Speed Operation (f max = f /2) XCK CK • Flexible Interrupt Generation 19.1 Overview Setting both UMSELn1:0 bits to one enables the USART in MSPIM logic. In this mode of operation the SPI master control logic takes direct control over the USART resources. These resources include the transmitter and receiver shift register and buffers, and the baud rate generator. The parity generator and checker, the data and clock recovery logic, and the RX and TX control logic is disabled. The USART RX and TX control logic is replaced by a common SPI transfer control logic. However, the pin control logic and interrupt generation logic is identical in both modes of operation. The I/O register locations are the same in both modes. However, some of the functionality of the control registers changes when using MSPIM. 19.2 Clock Generation The Clock Generation logic generates the base clock for the Transmitter and Receiver. For USART MSPIM mode of operation only internal clock generation (i.e. master operation) is supported. The Data Direction Register for the XCKn pin (DDR_XCKn) must therefore be set to one (i.e. as output) for the USART in MSPIM to operate correctly. Preferably the DDR_XCKn should be set up before the USART in MSPIM is enabled (i.e. TXENn and RXENn bit set to one). The internal clock generation used in MSPIM mode is identical to the USART synchronous master mode. The baud rate or UBRRn setting can therefore be calculated using the same equations in the following table: Equation for Calculating Baud Equation for Calculating UBRRn Operating Mode Rate(1) Value Synchronous Master BAUD = --------------f--O----S---C---------------- UBRRn = -----f--O----S---C------–1 mode 2UBRRn+1 2BAUD Note: 1. The baud rate is defined to be the transfer rate in bit per second (bps). BAUD: Baud rate (in bits per second, bps). f : System Oscillator clock frequency. OSC UBRRn: Contents of the UBRRnH and UBRRnL Registers, (0-4095). 19.3 SPI Data Modes and Timing There are four combinations of XCKn (SCK) phase and polarity with respect to serial data, which are determined by control bits UCPHAn and UCPOLn. The data transfer timing diagrams are shown in Figure 19-1. ATmega16U4/32U4 [DATASHEET] 214 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Data bits are shifted out and latched in on opposite edges of the XCKn signal, ensuring sufficient time for data signals to stabilize. The UCPOLn and UCPHAn functionality is summarized in the following table. Note that changing the setting of any of these bits will corrupt all ongoing communication for both the Receiver and Transmitter. Table 19-1. UCPOLn and UCPHAn Functionality UCPOLn UCPHAn SPI Mode Leading Edge Trailing Edge 0 0 0 Sample (Rising) Setup (Falling) 0 1 1 Setup (Rising) Sample (Falling) 1 0 2 Sample (Falling) Setup (Rising) 1 1 3 Setup (Falling) Sample (Rising) Figure 19-1. UCPHAn and UCPOLn data transfer timing diagrams. UCPOL=0 UCPOL=1 1 = A XCK XCK H P Data setup (TXD) Data setup (TXD) C U Data sample (RXD) Data sample (RXD) 0 = XCK XCK A H P Data setup (TXD) Data setup (TXD) C U Data sample (RXD) Data sample (RXD) 19.4 Frame Formats A serial frame for the MSPIM is defined to be one character of 8 data bits. The USART in MSPIM mode has two valid frame formats:  8-bit data with MSB first  8-bit data with LSB first A frame starts with the least or most significant data bit. Then the next data bits, up to a total of eight, are succeeding, ending with the most or least significant bit accordingly. When a complete frame is transmitted, a new frame can directly follow it, or the communication line can be set to an idle (high) state. The UDORDn bit in UCSRnC sets the frame format used by the USART in MSPIM mode. The Receiver and Transmitter use the same setting. Note that changing the setting of any of these bits will corrupt all ongoing communication for both the Receiver and Transmitter. 16-bit data transfer can be achieved by writing two data bytes to UDRn. A UART transmit complete interrupt will then signal that the 16-bit value has been shifted out. 19.4.1 USART MSPIM Initialization The USART in MSPIM mode has to be initialized before any communication can take place. The initialization process normally consists of setting the baud rate, setting master mode of operation (by setting DDR_XCKn to one), setting frame format and enabling the Transmitter and the Receiver. Only the transmitter can operate independently. For interrupt driven USART operation, the Global Interrupt Flag should be cleared (and thus interrupts globally disabled) when doing the initialization. ATmega16U4/32U4 [DATASHEET] 215 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Note: To ensure immediate initialization of the XCKn output the baud-rate register (UBRRn) must be zero at the time the transmitter is enabled. Contrary to the normal mode USART operation the UBRRn must then be written to the desired value after the transmitter is enabled, but before the first transmission is started. Setting UBRRn to zero before enabling the transmitter is not necessary if the initialization is done immediately after a reset since UBRRn is reset to zero. Before doing a re-initialization with changed baud rate, data mode, or frame format, be sure that there is no ongoing transmissions during the period the registers are changed. The TXCn Flag can be used to check that the Transmitter has completed all transfers, and the RXCn Flag can be used to check that there are no unread data in the receive buffer. Note that the TXCn Flag must be cleared before each transmission (before UDRn is written) if it is used for this purpose. The following simple USART initialization code examples show one assembly and one C function that are equal in functionality. The examples assume polling (no interrupts enabled). The baud rate is given as a function parameter. For the assembly code, the baud rate parameter is assumed to be stored in the r17:r16 registers. ATmega16U4/32U4 [DATASHEET] 216 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Assembly Code Example(1) USART_Init: clr r18 out UBRRnH,r18 out UBRRnL,r18 ; Setting the XCKn port pin as output, enables master mode. sbi XCKn_DDR, XCKn ; Set MSPI mode of operation and SPI data mode 0. ldi r18, (1<<UMSELn1)|(1<<UMSELn0)|(0<<UCPHAn)|(0<<UCPOLn) out UCSRnC,r18 ; Enable receiver and transmitter. ldi r18, (1<<RXENn)|(1<<TXENn) out UCSRnB,r18 ; Set baud rate. ; IMPORTANT: The Baud Rate must be set after the transmitter is enabled! out UBRRnH, r17 out UBRRnL, r18 ret C Code Example(1) void USART_Init( unsigned int baud ) { UBRRn = 0; /* Setting the XCKn port pin as output, enables master mode. */ XCKn_DDR |= (1<<XCKn); /* Set MSPI mode of operation and SPI data mode 0. */ UCSRnC = (1<<UMSELn1)|(1<<UMSELn0)|(0<<UCPHAn)|(0<<UCPOLn); /* Enable receiver and transmitter. */ UCSRnB = (1<<RXENn)|(1<<TXENn); /* Set baud rate. */ /* IMPORTANT: The Baud Rate must be set after the transmitter is enabled */ UBRRn = baud; } Note: 1. See “Code Examples” on page 8. ATmega16U4/32U4 [DATASHEET] 217 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

19.5 Data Transfer Using the USART in MSPI mode requires the Transmitter to be enabled, i.e. the TXENn bit in the UCSRnB register is set to one. When the Transmitter is enabled, the normal port operation of the TxDn pin is overridden and given the function as the Transmitter's serial output. Enabling the receiver is optional and is done by setting the RXENn bit in the UCSRnB register to one. When the receiver is enabled, the normal pin operation of the RxDn pin is overridden and given the function as the Receiver's serial input. The XCKn will in both cases be used as the transfer clock. After initialization the USART is ready for doing data transfers. A data transfer is initiated by writing to the UDRn I/O location. This is the case for both sending and receiving data since the transmitter controls the transfer clock. The data written to UDRn is moved from the transmit buffer to the shift register when the shift register is ready to send a new frame. Note: To keep the input buffer in sync with the number of data bytes transmitted, the UDRn register must be read once for each byte transmitted. The input buffer operation is identical to normal USART mode, i.e. if an overflow occurs the character last received will be lost, not the first data in the buffer. This means that if four bytes are transferred, byte 1 first, then byte 2, 3, and 4, and the UDRn is not read before all transfers are completed, then byte 3 to be received will be lost, and not byte 1. The following code examples show a simple USART in MSPIM mode transfer function based on polling of the Data Register Empty (UDREn) Flag and the Receive Complete (RXCn) Flag. The USART has to be initialized before the function can be used. For the assembly code, the data to be sent is assumed to be stored in Register R16 and the data received will be available in the same register (R16) after the function returns. The function simply waits for the transmit buffer to be empty by checking the UDREn Flag, before loading it with new data to be transmitted. The function then waits for data to be present in the receive buffer by checking the RXCn Flag, before reading the buffer and returning the value. ATmega16U4/32U4 [DATASHEET] 218 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Assembly Code Example(1) USART_MSPIM_Transfer: ; Wait for empty transmit buffer sbis UCSRnA, UDREn rjmp USART_MSPIM_Transfer ; Put data (r16) into buffer, sends the data out UDRn,r16 ; Wait for data to be received USART_MSPIM_Wait_RXCn: sbis UCSRnA, RXCn rjmp USART_MSPIM_Wait_RXCn ; Get and return received data from buffer in r16, UDRn ret C Code Example(1) unsigned char USART_Receive( void ) { /* Wait for empty transmit buffer */ while ( !( UCSRnA & (1<<UDREn)) ); /* Put data into buffer, sends the data */ UDRn = data; /* Wait for data to be received */ while ( !(UCSRnA & (1<<RXCn)) ); /* Get and return received data from buffer */ return UDRn; } Note: 1. See “Code Examples” on page 8. 19.5.1 Transmitter and Receiver Flags and Interrupts The RXCn, TXCn, and UDREn flags and corresponding interrupts in USART in MSPIM mode are identical in function to the normal USART operation. However, the receiver error status flags (FE, DOR, and PE) are not in use and is always read as zero. 19.5.2 Disabling the Transmitter or Receiver The disabling of the transmitter or receiver in USART in MSPIM mode is identical in function to the normal USART operation. ATmega16U4/32U4 [DATASHEET] 219 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

19.6 AVR USART MSPIM vs. AVR SPI The USART in MSPIM mode is fully compatible with the AVR SPI regarding:  Master mode timing diagram  The UCPOLn bit functionality is identical to the SPI CPOL bit  The UCPHAn bit functionality is identical to the SPI CPHA bit  The UDORDn bit functionality is identical to the SPI DORD bit However, since the USART in MSPIM mode reuses the USART resources, the use of the USART in MSPIM mode is somewhat different compared to the SPI. In addition to differences of the control register bits, and that only master operation is supported by the USART in MSPIM mode, the following features differ between the two modules:  The USART in MSPIM mode includes (double) buffering of the transmitter. The SPI has no buffer.  The USART in MSPIM mode receiver includes an additional buffer level  The SPI WCOL (Write Collision) bit is not included in USART in MSPIM mode  The SPI double speed mode (SPI2X) bit is not included. However, the same effect is achieved by setting UBRRn accordingly  Interrupt timing is not compatible  Pin control differs due to the master only operation of the USART in MSPIM mode A comparison of the USART in MSPIM mode and the SPI pins is shown in the table below. Table 19-2. Comparison of USART in MSPIM mode and SPI pins USART_MSPIM SPI Comment TxDn MOSI Master Out only RxDn MISO Master In only XCKn SCK (Functionally identical) (N/A) SS Not supported by USART in MSPIM ATmega16U4/32U4 [DATASHEET] 220 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

19.7 USART MSPIM Register Description The following section describes the registers used for SPI operation using the USART. 19.7.1 USART MSPIM I/O Data Register - UDRn The function and bit description of the USART data register (UDRn) in MSPI mode is identical to normal USART operation. See “USART I/O Data Register n– UDRn” on page 209. 19.7.2 USART MSPIM Control and Status Register n A - UCSRnA Bit 7 6 5 4 3 2 1 0 RXCn TXCn UDREn - - - - - UCSRnA Read/Write R/W R/W R/W R R R R R Initial Value 0 0 0 0 0 1 1 0 • Bit 7 - RXCn: USART Receive Complete This flag bit is set when there are unread data in the receive buffer and cleared when the receive buffer is empty (i.e., does not contain any unread data). If the Receiver is disabled, the receive buffer will be flushed and consequently the RXCn bit will become zero. The RXCn Flag can be used to generate a Receive Complete interrupt (see description of the RXCIEn bit). • Bit 6 - TXCn: USART Transmit Complete This flag bit is set when the entire frame in the Transmit Shift Register has been shifted out and there are no new data currently present in the transmit buffer (UDRn). The TXCn Flag bit is automatically cleared when a transmit complete interrupt is executed, or it can be cleared by writing a one to its bit location. The TXCn Flag can generate a Transmit Complete interrupt (see description of the TXCIEn bit). • Bit 5 - UDREn: USART Data Register Empty The UDREn Flag indicates if the transmit buffer (UDRn) is ready to receive new data. If UDREn is one, the buffer is empty, and therefore ready to be written. The UDREn Flag can generate a Data Register Empty interrupt (see description of the UDRIE bit). UDREn is set after a reset to indicate that the Transmitter is ready. • Bit 4:0 - Reserved Bits in MSPI mode When in MSPI mode, these bits are reserved for future use. For compatibility with future devices, these bits must be written to zero when UCSRnA is written. ATmega16U4/32U4 [DATASHEET] 221 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

19.7.3 USART MSPIM Control and Status Register n B - UCSRnB Bit 7 6 5 4 3 2 1 0 RXCIEn TXCIEn UDRIE RXENn TXENn - - - UCSRnB Read/Write R/W R/W R/W R/W R/W R R R Initial Value 0 0 0 0 0 1 1 0 • Bit 7 - RXCIEn: RX Complete Interrupt Enable Writing this bit to one enables interrupt on the RXCn Flag. A USART Receive Complete interrupt will be generated only if the RXCIEn bit is written to one, the Global Interrupt Flag in SREG is written to one and the RXCn bit in UCSRnA is set. • Bit 6 - TXCIEn: TX Complete Interrupt Enable Writing this bit to one enables interrupt on the TXCn Flag. A USART Transmit Complete interrupt will be generated only if the TXCIEn bit is written to one, the Global Interrupt Flag in SREG is written to one and the TXCn bit in UCSRnA is set. • Bit 5 - UDRIE: USART Data Register Empty Interrupt Enable Writing this bit to one enables interrupt on the UDREn Flag. A Data Register Empty interrupt will be generated only if the UDRIE bit is written to one, the Global Interrupt Flag in SREG is written to one and the UDREn bit in UCSRnA is set. • Bit 4 - RXENn: Receiver Enable Writing this bit to one enables the USART Receiver in MSPIM mode. The Receiver will override normal port operation for the RxDn pin when enabled. Disabling the Receiver will flush the receive buffer. Only enabling the receiver in MSPI mode (i.e. setting RXENn=1 and TXENn=0) has no meaning since it is the transmitter that controls the transfer clock and since only master mode is supported. • Bit 3 - TXENn: Transmitter Enable Writing this bit to one enables the USART Transmitter. The Transmitter will override normal port operation for the TxDn pin when enabled. The disabling of the Transmitter (writing TXENn to zero) will not become effective until ongoing and pending transmissions are completed, i.e., when the Transmit Shift Register and Transmit Buffer Register do not contain data to be transmitted. When disabled, the Transmitter will no longer override the TxDn port. • Bit 2:0 - Reserved Bits in MSPI mode When in MSPI mode, these bits are reserved for future use. For compatibility with future devices, these bits must be written to zero when UCSRnB is written. ATmega16U4/32U4 [DATASHEET] 222 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

19.7.4 USART MSPIM Control and Status Register n C - UCSRnC Bit 7 6 5 4 3 2 1 0 UMSELn1 UMSELn0 - - - UDORDn UCPHAn UCPOLn UCSRnC Read/Write R/W R/W R R R R/W R/W R/W Initial Value 0 0 0 0 0 1 1 0 • Bit 7:6 - UMSELn1:0: USART Mode Select These bits select the mode of operation of the USART as shown in the table. See “USART Control and Status Register n C – UCSRnC” on page211 for full description of the normal USART operation. The MSPIM is enabled when both UMSELn bits are set to one. The UDORDn, UCPHAn, and UCPOLn can be set in the same write operation where the MSPIM is enabled. Table 19-3. UMSELn Bits Settings UMSELn1 UMSELn0 Mode 0 0 Asynchronous USART 0 1 Synchronous USART 1 0 (Reserved) 1 1 Master SPI (MSPIM) • Bit 5:3 - Reserved When in MSPI mode, these bits are reserved for future use. For compatibility with future devices, these bits must be written to zero when UCSRnC is written. • Bit 2 - UDORDn: Data Order When set to one the LSB of the data word is transmitted first. When set to zero the MSB of the data word is transmitted first. For details, see “Frame Formats” on page 192. • Bit 1 - UCPHAn: Clock Phase The UCPHAn bit setting determine if data is sampled on the leasing edge (first) or tailing (last) edge of XCKn. For details, see “SPI Data Modes and Timing” on page 214. • Bit 0 - UCPOLn: Clock Polarity The UCPOLn bit sets the polarity of the XCKn clock. The combination of the UCPOLn and UCPHAn bit settings determine the timing of the data transfer. For details, see “SPI Data Modes and Timing” on page 214. 19.7.5 USART MSPIM Baud Rate Registers - UBRRnL and UBRRnH The function and bit description of the baud rate registers in MSPI mode is identical to normal USART operation. See “USART Baud Rate Registers – UBRRLn and UBRRHn” on page 213. ATmega16U4/32U4 [DATASHEET] 223 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

ATmega16U4/32U4 [DATASHEET] 224 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

20. 2-wire Serial Interface 20.1 Features • Simple Yet Powerful and Flexible Communication Interface, only two Bus Lines Needed • Both Master and Slave Operation Supported • Device can Operate as Transmitter or Receiver • 7-bit Address Space Allows up to 128 Different Slave Addresses • Multi-master Arbitration Support • Up to 400kHz Data Transfer Speed • Slew-rate Limited Output Drivers • Noise Suppression Circuitry Rejects Spikes on Bus Lines • Fully Programmable Slave Address with General Call Support • Address Recognition Causes Wake-up When AVR is in Sleep Mode 20.2 2-wire Serial Interface Bus Definition The 2-wire Serial Interface (TWI) is ideally suited for typical microcontroller applications. The TWI protocol allows the systems designer to interconnect up to 128 different devices using only two bi-directional bus lines, one for clock (SCL) and one for data (SDA). The only external hardware needed to implement the bus is a single pull-up resistor for each of the TWI bus lines. All devices connected to the bus have individual addresses, and mechanisms for resolving bus contention are inherent in the TWI protocol. Figure 20-1. TWI Bus Interconnection V CC Device 1 Device 2 Device 3 ........ Device n R1 R2 SDA SCL 20.2.1 TWI Terminology The following definitions are frequently encountered in this section. Term Description The device that initiates and terminates a transmission. The Master also generates the Master SCL clock. Slave The device addressed by a Master Transmitter The device placing data on the bus Receiver The device reading data from the bus ATmega16U4/32U4 [DATASHEET] 225 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The Power Reduction TWI bit, PRTWI bit in “Power Reduction Register 0 - PRR0” on page47 must be written to zero to enable the 2-wire Serial Interface. 20.2.2 Electrical Interconnection As depicted in Figure 20-1 on page 225, both bus lines are connected to the positive supply voltage through pull-up resistors. The bus drivers of all TWI-compliant devices are open-drain or open-collector. This implements a wired-AND function which is essential to the operation of the interface. A low level on a TWI bus line is generated when one or more TWI devices output a zero. A high level is output when all TWI devices trim- state their outputs, allowing the pull-up resistors to pull the line high. Note that all AVR devices connected to the TWI bus must be powered in order to allow any bus operation. The number of devices that can be connected to the bus is only limited by the bus capacitance limit of 400pF and the 7-bit slave address space. A detailed specification of the electrical characteristics of the TWI is given in “SPI Timing Characteristics” on page388. Two different sets of specifications are presented there, one relevant for bus speeds below 100kHz, and one valid for bus speeds up to 400kHz. 20.3 Data Transfer and Frame Format 20.3.1 Transferring Bits Each data bit transferred on the TWI bus is accompanied by a pulse on the clock line. The level of the data line must be stable when the clock line is high. The only exception to this rule is for generating start and stop conditions. Figure 20-2. Data Validity SDA SCL Data Stable Data Stable Data Change 20.3.2 START and STOP Conditions The Master initiates and terminates a data transmission. The transmission is initiated when the Master issues a START condition on the bus, and it is terminated when the Master issues a STOP condition. Between a START and a STOP condition, the bus is considered busy, and no other master should try to seize control of the bus. A special case occurs when a new START condition is issued between a START and STOP condition. This is referred to as a REPEATED START condition, and is used when the Master wishes to initiate a new transfer without relinquishing control of the bus. After a REPEATED START, the bus is considered busy until the next STOP. This is identical to the START behavior, and therefore START is used to describe both START and REPEATED START for the remainder of this datasheet, unless otherwise noted. As depicted below, START and STOP conditions are signalled by changing the level of the SDA line when the SCL line is high. ATmega16U4/32U4 [DATASHEET] 226 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 20-3. START, REPEATED START and STOP Conditions SDA SCL START STOP START REPEATED START STOP 20.3.3 Address Packet Format All address packets transmitted on the TWI bus are nine bits long, consisting of seven address bits, one READ/WRITE control bit and an acknowledge bit. If the READ/WRITE bit is set, a read operation is to be performed, otherwise a write operation should be performed. When a Slave recognizes that it is being addressed, it should acknowledge by pulling SDA low in the ninth SCL (ACK) cycle. If the addressed Slave is busy, or for some other reason can not service the Master’s request, the SDA line should be left high in the ACK clock cycle. The Master can then transmit a STOP condition, or a REPEATED START condition to initiate a new transmission. An address packet consisting of a slave address and a READ or a WRITE bit is called SLA+R or SLA+W, respectively. The MSB of the address byte is transmitted first. Slave addresses can freely be allocated by the designer, but the address 0000 000 is reserved for a general call. When a general call is issued, all slaves should respond by pulling the SDA line low in the ACK cycle. A general call is used when a Master wishes to transmit the same message to several slaves in the system. When the general call address followed by a Write bit is transmitted on the bus, all slaves set up to acknowledge the general call will pull the SDA line low in the ack cycle. The following data packets will then be received by all the slaves that acknowledged the general call. Note that transmitting the general call address followed by a Read bit is meaningless, as this would cause contention if several slaves started transmitting different data. All addresses of the format 1111 xxx should be reserved for future purposes. Figure 20-4. Address Packet Format Addr MSB Addr LSB R/W ACK SDA SCL 1 2 7 8 9 START 20.3.4 Data Packet Format All data packets transmitted on the TWI bus are nine bits long, consisting of one data byte and an acknowledge bit. During a data transfer, the Master generates the clock and the START and STOP conditions, while the Receiver is responsible for acknowledging the reception. An Acknowledge (ACK) is signalled by the Receiver pulling the SDA line low during the ninth SCL cycle. If the Receiver leaves the SDA line high, a NACK is signalled. When the Receiver has received the last byte, or for some reason cannot receive any more bytes, it ATmega16U4/32U4 [DATASHEET] 227 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

should inform the Transmitter by sending a NACK after the final byte. The MSB of the data byte is transmitted first. Figure 20-5. Data Packet Format Data MSB Data LSB ACK Aggregate SDA SDA from Transmitter SDA from Receiver SCL from Master 1 2 7 8 9 STOP, REPEATED SLA+R/W Data Byte START or Next Data Byte 20.3.5 Combining Address and Data Packets into a Transmission A transmission basically consists of a START condition, a SLA+R/W, one or more data packets and a STOP condition. An empty message, consisting of a START followed by a STOP condition, is illegal. Note that the Wired-ANDing of the SCL line can be used to implement handshaking between the Master and the Slave. The Slave can extend the SCL low period by pulling the SCL line low. This is useful if the clock speed set up by the Master is too fast for the Slave, or the Slave needs extra time for processing between the data transmissions. The Slave extending the SCL low period will not affect the SCL high period, which is determined by the Master. As a consequence, the Slave can reduce the TWI data transfer speed by prolonging the SCL duty cycle. Figure 20-6 shows a typical data transmission. Note that several data bytes can be transmitted between the SLA+R/W and the STOP condition, depending on the software protocol implemented by the application software. Figure 20-6. Typical Data Transmission Addr MSB Addr LSB R/W ACK Data MSB Data LSB ACK SDA SCL 1 2 7 8 9 1 2 7 8 9 START SLA+R/W Data Byte STOP 20.4 Multi-master Bus Systems, Arbitration and Synchronization The TWI protocol allows bus systems with several masters. Special concerns have been taken in order to ensure that transmissions will proceed as normal, even if two or more masters initiate a transmission at the same time. Two problems arise in multi-master systems:  An algorithm must be implemented allowing only one of the masters to complete the transmission. All other masters should cease transmission when they discover that they have lost the selection process. This selection process is called arbitration. When a contending master discovers that it has lost the arbitration process, it should immediately switch to Slave mode to check whether it is being addressed by ATmega16U4/32U4 [DATASHEET] 228 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

the winning master. The fact that multiple masters have started transmission at the same time should not be detectable to the slaves, i.e. the data being transferred on the bus must not be corrupted.  Different masters may use different SCL frequencies. A scheme must be devised to synchronize the serial clocks from all masters, in order to let the transmission proceed in a lockstep fashion. This will facilitate the arbitration process. The wired-ANDing of the bus lines is used to solve both these problems. The serial clocks from all masters will be wired-ANDed, yielding a combined clock with a high period equal to the one from the Master with the shortest high period. The low period of the combined clock is equal to the low period of the Master with the longest low period. Note that all masters listen to the SCL line, effectively starting to count their SCL high and low time-out periods when the combined SCL line goes high or low, respectively. Figure 20-7. SCL Synchronization Between Multiple Masters TA TA low high SCL from Master A SCL from Master B SCL Bus Line TB TB low high Masters Start Masters Start Counting Low Period Counting High Period Arbitration is carried out by all masters continuously monitoring the SDA line after outputting data. If the value read from the SDA line does not match the value the Master had output, it has lost the arbitration. Note that a Master can only lose arbitration when it outputs a high SDA value while another Master outputs a low value. The losing Master should immediately go to Slave mode, checking if it is being addressed by the winning Master. The SDA line should be left high, but losing masters are allowed to generate a clock signal until the end of the current data or address packet. Arbitration will continue until only one Master remains, and this may take many bits. If several masters are trying to address the same Slave, arbitration will continue into the data packet. ATmega16U4/32U4 [DATASHEET] 229 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 20-8. Arbitration Between Two Masters START Master A Loses Arbitration, SDA SDA SDA from A Master A SDA from Master B SDA Line Synchronized SCL Line Note that arbitration is not allowed between:  A REPEATED START condition and a data bit  A STOP condition and a data bit  A REPEATED START and a STOP condition It is the user software’s responsibility to ensure that these illegal arbitration conditions never occur. This implies that in multi-master systems, all data transfers must use the same composition of SLA+R/W and data packets. In other words: All transmissions must contain the same number of data packets, otherwise the result of the arbitration is undefined. 20.5 Overview of the TWI Module The TWI module is comprised of several submodules, as shown in Figure 20-9 on page 231. All registers drawn in a thick line are accessible through the AVR data bus. ATmega16U4/32U4 [DATASHEET] 230 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 20-9. Overview of the TWI Module SCL SDA Slew-rate Spike Slew-rate Spike Control Filter Control Filter Bus Interface Unit Bit Rate Generator START / STOP Spike Suppression Prescaler Control Address/Data Shift Bit Rate Register Arbitration detection Ack Register (TWDR) (TWBR) Address Match Unit Control Unit Address Register Status Register Control Register nit (TWAR) (TWSR) (TWCR) U WI State Machine and T Address Comparator Status control 20.5.1 SCL and SDA Pins These pins interface the AVR TWI with the rest of the MCU system. The output drivers contain a slew-rate limiter in order to conform to the TWI specification. The input stages contain a spike suppression unit removing spikes shorter than 50ns. Note that the internal pull-ups in the AVR pads can be enabled by setting the PORT bits corresponding to the SCL and SDA pins, as explained in the I/O Port section. The internal pull-ups can in some systems eliminate the need for external ones. 20.5.2 Bit Rate Generator Unit This unit controls the period of SCL when operating in a Master mode. The SCL period is controlled by settings in the TWI Bit Rate Register (TWBR) and the Prescaler bits in the TWI Status Register (TWSR). Slave operation does not depend on Bit Rate or Prescaler settings, but the CPU clock frequency in the Slave must be at least 16 times higher than the SCL frequency. Note that slaves may prolong the SCL low period, thereby reducing the average TWI bus clock period. The SCL frequency is generated according to the following equation:  TWBR = Value of the TWI Bit Rate Register CPU Clock frequency SCL frequency = ----------------------------------------------------------- TWPS 16+2(TWBR)4  TWPS = Value of the prescaler bits in the TWI Status Register Note: TWBR should be 10 or higher if the TWI operates in Master mode. If TWBR is lower than 10, the Master may produce an incorrect output on SDA and SCL for the reminder of the byte. The problem occurs when operating the TWI in Master mode, sending Start + SLA + R/W to a Slave (a Slave does not need to be connected to the bus for the condition to happen). ATmega16U4/32U4 [DATASHEET] 231 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

20.5.3 Bus Interface Unit This unit contains the Data and Address Shift Register (TWDR), a START/STOP Controller and Arbitration detection hardware. The TWDR contains the address or data bytes to be transmitted, or the address or data bytes received. In addition to the 8-bit TWDR, the Bus Interface Unit also contains a register containing the (N)ACK bit to be transmitted or received. This (N)ACK Register is not directly accessible by the application software. However, when receiving, it can be set or cleared by manipulating the TWI Control Register (TWCR). When in Transmitter mode, the value of the received (N)ACK bit can be determined by the value in the TWSR. The START/STOP Controller is responsible for generation and detection of START, REPEATED START, and STOP conditions. The START/STOP controller is able to detect START and STOP conditions even when the AVR MCU is in one of the sleep modes, enabling the MCU to wake up if addressed by a Master. If the TWI has initiated a transmission as Master, the Arbitration Detection hardware continuously monitors the transmission trying to determine if arbitration is in process. If the TWI has lost an arbitration, the Control Unit is informed. Correct action can then be taken and appropriate status codes generated. 20.5.4 Address Match Unit The Address Match unit checks if received address bytes match the seven-bit address in the TWI Address Register (TWAR). If the TWI General Call Recognition Enable (TWGCE) bit in the TWAR is written to one, all incoming address bits will also be compared against the General Call address. Upon an address match, the Control Unit is informed, allowing correct action to be taken. The TWI may or may not acknowledge its address, depending on settings in the TWCR. The Address Match unit is able to compare addresses even when the AVR MCU is in sleep mode, enabling the MCU to wake up if addressed by a Master. If another interrupt (e.g., INT0) occurs during TWI Power-down address match and wakes up the CPU, the TWI aborts operation and return to it’s idle state. If this cause any problems, ensure that TWI Address Match is the only enabled interrupt when entering Power-down. 20.5.5 Control Unit The Control unit monitors the TWI bus and generates responses corresponding to settings in the TWI Control Register (TWCR). When an event requiring the attention of the application occurs on the TWI bus, the TWI Interrupt Flag (TWINT) is asserted. In the next clock cycle, the TWI Status Register (TWSR) is updated with a status code identifying the event. The TWSR only contains relevant status information when the TWI Interrupt Flag is asserted. At all other times, the TWSR contains a special status code indicating that no relevant status information is available. As long as the TWINT Flag is set, the SCL line is held low. This allows the application software to complete its tasks before allowing the TWI transmission to continue. The TWINT Flag is set in the following situations:  After the TWI has transmitted a START/REPEATED START condition  After the TWI has transmitted SLA+R/W  After the TWI has transmitted an address byte  After the TWI has lost arbitration  After the TWI has been addressed by own slave address or general call  After the TWI has received a data byte  After a STOP or REPEATED START has been received while still addressed as a Slave  When a bus error has occurred due to an illegal START or STOP condition. 20.6 Using the TWI The AVR TWI is byte-oriented and interrupt based. Interrupts are issued after all bus events, like reception of a byte or transmission of a START condition. Because the TWI is interrupt-based, the application software is free to carry on other operations during a TWI byte transfer. Note that the TWI Interrupt Enable (TWIE) bit in TWCR ATmega16U4/32U4 [DATASHEET] 232 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

together with the Global Interrupt Enable bit in SREG allow the application to decide whether or not assertion of the TWINT Flag should generate an interrupt request. If the TWIE bit is cleared, the application must poll the TWINT Flag in order to detect actions on the TWI bus. When the TWINT Flag is asserted, the TWI has finished an operation and awaits application response. In this case, the TWI Status Register (TWSR) contains a value indicating the current state of the TWI bus. The application software can then decide how the TWI should behave in the next TWI bus cycle by manipulating the TWCR and TWDR Registers. Figure 20-10 on page 233 is a simple example of how the application can interface to the TWI hardware. In this example, a Master wishes to transmit a single data byte to a Slave. This description is quite abstract, a more detailed explanation follows later in this section. A simple code example implementing the desired behavior is also presented. Figure 20-10. Interfacing the Application to the TWI in a Typical Transmission 3. Check TWSR to see if START was 5. Check TWSR to see if SLA+W was 1. Application 7. Check TWSR to see if data was sent sent. Application loads SLA+W into sent and ACK received. n writes to TWCR to and ACK received. plicatioAction transinmitiisasteion of TsWigDnRalT,s Wa innItdNo TlTo aWisd Cws Rrait,pt empnrao tkpoirn ioa sntueer ,ce o tnhtarot l AlpoTpaWldicsCa aRtipo, pnmr olaopkaridniasgt eds auctroaen ittnrhotaol t s TTigWWnDIaNlRsT ,i naistnod Aspipglnicaalsti oton sloeandds S aTpOpPro pinrtioa tTeW coCnRtr,ol p START making sure that TWINT is written to one A and TWSTA is written to zero. written to one TWI bus START SLA+W A Data A STOP Indicates e 4. TWINT set. TWIardwarAction SSTtaAt2Ru.sT T cWcoodInNedT iitn isodenict .saetenst SStaLtuAs+ cWo dsee nint,d AicCaKtes daSttaa ts6ue.s nT ctW,o AdINCeT Kin srdeeictc.aetievesd TWINT set H received 1. The first step in a TWI transmission is to transmit a START condition. This is done by writing a specific value into TWCR, instructing the TWI hardware to transmit a START condition. Which value to write is described later on. However, it is important that the TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The TWI will not start any operation as long as the TWINT bit in TWCR is set. Immediately after the application has cleared TWINT, the TWI will initiate transmission of the START condition. 2. When the START condition has been transmitted, the TWINT Flag in TWCR is set, and TWSR is updated with a status code indicating that the START condition has successfully been sent. 3. The application software should now examine the value of TWSR, to make sure that the START condition was successfully transmitted. If TWSR indicates otherwise, the application software might take some special action, like calling an error routine. Assuming that the status code is as expected, the application must load SLA+W into TWDR. Remember that TWDR is used both for address and data. After TWDR has been loaded with the desired SLA+W, a specific value must be written to TWCR, instructing the TWI hardware to transmit the SLA+W present in TWDR. Which value to write is described later on. However, it is important that the TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The TWI will not start any operation as long as the TWINT bit in TWCR is set. Immediately after the application has cleared TWINT, the TWI will initiate transmission of the address packet. 4. When the address packet has been transmitted, the TWINT Flag in TWCR is set, and TWSR is updated with a status code indicating that the address packet has successfully been sent. The status code will also reflect whether a Slave acknowledged the packet or not. ATmega16U4/32U4 [DATASHEET] 233 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

5. The application software should now examine the value of TWSR, to make sure that the address packet was successfully transmitted, and that the value of the ACK bit was as expected. If TWSR indicates otherwise, the application software might take some special action, like calling an error routine. Assuming that the status code is as expected, the application must load a data packet into TWDR. Subsequently, a specific value must be written to TWCR, instructing the TWI hardware to transmit the data packet present in TWDR. Which value to write is described later on. However, it is important that the TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The TWI will not start any operation as long as the TWINT bit in TWCR is set. Immediately after the application has cleared TWINT, the TWI will initiate transmission of the data packet. 6. When the data packet has been transmitted, the TWINT Flag in TWCR is set, and TWSR is updated with a status code indicating that the data packet has successfully been sent. The status code will also reflect whether a Slave acknowledged the packet or not. 7. The application software should now examine the value of TWSR, to make sure that the data packet was successfully transmitted, and that the value of the ACK bit was as expected. If TWSR indicates otherwise, the application software might take some special action, like calling an error routine. Assuming that the status code is as expected, the application must write a specific value to TWCR, instructing the TWI hardware to transmit a STOP condition. Which value to write is described later on. However, it is important that the TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The TWI will not start any operation as long as the TWINT bit in TWCR is set. Immediately after the application has cleared TWINT, the TWI will initiate transmission of the STOP condition. Note that TWINT is NOT set after a STOP condition has been sent. Even though this example is simple, it shows the principles involved in all TWI transmissions. These can be summarized as follows:  When the TWI has finished an operation and expects application response, the TWINT Flag is set. The SCL line is pulled low until TWINT is cleared.  When the TWINT Flag is set, the user must update all TWI Registers with the value relevant for the next TWI bus cycle. As an example, TWDR must be loaded with the value to be transmitted in the next bus cycle.  After all TWI Register updates and other pending application software tasks have been completed, TWCR is written. When writing TWCR, the TWINT bit should be set. Writing a one to TWINT clears the flag. The TWI will then commence executing whatever operation was specified by the TWCR setting. In the following an assembly and C implementation of the example is given. Note that the code below assumes that several definitions have been made, for example by using include-files. ATmega16U4/32U4 [DATASHEET] 234 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Assembly Code Example C Example Comments ldi TWCR = r16, (1<<TWINT)|(1<<TWSTA) (1<<TWINT)|(1<<TWSTA)| | 1 (1<<TWEN) Send START condition (1<<TWEN) out TWCR, r16 wait1: while (!(TWCR & in (1<<TWINT))) r16,TWCR ; Wait for TWINT Flag set. This 2 sbrs indicates that the START r16,TWINT condition has been transmitted rjmp wait1 in if ((TWSR & 0xF8) != r16,TWSR START) andi Check value of TWI Status r16, 0xF8 ERROR(); Register. Mask prescaler bits. If cpi status different from START go to r16, START ERROR brne ERROR 3 ldi TWDR = SLA_W; r16, SLA_W TWCR = (1<<TWINT) | out (1<<TWEN); Load SLA_W into TWDR TWDR, r16 Register. Clear TWINT bit in ldi TWCR to start transmission of r16, (1<<TWINT) | address (1<<TWEN) out TWCR, r16 wait2: while (!(TWCR & in (1<<TWINT))) Wait for TWINT Flag set. This r16,TWCR ; indicates that the SLA+W has 4 sbrs been transmitted, and r16,TWINT ACK/NACK has been received. rjmp wait2 ATmega16U4/32U4 [DATASHEET] 235 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Assembly Code Example C Example Comments in if ((TWSR & 0xF8) != r16,TWSR MT_SLA_ACK) andi Check value of TWI Status r16, 0xF8 ERROR(); Register. Mask prescaler bits. If cpi status different from r16, MT_SLA_ACK MT_SLA_ACK go to ERROR brne ERROR 5 ldi TWDR = DATA; r16, DATA TWCR = (1<<TWINT) | out (1<<TWEN); TWDR, r16 Load DATA into TWDR Register. ldi Clear TWINT bit in TWCR to start r16, (1<<TWINT) | transmission of data (1<<TWEN) out TWCR, r16 wait3: while (!(TWCR & in (1<<TWINT))) Wait for TWINT Flag set. This r16,TWCR ; indicates that the DATA has been 6 sbrs transmitted, and ACK/NACK has r16,TWINT been received. rjmp wait3 in if ((TWSR & 0xF8) != r16,TWSR MT_DATA_ACK) andi Check value of TWI Status r16, 0xF8 ERROR(); Register. Mask prescaler bits. If cpi status different from r16, MT_DATA_ACK MT_DATA_ACK go to ERROR brne 7 ERROR ldi TWCR = r16, (1<<TWINT)|(1<<TWEN)| (1<<TWINT)|(1<<TWEN)| (1<<TWSTO); Transmit STOP condition (1<<TWSTO) out TWCR, r16 20.7 Transmission Modes The TWI can operate in one of four major modes. These are named Master Transmitter (MT), Master Receiver (MR), Slave Transmitter (ST) and Slave Receiver (SR). Several of these modes can be used in the same application. As an example, the TWI can use MT mode to write data into a TWI EEPROM, MR mode to read the data back from the EEPROM. If other masters are present in the system, some of these might transmit data to the TWI, and then SR mode would be used. It is the application software that decides which modes are legal. The following sections describe each of these modes. Possible status codes are described along with figures detailing data transmission in each of the modes. These figures contain the following abbreviations: S: START condition ATmega16U4/32U4 [DATASHEET] 236 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Rs: REPEATED START condition R: Read bit (high level at SDA) W: Write bit (low level at SDA) A: Acknowledge bit (low level at SDA) A: Not acknowledge bit (high level at SDA) Data: 8-bit data byte P: STOP condition SLA: Slave Address In Figure 20-12 on page 240 to Figure 20-18 on page 248, circles are used to indicate that the TWINT Flag is set. The numbers in the circles show the status code held in TWSR, with the prescaler bits masked to zero. At these points, actions must be taken by the application to continue or complete the TWI transfer. The TWI transfer is suspended until the TWINT Flag is cleared by software. When the TWINT Flag is set, the status code in TWSR is used to determine the appropriate software action. For each status code, the required software action and details of the following serial transfer are given in Table20-1 on page238 to Table20-4 on page248. Note that the prescaler bits are masked to zero in these tables. 20.7.1 Master Transmitter Mode In the Master Transmitter mode, a number of data bytes are transmitted to a Slave Receiver (see Figure 20-11). In order to enter a Master mode, a START condition must be transmitted. The format of the following address packet determines whether Master Transmitter or Master Receiver mode is to be entered. If SLA+W is transmitted, MT mode is entered, if SLA+R is transmitted, MR mode is entered. All the status codes mentioned in this section assume that the prescaler bits are zero or are masked to zero. Figure 20-11. Data Transfer in Master Transmitter Mode V CC Device 1 Device 2 MASTER SLAVE Device 3 ........ Device n R1 R2 TRANSMITTER RECEIVER SDA SCL A START condition is sent by writing the following value to TWCR: TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE value 1 X 1 0 X 1 0 X TWEN must be set to enable the 2-wire Serial Interface, TWSTA must be written to one to transmit a START condition and TWINT must be written to one to clear the TWINT Flag. The TWI will then test the 2-wire Serial Bus and generate a START condition as soon as the bus becomes free. After a START condition has been transmitted, the TWINT Flag is set by hardware, and the status code in TWSR will be 0x08 (see Table20-1 on page238). In order to enter MT mode, SLA+W must be transmitted. This is done by writing SLA+W to TWDR. ATmega16U4/32U4 [DATASHEET] 237 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Thereafter the TWINT bit should be cleared (by writing it to one) to continue the transfer. This is accomplished by writing the following value to TWCR: TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE value 1 X 0 0 X 1 0 X When SLA+W have been transmitted and an acknowledgement bit has been received, TWINT is set again and a number of status codes in TWSR are possible. Possible status codes in Master mode are 0x18, 0x20, or 0x38. The appropriate action to be taken for each of these status codes is detailed in Table20-1 on page238. When SLA+W has been successfully transmitted, a data packet should be transmitted. This is done by writing the data byte to TWDR. TWDR must only be written when TWINT is high. If not, the access will be discarded, and the Write Collision bit (TWWC) will be set in the TWCR Register. After updating TWDR, the TWINT bit should be cleared (by writing it to one) to continue the transfer. This is accomplished by writing the following value to TWCR: TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE value 1 X 0 0 X 1 0 X This scheme is repeated until the last byte has been sent and the transfer is ended by generating a STOP condition or a repeated START condition. A STOP condition is generated by writing the following value to TWCR: TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE value 1 X 0 1 X 1 0 X A REPEATED START condition is generated by writing the following value to TWCR: TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE value 1 X 1 0 X 1 0 X After a repeated START condition (state 0x10) the 2-wire Serial Interface can access the same Slave again, or a new Slave without transmitting a STOP condition. Repeated START enables the Master to switch between Slaves, Master Transmitter mode and Master Receiver mode without losing control of the bus Table 20-1. Status codes for Master Transmitter Mode Status Code Application Software Response (TWSR) Status of the 2-wire Serial Bus Prescaler Bits and 2-wire Serial Interface To/from TWDR To TWCR are 0 Hardware STA STO TWIN TWE Next Action Taken by TWI Hardware T A 0x08 A START condition has been Load SLA+W 0 0 1 X SLA+W will be transmitted; transmitted ACK or NOT ACK will be received 0x10 A repeated START condition Load SLA+W or 0 0 1 X SLA+W will be transmitted; has been transmitted ACK or NOT ACK will be received Load SLA+R 0 0 1 X SLA+R will be transmitted; Logic will switch to Master Receiver mode 0x18 SLA+W has been transmitted; Load data byte or 0 0 1 X Data byte will be transmitted and ACK or NOT ACK will ACK has been received be received No TWDR action or 1 0 1 X Repeated START will be transmitted No TWDR action or 0 1 1 X STOP condition will be transmitted and TWSTO Flag will be reset No TWDR action 1 1 1 X STOP condition followed by a START condition will be transmitted and TWSTO Flag will be reset ATmega16U4/32U4 [DATASHEET] 238 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 20-1. Status codes for Master Transmitter Mode 0x20 SLA+W has been transmitted; Load data byte or 0 0 1 X Data byte will be transmitted and ACK or NOT ACK will NOT ACK has been received be received No TWDR action or 1 0 1 X Repeated START will be transmitted No TWDR action or 0 1 1 X STOP condition will be transmitted and TWSTO Flag will be reset No TWDR action 1 1 1 X STOP condition followed by a START condition will be transmitted and TWSTO Flag will be reset 0x28 Data byte has been transmit- Load data byte or 0 0 1 X Data byte will be transmitted and ACK or NOT ACK will ted; be received ACK has been received No TWDR action or 1 0 1 X Repeated START will be transmitted No TWDR action or 0 1 1 X STOP condition will be transmitted and TWSTO Flag will be reset No TWDR action 1 1 1 X STOP condition followed by a START condition will be transmitted and TWSTO Flag will be reset 0x30 Data byte has been transmit- Load data byte or 0 0 1 X Data byte will be transmitted and ACK or NOT ACK will ted; be received NOT ACK has been received No TWDR action or 1 0 1 X Repeated START will be transmitted No TWDR action or 0 1 1 X STOP condition will be transmitted and TWSTO Flag will be reset No TWDR action 1 1 1 X STOP condition followed by a START condition will be transmitted and TWSTO Flag will be reset 0x38 Arbitration lost in SLA+W or No TWDR action or 0 0 1 X 2-wire Serial Bus will be released and not addressed data bytes Slave mode entered No TWDR action 1 0 1 X A START condition will be transmitted when the bus becomes free ATmega16U4/32U4 [DATASHEET] 239 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 20-12. Formats and States in the Master Transmitter Mode MT Successfull transmission S SLA W A DATA A P to a slave receiver $08 $18 $28 Next transfer started with a RS SLA W repeated start condition $10 Not acknowledge R received after the A P slave address $20 MR Not acknowledge received after a data A P byte $30 Aadrbdirterastsio onr ldoastt ain b syltaeve A or A Otchoenrt imnuaesster A or A Otchoenrt imnuaesster $38 $38 Aadrbdirterastsioend laoss ts alanvde A Otchoenrt imnuaesster $68 $78 $B0 To corresponding states in slave mode Any number of data bytes From master to slave DATA A and their associated acknowledge bits From slave to master This number (contained in TWSR) corresponds n to a defined state of the Two-Wire Serial Bus. The prescaler bits are zero or masked to zero 20.7.2 Master Receiver Mode In the Master Receiver mode, a number of data bytes are received from a Slave Transmitter (see Figure 20-13 on page 241). In order to enter a Master mode, a START condition must be transmitted. The format of the following address packet determines whether Master Transmitter or Master Receiver mode is to be entered. If SLA+W is transmitted, MT mode is entered, if SLA+R is transmitted, MR mode is entered. All the status codes mentioned in this section assume that the prescaler bits are zero or are masked to zero. ATmega16U4/32U4 [DATASHEET] 240 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 20-13. Data Transfer in Master Receiver Mode V CC Device 1 Device 2 MASTER SLAVE Device 3 ........ Device n R1 R2 RECEIVER TRANSMITTER SDA SCL A START condition is sent by writing the following value to TWCR: TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE value 1 X 1 0 X 1 0 X TWEN must be written to one to enable the 2-wire Serial Interface, TWSTA must be written to one to transmit a START condition and TWINT must be set to clear the TWINT Flag. The TWI will then test the 2-wire Serial Bus and generate a START condition as soon as the bus becomes free. After a START condition has been transmitted, the TWINT Flag is set by hardware, and the status code in TWSR will be 0x08 (See Table20-1 on page238). In order to enter MR mode, SLA+R must be transmitted. This is done by writing SLA+R to TWDR. Thereafter the TWINT bit should be cleared (by writing it to one) to continue the transfer. This is accomplished by writing the following value to TWCR: TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE value 1 X 0 0 X 1 0 X When SLA+R have been transmitted and an acknowledgement bit has been received, TWINT is set again and a number of status codes in TWSR are possible. Possible status codes in Master mode are 0x38, 0x40, or 0x48. The appropriate action to be taken for each of these status codes is detailed in Table20-2 on page242. Received data can be read from the TWDR Register when the TWINT Flag is set high by hardware. This scheme is repeated until the last byte has been received. After the last byte has been received, the MR should inform the ST by sending a NACK after the last received data byte. The transfer is ended by generating a STOP condition or a repeated START condition. A STOP condition is generated by writing the following value to TWCR: TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE value 1 X 0 1 X 1 0 X A REPEATED START condition is generated by writing the following value to TWCR: TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE value 1 X 1 0 X 1 0 X After a repeated START condition (state 0x10) the 2-wire Serial Interface can access the same Slave again, or a new Slave without transmitting a STOP condition. Repeated START enables the Master to switch between Slaves, Master Transmitter mode and Master Receiver mode without losing control over the bus. ATmega16U4/32U4 [DATASHEET] 241 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 20-2. Status codes for Master Receiver Mode Status Code Application Software Response (TWSR) Status of the 2-wire Serial Bus Prescaler Bits and 2-wire Serial Interface To TWCR are 0 Hardware To/from TWDR STA STO TWIN TWE Next Action Taken by TWI Hardware T A 0x08 A START condition has been Load SLA+R 0 0 1 X SLA+R will be transmitted transmitted ACK or NOT ACK will be received 0x10 A repeated START condition Load SLA+R or 0 0 1 X SLA+R will be transmitted has been transmitted ACK or NOT ACK will be received Load SLA+W 0 0 1 X SLA+W will be transmitted Logic will switch to Master Transmitter mode 0x38 Arbitration lost in SLA+R or No TWDR action or 0 0 1 X 2-wire Serial Bus will be released and not addressed NOT ACK bit Slave mode will be entered No TWDR action 1 0 1 X A START condition will be transmitted when the bus becomes free 0x40 SLA+R has been transmitted; No TWDR action or 0 0 1 0 Data byte will be received and NOT ACK will be ACK has been received returned No TWDR action 0 0 1 1 Data byte will be received and ACK will be returned 0x48 SLA+R has been transmitted; No TWDR action or 1 0 1 X Repeated START will be transmitted NOT ACK has been received No TWDR action or 0 1 1 X STOP condition will be transmitted and TWSTO Flag will be reset No TWDR action 1 1 1 X STOP condition followed by a START condition will be transmitted and TWSTO Flag will be reset 0x50 Data byte has been received; Read data byte or 0 0 1 0 Data byte will be received and NOT ACK will be ACK has been returned returned Read data byte 0 0 1 1 Data byte will be received and ACK will be returned 0x58 Data byte has been received; Read data byte or 1 0 1 X Repeated START will be transmitted NOT ACK has been returned Read data byte or 0 1 1 X STOP condition will be transmitted and TWSTO Flag will be reset Read data byte 1 1 1 X STOP condition followed by a START condition will be transmitted and TWSTO Flag will be reset ATmega16U4/32U4 [DATASHEET] 242 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 20-14. Formats and States in the Master Receiver Mode MR Successfull reception S SLA R A DATA A DATA A P from a slave receiver $08 $40 $50 $58 Next transfer started with a RS SLA R repeated start condition $10 Not acknowledge W received after the A P slave address $48 MT Aadrbdirterastsio onr ldoastt ain b syltaeve A or A Otchoenrt imnuaesster A Otchoenrt imnuaesster $38 $38 Aadrbdirterastsioend laoss ts alanvde A Otchoenrt imnuaesster $68 $78 $B0 Tsota cteosr riens sploanved inmgode Any number of data bytes From master to slave DATA A and their associated acknowledge bits From slave to master This number (contained in TWSR) corresponds n to a defined state of the Two-Wire Serial Bus. The prescaler bits are zero or masked to zero 20.7.3 Slave Receiver Mode In the Slave Receiver mode, a number of data bytes are received from a Master Transmitter (see Figure 20-15). All the status codes mentioned in this section assume that the prescaler bits are zero or are masked to zero. Figure 20-15. Data transfer in Slave Receiver mode V CC Device 1 Device 2 SLAVE MASTER Device 3 ........ Device n R1 R2 RECEIVER TRANSMITTER SDA SCL To initiate the Slave Receiver mode, TWAR and TWCR must be initialized as follows: TWAR TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE value Device’s Own Slave Address ATmega16U4/32U4 [DATASHEET] 243 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The upper seven bits are the address to which the 2-wire Serial Interface will respond when addressed by a Master. If the LSB is set, the TWI will respond to the general call address (0x00), otherwise it will ignore the general call address. TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE value 0 1 0 0 0 1 0 X TWEN must be written to one to enable the TWI. The TWEA bit must be written to one to enable the acknowledgement of the device’s own slave address or the general call address. TWSTA and TWSTO must be written to zero. When TWAR and TWCR have been initialized, the TWI waits until it is addressed by its own slave address (or the general call address if enabled) followed by the data direction bit. If the direction bit is “0” (write), the TWI will operate in SR mode, otherwise ST mode is entered. After its own slave address and the write bit have been received, the TWINT Flag is set and a valid status code can be read from TWSR. The status code is used to determine the appropriate software action. The appropriate action to be taken for each status code is detailed in Table20-3 on page245. The Slave Receiver mode may also be entered if arbitration is lost while the TWI is in the Master mode (see states 0x68 and 0x78). If the TWEA bit is reset during a transfer, the TWI will return a “Not Acknowledge” (“1”) to SDA after the next received data byte. This can be used to indicate that the Slave is not able to receive any more bytes. While TWEA is zero, the TWI does not acknowledge its own slave address. However, the 2-wire Serial Bus is still monitored and address recognition may resume at any time by setting TWEA. This implies that the TWEA bit may be used to temporarily isolate the TWI from the 2-wire Serial Bus. In all sleep modes other than Idle mode, the clock system to the TWI is turned off. If the TWEA bit is set, the interface can still acknowledge its own slave address or the general call address by using the 2-wire Serial Bus clock as a clock source. The part will then wake up from sleep and the TWI will hold the SCL clock low during the wake up and until the TWINT Flag is cleared (by writing it to one). Further data reception will be carried out as normal, with the AVR clocks running as normal. Observe that if the AVR is set up with a long start-up time, the SCL line may be held low for a long time, blocking other data transmissions. Note that the 2-wire Serial Interface Data Register – TWDR does not reflect the last byte present on the bus when waking up from these Sleep modes. ATmega16U4/32U4 [DATASHEET] 244 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 20-3. Status Codes for Slave Receiver Mode Status Code Application Software Response (TWSR) Status of the 2-wire Serial Bus Prescaler Bits and 2-wire Serial Interface Hard- To TWCR are 0 ware To/from TWDR STA STO TWIN TWE Next Action Taken by TWI Hardware T A 0x60 Own SLA+W has been received; No TWDR action or X 0 1 0 Data byte will be received and NOT ACK will be ACK has been returned returned No TWDR action X 0 1 1 Data byte will be received and ACK will be returned 0x68 Arbitration lost in SLA+R/W as No TWDR action or X 0 1 0 Data byte will be received and NOT ACK will be Master; own SLA+W has been returned received; ACK has been returned No TWDR action X 0 1 1 Data byte will be received and ACK will be returned 0x70 General call address has been No TWDR action or X 0 1 0 Data byte will be received and NOT ACK will be received; ACK has been returned returned No TWDR action X 0 1 1 Data byte will be received and ACK will be returned 0x78 Arbitration lost in SLA+R/W as No TWDR action or X 0 1 0 Data byte will be received and NOT ACK will be Master; General call address has returned been received; ACK has been re- No TWDR action X 0 1 1 Data byte will be received and ACK will be returned turned 0x80 Previously addressed with own Read data byte or X 0 1 0 Data byte will be received and NOT ACK will be SLA+W; data has been received; returned ACK has been returned Read data byte X 0 1 1 Data byte will be received and ACK will be returned 0x88 Previously addressed with own Read data byte or 0 0 1 0 Switched to the not addressed Slave mode; SLA+W; data has been received; no recognition of own SLA or GCA NOT ACK has been returned Read data byte or 0 0 1 1 Switched to the not addressed Slave mode; own SLA will be recognized; GCA will be recognized if TWGCE = “1” Read data byte or 1 0 1 0 Switched to the not addressed Slave mode; no recognition of own SLA or GCA; a START condition will be transmitted when the bus becomes free Read data byte 1 0 1 1 Switched to the not addressed Slave mode; own SLA will be recognized; GCA will be recognized if TWGCE = “1”; a START condition will be transmitted when the bus becomes free 0x90 Previously addressed with gener- Read data byte or X 0 1 0 Data byte will be received and NOT ACK will be al call; data has been received; returned ACK has been returned Read data byte X 0 1 1 Data byte will be received and ACK will be returned 0x98 Previously addressed with gener- Read data byte or 0 0 1 0 Switched to the not addressed Slave mode; al call; data has been received; no recognition of own SLA or GCA NOT ACK has been returned Read data byte or 0 0 1 1 Switched to the not addressed Slave mode; own SLA will be recognized; GCA will be recognized if TWGCE = “1” Read data byte or 1 0 1 0 Switched to the not addressed Slave mode; no recognition of own SLA or GCA; a START condition will be transmitted when the bus becomes free Read data byte 1 0 1 1 Switched to the not addressed Slave mode; own SLA will be recognized; GCA will be recognized if TWGCE = “1”; a START condition will be transmitted when the bus becomes free 0xA0 A STOP condition or repeated No action 0 0 1 0 Switched to the not addressed Slave mode; START condition has been re- no recognition of own SLA or GCA ceived while still addressed as 0 0 1 1 Switched to the not addressed Slave mode; Slave own SLA will be recognized; GCA will be recognized if TWGCE = “1” 1 0 1 0 Switched to the not addressed Slave mode; no recognition of own SLA or GCA; a START condition will be transmitted when the bus becomes free 1 0 1 1 Switched to the not addressed Slave mode; own SLA will be recognized; GCA will be recognized if TWGCE = “1”; a START condition will be transmitted when the bus becomes free ATmega16U4/32U4 [DATASHEET] 245 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 20-16. Formats and States in the Slave Receiver Mode Reception of the own slave address and one or S SLA W A DATA A DATA A P or S more data bytes. All are acknowledged $60 $80 $80 $A0 Last data byte received is not acknowledged A P or S $88 Arbitration lost as master and addressed as slave A $68 Reception of the general call address and one or more data General Call A DATA A DATA A P or S bytes $70 $90 $90 $A0 Last data byte received is not acknowledged A P or S $98 Arbitration lost as master and addressed as slave by general call A $78 Any number of data bytes From master to slave DATA A and their associated acknowledge bits From slave to master This number (contained in TWSR) corresponds n to a defined state of the Two-Wire Serial Bus. The prescaler bits are zero or masked to zero 20.7.4 Slave Transmitter Mode In the Slave Transmitter mode, a number of data bytes are transmitted to a Master Receiver (see Figure 20-17). All the status codes mentioned in this section assume that the prescaler bits are zero or are masked to zero. Figure 20-17. Data Transfer in Slave Transmitter Mode V CC Device 1 Device 2 SLAVE MASTER Device 3 ........ Device n R1 R2 TRANSMITTER RECEIVER SDA SCL To initiate the Slave Transmitter mode, TWAR and TWCR must be initialized as follows: TWAR TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE value Device’s Own Slave Address ATmega16U4/32U4 [DATASHEET] 246 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The upper seven bits are the address to which the 2-wire Serial Interface will respond when addressed by a Master. If the LSB is set, the TWI will respond to the general call address (0x00), otherwise it will ignore the general call address. TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE value 0 1 0 0 0 1 0 X TWEN must be written to one to enable the TWI. The TWEA bit must be written to one to enable the acknowledgement of the device’s own slave address or the general call address. TWSTA and TWSTO must be written to zero. When TWAR and TWCR have been initialized, the TWI waits until it is addressed by its own slave address (or the general call address if enabled) followed by the data direction bit. If the direction bit is “1” (read), the TWI will operate in ST mode, otherwise SR mode is entered. After its own slave address and the write bit have been received, the TWINT Flag is set and a valid status code can be read from TWSR. The status code is used to determine the appropriate software action. The appropriate action to be taken for each status code is detailed in Table20-4 on page248. The Slave Transmitter mode may also be entered if arbitration is lost while the TWI is in the Master mode (see state 0xB0). If the TWEA bit is written to zero during a transfer, the TWI will transmit the last byte of the transfer. State 0xC0 or state 0xC8 will be entered, depending on whether the Master Receiver transmits a NACK or ACK after the final byte. The TWI is switched to the not addressed Slave mode, and will ignore the Master if it continues the transfer. Thus the Master Receiver receives all “1” as serial data. State 0xC8 is entered if the Master demands additional data bytes (by transmitting ACK), even though the Slave has transmitted the last byte (TWEA zero and expecting NACK from the Master). While TWEA is zero, the TWI does not respond to its own slave address. However, the 2-wire Serial Bus is still monitored and address recognition may resume at any time by setting TWEA. This implies that the TWEA bit may be used to temporarily isolate the TWI from the 2-wire Serial Bus. In all sleep modes other than Idle mode, the clock system to the TWI is turned off. If the TWEA bit is set, the interface can still acknowledge its own slave address or the general call address by using the 2-wire Serial Bus clock as a clock source. The part will then wake up from sleep and the TWI will hold the SCL clock will low during the wake up and until the TWINT Flag is cleared (by writing it to one). Further data transmission will be carried out as normal, with the AVR clocks running as normal. Observe that if the AVR is set up with a long start- up time, the SCL line may be held low for a long time, blocking other data transmissions. Note that the 2-wire Serial Interface Data Register – TWDR does not reflect the last byte present on the bus when waking up from these sleep modes. ATmega16U4/32U4 [DATASHEET] 247 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 20-4. Status Codes for Slave Transmitter Mode Status Code Application Software Response (TWSR) Status of the 2-wire Serial Bus Prescaler and 2-wire Serial Interface Hard- To TWCR Bits ware To/from TWDR STA STO TWIN TWE Next Action Taken by TWI Hardware are 0 T A 0xA8 Own SLA+R has been received; Load data byte or X 0 1 0 Last data byte will be transmitted and NOT ACK should ACK has been returned be received Load data byte X 0 1 1 Data byte will be transmitted and ACK should be re- ceived 0xB0 Arbitration lost in SLA+R/W as Load data byte or X 0 1 0 Last data byte will be transmitted and NOT ACK should Master; own SLA+R has been re- be received ceived; ACK has been returned Load data byte X 0 1 1 Data byte will be transmitted and ACK should be re- ceived 0xB8 Data byte in TWDR has been Load data byte or X 0 1 0 Last data byte will be transmitted and NOT ACK should transmitted; ACK has been re- be received ceived Load data byte X 0 1 1 Data byte will be transmitted and ACK should be re- ceived 0xC0 Data byte in TWDR has been No TWDR action or 0 0 1 0 Switched to the not addressed Slave mode; transmitted; NOT ACK has been no recognition of own SLA or GCA received No TWDR action or 0 0 1 1 Switched to the not addressed Slave mode; own SLA will be recognized; GCA will be recognized if TWGCE = “1” No TWDR action or 1 0 1 0 Switched to the not addressed Slave mode; no recognition of own SLA or GCA; a START condition will be transmitted when the bus becomes free No TWDR action 1 0 1 1 Switched to the not addressed Slave mode; own SLA will be recognized; GCA will be recognized if TWGCE = “1”; a START condition will be transmitted when the bus becomes free 0xC8 Last data byte in TWDR has been No TWDR action or 0 0 1 0 Switched to the not addressed Slave mode; transmitted (TWEA = “0”); ACK no recognition of own SLA or GCA has been received No TWDR action or 0 0 1 1 Switched to the not addressed Slave mode; own SLA will be recognized; GCA will be recognized if TWGCE = “1” No TWDR action or 1 0 1 0 Switched to the not addressed Slave mode; no recognition of own SLA or GCA; a START condition will be transmitted when the bus becomes free No TWDR action 1 0 1 1 Switched to the not addressed Slave mode; own SLA will be recognized; GCA will be recognized if TWGCE = “1”; a START condition will be transmitted when the bus becomes free Figure 20-18. Formats and States in the Slave Transmitter Mode Reception of the own slave address and one or S SLA R A DATA A DATA A P or S more data bytes $A8 $B8 $C0 Arbitration lost as master and addressed as slave A $B0 Last data byte transmitted. Switched to not addressed A All 1's P or S slave (TWEA = '0') $C8 Any number of data bytes From master to slave DATA A and their associated acknowledge bits From slave to master This number (contained in TWSR) corresponds n to a defined state of the Two-Wire Serial Bus. The prescaler bits are zero or masked to zero ATmega16U4/32U4 [DATASHEET] 248 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

20.7.5 Miscellaneous States There are two status codes that do not correspond to a defined TWI state, see the table below. Status 0xF8 indicates that no relevant information is available because the TWINT Flag is not set. This occurs between other states, and when the TWI is not involved in a serial transfer. Status 0x00 indicates that a bus error has occurred during a 2-wire Serial Bus transfer. A bus error occurs when a START or STOP condition occurs at an illegal position in the format frame. Examples of such illegal positions are during the serial transfer of an address byte, a data byte, or an acknowledge bit. When a bus error occurs, TWINT is set. To recover from a bus error, the TWSTO Flag must set and TWINT must be cleared by writing a logic one to it. This causes the TWI to enter the not addressed Slave mode and to clear the TWSTO Flag (no other bits in TWCR are affected). The SDA and SCL lines are released, and no STOP condition is transmitted. Table 20-5. Miscellaneous States Status Code Application Software Response (TWSR) Status of the 2-wire Serial Bus Prescaler Bits and 2-wire Serial Interface To TWCR are 0 Hardware To/from TWDR STA STO TWIN TWE Next Action Taken by TWI Hardware T A 0xF8 No relevant state information No TWDR action No TWCR action Wait or proceed current transfer available; TWINT = “0” 0x00 Bus error due to an illegal No TWDR action 0 1 1 X Only the internal hardware is affected, no STOP condi- START or STOP condition tion is sent on the bus. In all cases, the bus is released and TWSTO is cleared. 20.7.6 Combining Several TWI Modes In some cases, several TWI modes must be combined in order to complete the desired action. Consider for example reading data from a serial EEPROM. Typically, such a transfer involves the following steps: 1. The transfer must be initiated. 2. The EEPROM must be instructed what location should be read. 3. The reading must be performed. 4. The transfer must be finished. Note that data is transmitted both from Master to Slave and vice versa. The Master must instruct the Slave what location it wants to read, requiring the use of the MT mode. Subsequently, data must be read from the Slave, implying the use of the MR mode. Thus, the transfer direction must be changed. The Master must keep control of the bus during all these steps, and the steps should be carried out as an atomical operation. If this principle is violated in a multi master system, another Master can alter the data pointer in the EEPROM between steps 2 and 3, and the Master will read the wrong data location. Such a change in transfer direction is accomplished by transmitting a REPEATED START between the transmission of the address byte and reception of the data. After a REPEATED START, the Master keeps ownership of the bus. The following figure shows the flow in this transfer. Figure 20-19. Combining Several TWI Modes to Access a Serial EEPROM Master Transmitter Master Receiver S SLA+W A ADDRESS A Rs SLA+R A DATA A P S = START Rs = REPEATED START P = STOP Transmitted from master to slave Transmitted from slave to master ATmega16U4/32U4 [DATASHEET] 249 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

20.8 Multi-master Systems and Arbitration If multiple masters are connected to the same bus, transmissions may be initiated simultaneously by one or more of them. The TWI standard ensures that such situations are handled in such a way that one of the masters will be allowed to proceed with the transfer, and that no data will be lost in the process. An example of an arbitration situation is depicted below, where two masters are trying to transmit data to a Slave Receiver. Figure 20-20. An Arbitration Example V CC Device 1 Device 2 Device 3 MASTER MASTER SLAVE ........ Device n R1 R2 TRANSMITTER TRANSMITTER RECEIVER SDA SCL Several different scenarios may arise during arbitration, as described below:  Two or more masters are performing identical communication with the same Slave. In this case, neither the Slave nor any of the masters will know about the bus contention.  Two or more masters are accessing the same Slave with different data or direction bit. In this case, arbitration will occur, either in the READ/WRITE bit or in the data bits. The masters trying to output a one on SDA while another Master outputs a zero will lose the arbitration. Losing masters will switch to not addressed Slave mode or wait until the bus is free and transmit a new START condition, depending on application software action.  Two or more masters are accessing different slaves. In this case, arbitration will occur in the SLA bits. Masters trying to output a one on SDA while another Master outputs a zero will lose the arbitration. Masters losing arbitration in SLA will switch to Slave mode to check if they are being addressed by the winning Master. If addressed, they will switch to SR or ST mode, depending on the value of the READ/WRITE bit. If they are not being addressed, they will switch to not addressed Slave mode or wait until the bus is free and transmit a new START condition, depending on application software action. This is summarized in Figure 20-21. Possible status values are given in circles. ATmega16U4/32U4 [DATASHEET] 250 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 20-21. Possible Status Codes Caused by Arbitration START SLA Data STOP Arbitration lost in SLA Arbitration lost in Data Own No 38 TWI bus will be released and not addressed slave mode will be entered Address / General Call A START condition will be transmitted when the bus becomes free received Yes Write 68/78 Data byte will be received and NOT ACK will be returned Direction Data byte will be received and ACK will be returned Read Last data byte will be transmitted and NOT ACK should be received B0 Data byte will be transmitted and ACK should be received ATmega16U4/32U4 [DATASHEET] 251 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

20.9 TWI Register Description 20.9.1 TWI Bit Rate Register – TWBR Bit 7 6 5 4 3 2 1 0 TWBR7 TWBR6 TWBR5 TWBR4 TWBR3 TWBR2 TWBR1 TWBR0 TWBR Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bits 7..0 – TWI Bit Rate Register TWBR selects the division factor for the bit rate generator. The bit rate generator is a frequency divider which generates the SCL clock frequency in the Master modes. See “Bit Rate Generator Unit” on page231 for calculating bit rates. 20.9.2 TWI Control Register – TWCR Bit 7 6 5 4 3 2 1 0 TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE TWCR Read/Write R/W R/W R/W R/W R R/W R R/W Initial Value 0 0 0 0 0 0 0 0 The TWCR is used to control the operation of the TWI. It is used to enable the TWI, to initiate a Master access by applying a START condition to the bus, to generate a Receiver acknowledge, to generate a stop condition, and to control halting of the bus while the data to be written to the bus are written to the TWDR. It also indicates a write collision if data is attempted written to TWDR while the register is inaccessible. • Bit 7 – TWINT: TWI Interrupt Flag This bit is set by hardware when the TWI has finished its current job and expects application software response. If the I-bit in SREG and TWIE in TWCR are set, the MCU will jump to the TWI Interrupt Vector. While the TWINT Flag is set, the SCL low period is stretched. The TWINT Flag must be cleared by software by writing a logic one to it. Note that this flag is not automatically cleared by hardware when executing the interrupt routine. Also note that clearing this flag starts the operation of the TWI, so all accesses to the TWI Address Register (TWAR), TWI Status Register (TWSR), and TWI Data Register (TWDR) must be complete before clearing this flag. • Bit 6 – TWEA: TWI Enable Acknowledge Bit The TWEA bit controls the generation of the acknowledge pulse. If the TWEA bit is written to one, the ACK pulse is generated on the TWI bus if the following conditions are met: 1. The device’s own slave address has been received. 2. A general call has been received, while the TWGCE bit in the TWAR is set. 3. A data byte has been received in Master Receiver or Slave Receiver mode. By writing the TWEA bit to zero, the device can be virtually disconnected from the 2-wire Serial Bus temporarily. Address recognition can then be resumed by writing the TWEA bit to one again. • Bit 5 – TWSTA: TWI START Condition Bit The application writes the TWSTA bit to one when it desires to become a Master on the 2-wire Serial Bus. The TWI hardware checks if the bus is available, and generates a START condition on the bus if it is free. However, if the bus is not free, the TWI waits until a STOP condition is detected, and then generates a new START condition to claim the bus Master status. TWSTA must be cleared by software when the START condition has been transmitted. ATmega16U4/32U4 [DATASHEET] 252 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 4 – TWSTO: TWI STOP Condition Bit Writing the TWSTO bit to one in Master mode will generate a STOP condition on the 2-wire Serial Bus. When the STOP condition is executed on the bus, the TWSTO bit is cleared automatically. In Slave mode, setting the TWSTO bit can be used to recover from an error condition. This will not generate a STOP condition, but the TWI returns to a well-defined unaddressed Slave mode and releases the SCL and SDA lines to a high impedance state. • Bit 3 – TWWC: TWI Write Collision Flag The TWWC bit is set when attempting to write to the TWI Data Register – TWDR when TWINT is low. This flag is cleared by writing the TWDR Register when TWINT is high. • Bit 2 – TWEN: TWI Enable Bit The TWEN bit enables TWI operation and activates the TWI interface. When TWEN is written to one, the TWI takes control over the I/O pins connected to the SCL and SDA pins, enabling the slew-rate limiters and spike filters. If this bit is written to zero, the TWI is switched off and all TWI transmissions are terminated, regardless of any ongoing operation. • Bit 1 – Res: Reserved Bit This bit is a reserved bit and will always read as zero. • Bit 0 – TWIE: TWI Interrupt Enable When this bit is written to one, and the I-bit in SREG is set, the TWI interrupt request will be activated for as long as the TWINT Flag is high. 20.9.3 TWI Status Register – TWSR Bit 7 6 5 4 3 2 1 0 TWS7 TWS6 TWS5 TWS4 TWS3 – TWPS1 TWPS0 TWSR Read/Write R R R R R R R/W R/W Initial Value 1 1 1 1 1 0 0 0 • Bits 7..3 – TWS: TWI Status These 5 bits reflect the status of the TWI logic and the 2-wire Serial Bus. The different status codes are described later in this section. Note that the value read from TWSR contains both the 5-bit status value and the 2-bit prescaler value. The application designer should mask the prescaler bits to zero when checking the Status bits. This makes status checking independent of prescaler setting. This approach is used in this datasheet, unless otherwise noted. • Bit 2 – Res: Reserved Bit This bit is reserved and will always read as zero. • Bits 1..0 – TWPS: TWI Prescaler Bits These bits can be read and written, and control the bit rate prescaler. ATmega16U4/32U4 [DATASHEET] 253 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 20-6. TWI Prescaler Bits TWPS1 TWPS0 Prescaler Value 0 0 1 0 1 4 1 0 16 1 1 64 To calculate bit rates, see “Bit Rate Generator Unit” on page231. The value of TWPS1..0 is used in the equation. 20.9.4 TWI Data Register – TWDR Bit 7 6 5 4 3 2 1 0 TWD7 TWD6 TWD5 TWD4 TWD3 TWD2 TWD1 TWD0 TWDR Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 1 1 1 1 1 1 1 1 In Transmit mode, TWDR contains the next byte to be transmitted. In Receive mode, the TWDR contains the last byte received. It is writable while the TWI is not in the process of shifting a byte. This occurs when the TWI Interrupt Flag (TWINT) is set by hardware. Note that the Data Register cannot be initialized by the user before the first interrupt occurs. The data in TWDR remains stable as long as TWINT is set. While data is shifted out, data on the bus is simultaneously shifted in. TWDR always contains the last byte present on the bus, except after a wake up from a sleep mode by the TWI interrupt. In this case, the contents of TWDR is undefined. In the case of a lost bus arbitration, no data is lost in the transition from Master to Slave. Handling of the ACK bit is controlled automatically by the TWI logic, the CPU cannot access the ACK bit directly. • Bits 7..0 – TWD: TWI Data Register These eight bits constitute the next data byte to be transmitted, or the latest data byte received on the 2-wire Serial Bus. 20.9.5 TWI (Slave) Address Register – TWAR Bit 7 6 5 4 3 2 1 0 TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE TWAR Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 1 1 1 1 1 1 1 0 The TWAR should be loaded with the 7-bit Slave address (in the seven most significant bits of TWAR) to which the TWI will respond when programmed as a Slave Transmitter or Receiver, and not needed in the Master modes. In multi master systems, TWAR must be set in masters which can be addressed as Slaves by other Masters. The LSB of TWAR is used to enable recognition of the general call address (0x00). There is an associated address comparator that looks for the slave address (or general call address if enabled) in the received serial address. If a match is found, an interrupt request is generated. • Bits 7..1 – TWA: TWI (Slave) Address Register These seven bits constitute the slave address of the TWI unit. ATmega16U4/32U4 [DATASHEET] 254 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 0 – TWGCE: TWI General Call Recognition Enable Bit If set, this bit enables the recognition of a General Call given over the 2-wire Serial Bus. 20.9.6 TWI (Slave) Address Mask Register – TWAMR Bit 7 6 5 4 3 2 1 0 TWAM[6:0] – TWAMR Read/Write R/W R/W R/W R/W R/W R/W R/W R Initial Value 0 0 0 0 0 0 0 0 • Bits 7..1 – TWAM: TWI Address Mask The TWAMR can be loaded with a 7-bit Slave Address mask. Each of the bits in TWAMR can mask (disable) the corresponding address bit in the TWI Address Register (TWAR). If the mask bit is set to one then the address match logic ignores the compare between the incoming address bit and the corresponding bit in TWAR. Figure 20-22 shows the address match logic in detail. Figure 20-22. TWI Address Match Logic, Block Diagram TWAR0 Address Address Match Bit 0 TWAMR0 Address Bit Comparator 0 Address Bit Comparator 6..1 • Bit 0 – Res: Reserved Bit This bit is reserved and will always read as zero. ATmega16U4/32U4 [DATASHEET] 255 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

21. USB Controller 21.1 Features • Supports full-speed and low-speed Device role • Complies with USB Specification v2.0 • Supports ping-pong mode (dual bank) • 832 bytes of DPRAM: – 1 endpoint 64 bytes max. (default control endpoint) – 1 endpoints of 256 bytes max., (one or two banks) – 5 endpoints of 64 bytes max., (one or two banks) • Crystal-less operation for low-speed mode 21.2 Block Diagram The USB controller provides the hardware to interface a USB link to a data flow stored in a double port memory (DPRAM). The USB controller requires a 48MHz ±0.25% reference clock (for Full-Speed operation), which is the output of an internal PLL. The on-chip PLL generates the internal high frequency (48MHz) clock for USB interface. The PLL clock input can be configured to use external low-power crystal oscillator, external source clock or internal RC (see Section“Crystal-less Operation”, page259). The 48MHz clock is used to generate a 12MHz Full-speed (or 1.5MHz Low-Speed) bit clock from the received USB differential data and to transmit data according to full or low speed USB device tolerance. Clock recovery is done by a Digital Phase Locked Loop (DPLL) block, which is compliant with the jitter specification of the USB bus. To comply with the USB Electrical specification, USB buffers (D+ or D-) should be powered within the 3.0 to 3.6V range. As ATmega16U4/ATmega32U4 can be powered up to 5.5V, an internal regulator provides the USB buffers power supply. Figure 21-1. USB controller Block Diagram Overview UVCC AVCC XT1 IntRC Clock Mux UCAP USB Regulator P&LL c8lkMHz PPLreLs ccaloleckr Div-by-2 clk 48MHz D- CPU DPLL Clock Recovery D+ USB Interface VBUS On-Chip USB DPRAM ATmega16U4/32U4 [DATASHEET] 256 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

21.3 Typical Application Implementation Depending on the target application power supply, the ATmega16U4/ATmega32U4 requires different hardware typical implementations. Figure 21-2. Operating Modes versus Frequency and Power-supply Max VCC (V) Operating Frequency (MHz) 5.5 16 MHz 4.5 USB compliant, with internal regulator 3.6 8 MHz 3.4 USB compliant, without internal regulator 3.0 2.7 USB not operational 2 MHz VCC min 0 21.3.1 Bus Powered Device Figure 21-3. Typical Bus Powered Application with 5V I/O UVCC AVCC VCC UCAP 1µF VBUS VBUS UDP D+ Rs=22 UDM D- Rs=22 UVSS UGND UID UID XTAL1 XTAL2 GND GND ATmega16U4/32U4 [DATASHEET] 257 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 21-4. Typical Bus Powered Application with 3V I/O 3V ERxetgeurnlaatlor UVCC AVCC VCC UCAP 1µF VBUS VBUS UDP D+ Rs=22 UDM D- Rs=22 UVSS UUVGSSND UID UID XTAL1 XTAL2 GND GND 21.3.2 Self Powered Device Figure 21-5. Typical Self Powered Application with 3.4V to 5.5V I/O EPxotweernr aSl u3p.4pVly - 5.5V UVCC AVCC VCC UCAP 1µF VBUS VBUS UDP D+ Rs=22 UDM D- Rs=22 UVSS UGND UID UID XTAL1 XTAL2 GND GND ATmega16U4/32U4 [DATASHEET] 258 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 21-6. Typical Self Powered Application with 3.0V to 3.6 I/O EPxotweernr aSl u3p.0pVly - 3.6V UVCC AVCC VCC UCAP 1µF VBUS VBUS UDP D+ Rs=22 UDM D- Rs=22 UVSS UGND UID UID XTAL1 XTAL2 GND GND 21.4 Crystal-less Operation To reduce external components count and BOM cost, the USB module can be configured to operate in low- speed mode with internal RC oscillator as input source clock for the PLL. The internal RC oscillator is factory calibrated to satisfy the USB low speed frequency accuracy within the 0°C and +40°C temperature range. For USB full-speed operation only external crystal oscillator or external source clock can be used. 21.5 Design Guidelines  Serial resistors on USB Data lines must have 22 value (±5%)  Traces from the input USB receptable (or from the cable connection in the case of a tethered device) to the USB microcontroller pads should be as short as possible, and follow differential traces routing rules (same length, as near as possible, avoid via accumulation)  Voltage transient / ESD suppressors may also be used to prevent USB pads to be damaged by external disturbances  U capacitor should be 1µF (±10%) for correct operation cap  A 10µF capacitor is highly recommended on VBUS line ATmega16U4/32U4 [DATASHEET] 259 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

21.6 General Operating Modes 21.6.1 Introduction The USB controller is disabled and reset after an hardware reset generated by: ̶ Power on reset ̶ External reset ̶ Watchdog reset ̶ Brown out reset ̶ JTAG reset But another available and optional CPU reset source is: ̶ USB End Of Reset In this case, the USB controller is reset, but not disabled (so that the device remains attached). 21.6.2 Power-on and Reset The next diagram explains the USB controller main states on power-on: Figure 21-7. USB Controller States after Reset Clock stopped <any other FRZCLK=1 USBE=0 state> Macro off Reset HW RESET USBE=1 USBE=0 USBE=0 Device USB Controller state after an hardware reset is ‘Reset’. In this state:  USBE is not set  the USB controller clock is stopped in order to minimize the power consumption (FRZCLK=1),  the USB controller is disabled,  the USB pad is in the suspend mode,  the Device USB controller internal state is reset. After setting USBE, the USB Controller enters the Device state. The controller is ‘Idle’. The USB Controller can at any time be stopped by clearing USBE. In fact, clearing USBE acts as an hardware reset. 21.6.3 Interrupts Two interrupts vectors are assigned to USB interface. ATmega16U4/32U4 [DATASHEET] 260 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 21-8. USB Interrupt System USB General Interrupt USB General Interrupt Vector USB Device Interrupt Endpoint USB Endpoint/Pipe Interrupt Interrupt Vector The USB hardware module distinguishes between USB General events and USB Endpoint events that are relevant with data transfers relative to each endpoint. Figure 21-9. USB General Interrupt Vector Sources USB General VBUSTI Interrupt Vector USBINT.0 VBUSTE USBCON.0 UPRSMI UDINT.6 UPRSME UDIEN.6 EORSMI UDINT.5 EORSME UDIEN.5 WAKEUPI UDINT.4 WAKEUPE USB Device USB General UDIEN.4 Interrupt Interrupt Vector EORSTI UDINT.3 EORSTE UDIEN.3 SOFI UDINT.2 SOFE UDIEN.2 SUSPI UDINT.0 Asynchronous Interrupt source SUSPE (allows the CPU to wake up from power down mode) UDIEN.0 Almost all these interrupts are time-relative events that will be detected only if the USB clock is enabled (FRZCLK bit set), except for:  VBUS plug-in detection (insert, remove)  WAKEUP interrupt that will trigger each time a state change is detected on the data lines This asynchronous interrupts allow to wake-up a device that is in power-down mode, generally after that the USB has entered the Suspend state. ATmega16U4/32U4 [DATASHEET] 261 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 21-10. USB Endpoint Interrupt Vector Sources Endpoint 6 Endpoint 5 Endpoint 4 Endpoint 3 Endpoint 2 Endpoint 1 Endpoint 0 OVERFI UESTAX.6 UNDERFI FLERRE UESTAX.5 UEIENX.7 NAKINI UEINTX.6 NAKINE UEIENX.6 NAKOUTI UEINTX.4 TXSTPE UEIENX.4 USB Endpoint Interrupt Vector RXSTPI EPINT UEINTX.3 RXSTPE UEINT.X UEIENX.3 RXOUTI UEINTX.2 RXOUTE UEIENX.2 STALLEDI UEINTX.1 STALLEDE UEIENX.1 TXINI UEINTX.0 TXINE UEIENX.0 Each endpoint has eight interrupts sources associated with flags, and each source can be enabled or not to trigger the corresponding endpoint interrupt. If, for an endpoint, at least one of the sources is enabled to trigger interrupt, the corresponding event(s) will make the program branch to the USB Endpoint Interrupt vector. The user may determine the source (endpoint) of the interrupt by reading the UEINT register, and then handle the event detected by polling the different flags. 21.7 Power Modes 21.7.1 Idle Mode In this mode, the CPU core is halted (CPU clock stopped). The Idle mode is taken wether the USB controller is running or not. The CPU “wakes up” on any USB interrupts. 21.7.2 Power Down In this mode, the oscillator is stopped and halts all the clocks (CPU and peripherals). The USB controller “wakes up” when:  the WAKEUPI interrupt is triggered  the VBUSTI interrupt is triggered 21.7.3 Freeze Clock The firmware has the ability to reduce the power consumption by setting the FRZCLK bit, which freeze the clock of USB controller. When FRZCLK is set, it is still possible to access to the following registers: ATmega16U4/32U4 [DATASHEET] 262 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

 USBCON, USBSTA, USBINT  UDCON (detach, ..)  UDINT  UDIEN Moreover, when FRZCLK is set, only the following interrupts may be triggered:  WAKEUPI  VBUSTI 21.8 Speed Control The speed selection (Full Speed or Low Speed) depends on the D+/D- pull-up. The LSM bit in UDCON register allows to select an internal pull up on D- (Low Speed mode) or D+ (Full Speed mode) data lines. Figure 21-11. Device Mode Speed Selection UCAP USB Regulator DETACH UDCON.0 LSM UDCON.2 RPU RPU D+ D- 21.9 Memory Management The controller only supports the following memory allocation management. The reservation of a Pipe or an Endpoint can only be made in the increasing order (Pipe/Endpoint 0 to the last Pipe/Endpoint). The firmware shall thus configure them in the same order. The reservation of a Pipe or an Endpoint “ki” is done when its ALLOC bit is set. Then, the hardware allocates the memory and inserts it between the Pipe/Endpoints “ki-1” and “ki+1”. The “ki+1” Pipe/Endpoint memory “slides” up and its data is lost. Note that the “ki+2” and upper Pipe/Endpoint memory does not slide. Clearing a Pipe enable (PEN) or an Endpoint enable (EPEN) does not clear either its ALLOC bit, or its configuration (EPSIZE/PSIZE, EPBK/PBK). To free its memory, the firmware should clear ALLOC. Then, the “ki+1” Pipe/Endpoint memory automatically “slides” down. Note that the “ki+2” and upper Pipe/Endpoint memory does not slide. The following figure illustrates the allocation and reorganization of the USB memory in a typical example: ATmega16U4/32U4 [DATASHEET] 263 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 21-1. Allocation and Reorganization USB Memory Flow Free memory Free memory Free memory Free memory 5 5 5 5 4 Conflict 4 4 Lost memory 3 EPEN=0 4 3 (bigger size) (ALLOC=1) 2 2 2 2 1 1 1 1 0 0 0 0 EPEN=1 ALLOC=1 Endpoints Free its memory Endpoint Endpoint Disable activation (ALLOC=0) Activatation  First, Endpoint 0 to Endpoint 5 are configured, in the growing order. The memory of each is reserved in the DPRAM.  Then, the Endpoint 3 is disabled (EPEN=0), but its memory reservation is internally kept by the controller  Its ALLOC bit is cleared: the Endpoint 4 “slides” down, but the Endpoint 5 does not “slide”  Finally, if the firmware chooses to reconfigure the Endpoint 3, with a bigger size. The controller reserved the memory after the Endpoint 2 memory and automatically “slide” the Endpoint 4. The Endpoint 5 does not move and a memory conflict appear, in that both Endpoint 4 and 5 use a common area. The data of those endpoints are potentially lost. Note that:  the data of Endpoint 0 are never lost whatever the activation or deactivation of the higher Endpoint. Its data is lost if it is deactivated.  Deactivate and reactivate the same Endpoint with the same parameters does not lead to a “slide” of the higher endpoints. For those endpoints, the data are preserved.  CFGOK is set by hardware even in the case where there is a “conflict” in the memory allocation 21.10 PAD Suspend The next figures illustrates the pad behaviour:  In the “idle” mode, the pad is put in low power consumption mode  In the “active” mode, the pad is working ATmega16U4/32U4 [DATASHEET] 264 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 21-12. Pad Behaviour USBE=1 & DETACH=0 & suspend Idle mode USBE=0 Active mode | DETACH=1 | suspend The SUSPI flag indicated that a suspend state has been detected on the USB bus. This flag automatically put the USB pad in Idle. The detection of a non-idle event sets the WAKEUPI flag and wakes-up the USB pad. SUSPI Suspend detected = USB pad power down Clear Suspend by software WAKEUPI Clear Resume by software Resume = USB pad wake-up PAD status Active Power Down Active Moreover, the pad can also be put in the “idle” mode if the DETACH bit is set. It come back in the active mode when the DETACH bit is cleared. 21.11 Plug-in Detection The USB connection is detected by the VBUS pad, thanks to the following architecture: Figure 21-13. Plug-in Detection Input Block Diagram VDD RPU Session_valid VBUS VBUS VBUSTI USBSTA.0 USBINT.0 RPU VSS Pad logic The control logic of the VBUS pad outputs a signal regarding the VBUS voltage level: ATmega16U4/32U4 [DATASHEET] 265 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

 The “Session_valid” signal is active high when the voltage on the UVBUS pad is higher or equal to 1.4V. If lower than 1.4V, the signal is not active  The VBUS status bit is set when “Session_valid” signal is active (VBUS > 1.4V)  The VBUSTI flag is set each time the VBUS state changes  The USB peripheral cannot attach to the bus while VBUS bit is not set 21.12 USB Software Operating Modes Depending on the USB operating mode, the software should perform some the following operations: Power On the USB interface  Power-On USB pads regulator  Configure PLL interface  Enable PLL  Check PLL lock  Enable USB interface  Configure USB interface (USB speed, Endpoints configuration...)  Wait for USB VBUS information connection  Attach USB device Power Off the USB interface  Detach USB interface  Disable USB interface  Disable PLL  Disable USB pad regulator Suspending the USB interface  Clear Suspend Bit  Freeze USB clock  Disable PLL  Be sure to have interrupts enable to exit sleep mode  Make the MCU enter sleep mode Resuming the USB interface  Enable PLL  Wait PLL lock  Unfreeze USB clock  Clear Resume information ATmega16U4/32U4 [DATASHEET] 266 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

21.13 Registers Description 21.13.1USB General Registers Bit 7 6 5 4 3 2 1 0 - - - - - - - UVREGE UHWCON Read/Write R/W R/W R R/W R R R R/W Initial Value 0 0 0 0 0 0 0 0 • Bits 7:1 – Reserved These bits are reserved. Do not modify these bits. • Bit 0 – UVREGE: USB pad regulator Enable Set to enable the USB pad regulator. Clear to disable the USB pad regulator. Bit 7 6 5 4 3 2 1 0 USBE - FRZCLK OTGPADE - - - VBUSTE USBCON Read/Write R/W R/W R/W R/W R R R/W R/W Initial Val- 0 0 1 0 0 0 0 0 ue • Bit 7 – USBE: USB macro Enable Bit Set to enable the USB controller. Clear to disable and reset the USB controller, to disable the USB transceiver and to disable the USB controller clock inputs. • Bit 6 –Reserved The value read from these bits is always 0. Do not set these bits. • Bit 5 – FRZCLK: Freeze USB Clock Bit Set to disable the clock inputs (the ”Resume Detection” is still active). This reduces the power consumption. Clear to enable the clock inputs. • Bit 4 –OTGPADE: VBUS Pad Enable Set to enable the VBUS pad. Clear to disable the VBUS pad. Note that this bit can be set/cleared even if USBE=0. That allows the VBUS detection even if the USB macro is disable. • Bits 3:1 –Reserved The value read from these bits is always 0. Do not set these bits. • Bit 0 – VBUSTE: VBUS Transition Interrupt Enable Bit Set this bit to enable the VBUS Transition interrupt generation. Clear this bit to disable the VBUS Transition interrupt generation. ATmega16U4/32U4 [DATASHEET] 267 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Bit 7 6 5 4 3 2 1 0 - - - - - - ID VBUS USBSTA Read/Write R R R R R R R R Initial Val- 0 0 0 0 0 0 1 0 ue • Bits 7:2 - Reserved The value read from these bits is always 0. Do not set these bits. • Bit 1 - ID: ID status This bit is always read as “1”, it has been conserved for compatibility with AT90USB64/128 (in which it indicates the value of the OTG ID pin). • Bit 0 – VBUS: VBus Flag The value read from this bit indicates the state of the VBUS pin. This bit can be used in device mode to monitor the USB bus connection state of the application. See “Plug-in Detection” on page265 for more details. Bit 7 6 5 4 3 2 1 0 - - - - - - - VBUSTI USBINT Read/Write R R R R R R R/W R/W Initial Val- 0 0 0 0 0 0 0 0 ue • Bits 7:1 - Reserved The value read from these bits is always 0. Do not set these bits. • Bit 0 – VBUSTI: IVBUS Transition Interrupt Flag Set by hardware when a transition (high to low, low to high) has been detected on the VBUS pad. This shall be cleared by software (see “USB Software Operating Modes” on page266). ATmega16U4/32U4 [DATASHEET] 268 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

ATmega16U4/32U4 [DATASHEET] 269 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

22. USB Device Operating Modes 22.1 Introduction The USB device controller supports full speed and low speed data transfers. In addition to the default control endpoint, it provides six other endpoints, which can be configured in control, bulk, interrupt or isochronous modes:  Endpoint 0:programmable size FIFO up to 64 bytes, default control endpoint  Endpoints 1 programmable size FIFO up to 256 bytes in ping-pong mode  Endpoints 2 to 6: programmable size FIFO up to 64 bytes in ping-pong mode The controller starts in the “idle” mode. In this mode, the pad consumption is reduced to the minimum. 22.2 Power-on and Reset The next diagram explains the USB device controller main states on power-on: Figure 22-1. USB Device Controller States after Reset <any other USBE=0 state> USBE=0 Idle Reset USBE=1 HW RESET The reset state of the Device controller is:  the macro clock is stopped in order to minimize the power consumption (FRZCLK set)  the USB device controller internal state is reset (all the registers are reset to their default value. Note that DETACH is set.)  the endpoint banks are reset  the D+ or D- pull up are not activated (mode Detach) The D+ or D- pull-up will be activated as soon as the DETACH bit is cleared and VBUS is present. The macro is in the ‘Idle’ state after reset with a minimum power consumption and does not need to have the PLL activated to enter this state. The USB device controller can at any time be reset by clearing USBE (disable USB interface). 22.3 Endpoint Reset An endpoint can be reset at any time by setting in the UERST register the bit corresponding to the endpoint (EPRSTx). This resets:  the internal state machine on that endpoint  the Rx and Tx banks are cleared and their internal pointers are restored  the UEINTX, UESTA0X and UESTA1X are restored to their reset value The data toggle field remains unchanged. The other registers remain unchanged. The endpoint configuration remains active and the endpoint is still enabled. ATmega16U4/32U4 [DATASHEET] 270 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The endpoint reset may be associated with a clear of the data toggle command (RSTDT bit) as an answer to the CLEAR_FEATURE USB command. 22.4 USB Reset When an USB reset is detected on the USB line (SE0 state with a minimum duration of 2.5µs), the next operations are performed by the controller:  all the endpoints are disabled  the default control endpoint remains configured (see “Endpoint Reset” on page270 for more details) If the CPU hardware reset function is activated (RSTCPU bit set in UDCON register), a reset is generated to the CPU core without disabling the USB controller (that follows the same behavior than after a standard USB End of Reset, and remains attached). That feature may be used to enhance device reliability. 22.5 Endpoint Selection Prior to any operation performed by the CPU, the endpoint must first be selected. This is done by setting the EPNUM2:0 bits (UENUM register) with the endpoint number which will be managed by the CPU. The CPU can then access to the various endpoint registers and data. 22.6 Endpoint Activation The endpoint is maintained under reset as long as the EPEN bit is not set. The following flow must be respected in order to activate an endpoint: Figure 22-2. Endpoint Activation Flow: Endpoint Activation UENUM Select the endpoint EPNUM=x EPEN=1 Activate the endpoint UECFG0X Configure: EPDIR - the endpoint direction EPTYPE - the endpoint type ... UECFG1X Configure: - the endpoint size ALLOC - the bank parametrization EPSIZE Allocation and reorganization of EPBK the memory is made on-the-fly Test the correct endpoint CFGOK=1 configuration No Yes Endpoint activated ERROR As long as the endpoint is not correctly configured (CFGOK cleared), the hardware does not acknowledge the packets sent by the host. CFGOK is will not be sent if the Endpoint size parameter is bigger than the DPRAM size. ATmega16U4/32U4 [DATASHEET] 271 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

A clear of EPEN acts as an endpoint reset (see “Endpoint Reset” on page270 for more details). It also performs the next operation:  The configuration of the endpoint is kept (EPSIZE, EPBK, ALLOC kept)  It resets the data toggle field  The DPRAM memory associated to the endpoint is still reserved See “Memory Management” on page263 for more details about the memory allocation/reorganization. 22.7 Address Setup The USB device address is set up according to the USB protocol:  the USB device, after power-up, responds at address 0  the host sends a SETUP command (SET_ADDRESS(addr))  the firmware handles this request, and records that address in UADD, but keep ADDEN cleared  the USB device firmware sends an IN command of 0 bytes (IN 0 Zero Length Packet)  then, the firmware can enable the USB device address by setting ADDEN. The only accepted address by the controller is the one stored in UADD. ADDEN and UADD shall not be written at the same time. UADD contains the default address 00h after a power-up or USB reset. ADDEN is cleared by hardware:  after a power-up reset  when an USB reset is received  or when the macro is disabled (USBE cleared) When this bit is cleared, the default device address 00h is used. 22.8 Suspend, Wake-up and Resume After a period of 3ms during which the USB line was inactive, the controller switches to the full-speed mode and triggers (if enabled) the SUSPI (suspend) interrupt. The firmware may then set the FRZCLK bit. The CPU can also, depending on software architecture, enter in the idle mode to lower again the power consumption. There are two ways to recover from the “Suspend” mode:  First one is to clear the FRZCLK bit. This is possible if the CPU is not in the Idle mode.  Second way, if the CPU is “idle”, is to enable the WAKEUPI interrupt (WAKEUPE set). Then, as soon as an non-idle signal is seen by the controller, the WAKEUPI interrupt is triggered. The firmware shall then clear the FRZCLK bit to restart the transfer. There are no relationship between the SUSPI interrupt and the WAKEUPI interrupt: the WAKEUPI interrupt is triggered as soon as there are non-idle patterns on the data lines. Thus, the WAKEUPI interrupt can occurs even if the controller is not in the “suspend” mode. When the WAKEUPI interrupt is triggered, if the SUSPI interrupt bit was already set, it is cleared by hardware. When the SUSPI interrupt is triggered, if the WAKEUPI interrupt bit was already set, it is cleared by hardware. 22.9 Detach The reset value of the DETACH bit is 1. It is possible to re-enumerate a device, simply by setting and clearing the DETACH bit (but firmware must take in account a debouncing delay of some milliseconds). ATmega16U4/32U4 [DATASHEET] 272 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

 Setting DETACH will disconnect the pull-up on the D+ or D- pad (depending on full or low speed mode selected). Then, clearing DETACH will connect the pull-up on the D+ or D- pad. Figure 22-3. Detach a Device in Full-speed UVREF UVREF D + D + D - D - Detach, then EN=1 Attach EN=1 22.10 Remote Wake-up The “Remote Wake-up” (or “upstream resume”) feature is the only operation allowed to be sent by the device on its own initiative. Anyway, to do that, the device should first have received a DEVICE_REMOTE_WAKEUP request from the host.  First, the USB controller must have detected the “suspend” state of the line: the remote wake-up can only be sent when a SUSPI flag is set  The firmware has then the ability to set RMWKUP to send the “upstream resume” stream. This will automatically be done by the controller after 5ms of inactivity on the USB line.  When the controller starts to send the “upstream resume”, the UPRSMI interrupt is triggered (if enabled). SUSPI is cleared by hardware  RMWKUP is cleared by hardware at the end of the “upstream resume”  If the controller detects a good “End Of Resume” signal from the host, an EORSMI interrupt is triggered (if enabled) 22.11 STALL Request For each endpoint, the STALL management is performed using two bits: ̶ STALLRQ (enable stall request) ̶ STALLRQC (disable stall request) ̶ STALLEDI (stall sent interrupt) To send a STALL handshake at the next request, the STALLRQ request bit has to be set. All following requests will be handshak’ed with a STALL until the STALLRQC bit is set. Setting STALLRQC automatically clears the STALLRQ bit. The STALLRQC bit is also immediately cleared by hardware after being set by software. Thus, the firmware will never read this bit as set. Each time the STALL handshake is sent, the STALLEDI flag is set by the USB controller and the EPINTx interrupt will be triggered (if enabled). The incoming packets will be discarded (RXOUTI and RWAL will not be set). The host will then send a command to reset the STALL: the firmware just has to set the STALLRQC bit and to reset the endpoint. 22.11.1 Special Consideration for Control Endpoints A SETUP request is always ACK’ed. ATmega16U4/32U4 [DATASHEET] 273 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

If a STALL request is set for a Control Endpoint and if a SETUP request occurs, the SETUP request has to be ACK’ed and the STALLRQ request and STALLEDI sent flags are automatically reset (RXSETUPI set, TXIN cleared, STALLED cleared, TXINI cleared...). This management simplifies the enumeration process management. If a command is not supported or contains an error, the firmware set the STALL request flag and can return to the main task, waiting for the next SETUP request. This function is compliant with the Chapter 8 test that may send extra status for a GET_DESCRIPTOR. The firmware sets the STALL request just after receiving the status. All extra status will be automatically STALL’ed until the next SETUP request. 22.11.2 STALL Handshake and Retry Mechanism The Retry mechanism has priority over the STALL handshake. A STALL handshake is sent if the STALLRQ request bit is set and if there is no retry required. 22.12 CONTROL Endpoint Management A SETUP request is always ACK’ed. When a new setup packet is received, the RXSTPI interrupt is triggered (if enabled). The RXOUTI interrupt is not triggered. The FIFOCON and RWAL fields are irrelevant with CONTROL endpoints. The firmware shall thus never use them on that endpoints. When read, their value is always 0. CONTROL endpoints are managed by the following bits:  RXSTPI is set when a new SETUP is received. It shall be cleared by firmware to acknowledge the packet and to clear the endpoint bank.  RXOUTI is set when a new OUT data is received. It shall be cleared by firmware to acknowledge the packet and to clear the endpoint bank.  TXINI is set when the bank is ready to accept a new IN packet. It shall be cleared by firmware to send the packet and to clear the endpoint bank. 22.12.1Control Write The next figure shows a control write transaction. During the status stage, the controller will not necessary send a NAK at the first IN token:  If the firmware knows the exact number of descriptor bytes that must be read, it can then anticipate on the status stage and send a ZLP for the next IN token  or it can read the bytes and poll NAKINI, which tells that all the bytes have been sent by the host, and the transaction is now in the status stage SETUP DATA STATUS USB line SETUP OUT OUT IN IN NAK RXSTPI HW SW RXOUTI HW SW HW SW TXINI SW ATmega16U4/32U4 [DATASHEET] 274 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

22.12.2Control Read The next figure shows a control read transaction. The USB controller has to manage the simultaneous write requests from the CPU and the USB host: SETUP DATA STATUS USB line SETUP IN IN OUT OUT NAK RXSTPI HW SW RXOUTI HW SW TXINI SW HW SW Wr Enable HOST Wr Enable CPU A NAK handshake is always generated at the first status stage command. When the controller detect the status stage, all the data written by the CPU are erased, and clearing TXINI has no effects. The firmware checks if the transmission is complete or if the reception is complete. The OUT retry is always ack’ed. This reception: - set the RXOUTI flag (received OUT data) - set the TXINI flag (data sent, ready to accept new data) software algorithm: set transmit ready wait (transmit complete OR Receive complete) if receive complete, clear flag and return if transmit complete, continue Once the OUT status stage has been received, the USB controller waits for a SETUP request. The SETUP request have priority over any other request and has to be ACK’ed. This means that any other flag should be cleared and the fifo reset when a SETUP is received. WARNING: the byte counter is reset when the OUT Zero Length Packet is received. The firmware has to take care of this. 22.13 OUT Endpoint Management OUT packets are sent by the host. All the data can be read by the CPU, which acknowledges or not the bank when it is empty. 22.13.1Overview The Endpoint must be configured first. Each time the current bank is full, the RXOUTI and the FIFOCON bits are set. This triggers an interrupt if the RXOUTE bit is set. The firmware can acknowledge the USB interrupt by clearing the RXOUTI bit. The Firmware read the data and clear the FIFOCON bit in order to free the current bank. If the OUT Endpoint is composed of multiple banks, clearing the FIFOCON bit will switch to the next bank. The RXOUTI and FIFOCON bits are then updated by hardware in accordance with the status of the new bank. RXOUTI shall always be cleared before clearing FIFOCON. ATmega16U4/32U4 [DATASHEET] 275 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The RWAL bit always reflects the state of the current bank. This bit is set if the firmware can read data from the bank, and cleared by hardware when the bank is empty. Example with 1 OUT data bank DATA NAK DATA OUT ACK OUT ACK (to bank 0) (to bank 0) HW HW RXOUTI SW SW FIFOCON read data from CPU SW BANK 0 read data from CPU BANK 0 Example with 2 OUT data banks DATA DATA OUT ACK OUT ACK (to bank 0) (to bank 1) HW HW RXOUTI SW SW FIFOCON read data from CPU SW BANK 0 read data from CPU BANK 1 22.13.2Detailed description 22.13.2.1 The data are read by the CPU, following the next flow:  When the bank is filled by the host, an endpoint interrupt (EPINTx) is triggered, if enabled (RXOUTE set) and RXOUTI is set. The CPU can also poll RXOUTI or FIFOCON, depending on the software architecture  The CPU acknowledges the interrupt by clearing RXOUTI  The CPU can read the number of byte (N) in the current bank (N=BYCT)  The CPU can read the data from the current bank (“N” read of UEDATX)  The CPU can free the bank by clearing FIFOCON when all the data is read, that is: ̶ after “N” read of UEDATX ̶ as soon as RWAL is cleared by hardware If the endpoint uses 2 banks, the second one can be filled by the HOST while the current one is being read by the CPU. Then, when the CPU clear FIFOCON, the next bank may be already ready and RXOUTI is set immediately. 22.14 IN endpoint management IN packets are sent by the USB device controller, upon an IN request from the host. All the data can be written by the CPU, which acknowledge or not the bank when it is full.Overview The Endpoint must be configured first. The TXINI bit is set by hardware when the current bank becomes free. This triggers an interrupt if the TXINE bit is set. The FIFOCON bit is set at the same time. The CPU writes into the FIFO and clears the FIFOCON bit to allow the USB controller to send the data. If the IN Endpoint is composed of multiple banks, this also switches to the next data bank. The TXINI and FIFOCON bits are automatically updated by hardware regarding the status of the next bank. TXINI shall always be cleared before clearing FIFOCON. ATmega16U4/32U4 [DATASHEET] 276 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The RWAL bit always reflects the state of the current bank. This bit is set if the firmware can write data to the bank, and cleared by hardware when the bank is full . Example with 1 IN data bank NAK DATA IN ACK IN (bank 0) HW TXINI SW SW FIFOCON write data from CPU SW SW BANK 0 write data from CPU BANK 0 Example with 2 IN data banks DATA DATA IN ACK IN ACK (bank 0) (bank 1) HW TXINI SW SW SW FIFOCON write data from CPU SW write data from CPU SW write data from CPU BANK 0 BANK 1 BANK0 22.14.1Detailed Description The data are written by the CPU, following the next flow:  When the bank is empty, an endpoint interrupt (EPINTx) is triggered, if enabled (TXINE set) and TXINI is set. The CPU can also poll TXINI or FIFOCON, depending the software architecture choice  The CPU acknowledges the interrupt by clearing TXINI  The CPU can write the data into the current bank (write in UEDATX)  The CPU can free the bank by clearing FIFOCON when all the data are written, that is:  after “N” write into UEDATX  as soon as RWAL is cleared by hardware If the endpoint uses two banks, the second one can be read by the HOST while the current is being written by the CPU. Then, when the CPU clears FIFOCON, the next bank may be already ready (free) and TXINI is set immediately. 22.14.1.1Abort An “abort” stage can be produced by the host in some situations:  In a control transaction: ZLP data OUT received during a IN stage  In an isochronous IN transaction: ZLP data OUT received on the OUT endpoint during a IN stage on the IN endpoint  ... The KILLBK bit is used to kill the last “written” bank. The best way to manage this abort is to perform the following operations: ATmega16U4/32U4 [DATASHEET] 277 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 22-1. Abort Flow Endpoint Abort Clear Disable the TXINI interrupt. UEIENX. TXINE Abort is based on the fact NBUSYBK No that no banks are busy, =0 meaning that nothing has to be sent. Yes Endpoint Kill the last written KILLBK=1 bank. reset Yes Wait for the end of the KILLBK=1 procedure. No Abort done 22.15 Isochronous Mode 22.15.1Underflow An underflow can occur during IN stage if the host attempts to read a bank which is empty. In this situation, the UNDERFI interrupt is triggered. An underflow can also occur during OUT stage if the host send a packet while the banks are already full. Typically, the CPU is not fast enough. The packet is lost. It is not possible to have underflow error during OUT stage, in the CPU side, since the CPU should read only if the bank is ready to give data (RXOUTI=1 or RWAL=1) 22.15.2CRC Error A CRC error can occur during OUT stage if the USB controller detects a bad received packet. In this situation, the STALLEDI interrupt is triggered. This does not prevent the RXOUTI interrupt from being triggered. 22.16 Overflow In Control, Isochronous, Bulk or Interrupt Endpoint, an overflow can occur during OUT stage, if the host attempts to write in a bank that is too small for the packet. In this situation, the OVERFI interrupt is triggered (if enabled). The packet is acknowledged and the RXOUTI interrupt is also triggered (if enabled). The bank is filled with the first bytes of the packet. It is not possible to have overflow error during IN stage, in the CPU side, since the CPU should write only if the bank is ready to access data (TXINI=1 or RWAL=1). 22.17 Interrupts Figure 22-4 shows all the interrupts sources. ATmega16U4/32U4 [DATASHEET] 278 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 22-4. USB Device Controller Interrupt System UPRSMI UDINT.6 UPRSME UDIEN.6 EORSMI UDINT.5 EORSME UDIEN.5 WAKEUPI UDINT.4 WAKEUPE USB Device UDIEN.4 Interrupt EORSTI UDINT.3 EORSTE UDIEN.3 SOFI UDINT.2 SOFE UDIEN.2 SUSPI UDINT.0 SUSPE UDIEN.0 There are two kind of interrupts: processing (i.e. their generation are part of the normal processing) and exception (errors). Processing interrupts are generated when:  VBUS plug-in detection (insert, remove)(VBUSTI)  Upstream resume(UPRSMI)  End of resume(EORSMI)  Wake up(WAKEUPI)  End of reset (Speed Initialization)(EORSTI)  Start of frame(SOFI, if FNCERR=0)  Suspend detected after 3ms of inactivity(SUSPI) Exception Interrupts are generated when:  CRC error in frame number of SOF(SOFI, FNCERR=1) ATmega16U4/32U4 [DATASHEET] 279 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 22-5. USB Device Controller Endpoint Interrupt System Endpoint 6 Endpoint 5 Endpoint 4 Endpoint 3 Endpoint 2 Endpoint 1 Endpoint 0 OVERFI UESTAX.6 UNDERFI FLERRE UESTAX.5 UEIENX.7 NAKINI UEINTX.6 NAKINE UEIENX.6 NAKOUTI UEINTX.4 TXSTPE Endpoint Interrupt UEIENX.4 RXSTPI EPINT UEINTX.3 TXOUTE UEINT.X UEIENX.3 RXOUTI UEINTX.2 RXOUTE UEIENX.2 STALLEDI UEINTX.1 STALLEDE UEIENX.1 TXINI UEINTX.0 TXINE UEIENX.0 Processing interrupts are generated when:  Ready to accept IN data(EPINTx, TXINI=1)  Received OUT data(EPINTx, RXOUTI=1)  Received SETUP(EPINTx, RXSTPI=1) Exception Interrupts are generated when:  Stalled packet(EPINTx, STALLEDI=1)  CRC error on OUT in isochronous mode(EPINTx, STALLEDI=1)  Overflow in isochronous mode(EPINTx, OVERFI=1)  Underflow in isochronous mode(EPINTx, UNDERFI=1)  NAK IN sent(EPINTx, NAKINI=1)  NAK OUT sent(EPINTx, NAKOUTI=1) ATmega16U4/32U4 [DATASHEET] 280 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

22.18 Registers 22.18.1USB Device General Registers Bit 7 6 5 4 3 2 1 0 - - - - RSTCPU LSM RMWKUP DETACH UDCON Read/Write R R R R R R/W R/W R/W Initial Value 0 0 0 0 0 0 0 1 • Bits 7:4 - Reserved The value read from these bits is always 0. Do not set these bits. • Bit 3 - RSTCPU - USB Reset CPU bit Set this bit to 1 by firmware in order to reset the CPU on the detection of a USB End of Reset signal (without disabling the USB controller and Attached state). This bit is reset when the USB controller is disabled, but is not affected by the CPU reset generated after a USB End of Reset (remains enabled). • Bit 2 - LSM - USB Device Low Speed Mode Selection When configured USB is configured in device mode, this bit allows to select the USB the USB Low Speed or Full Speed Mod. Clear to select full speed mode (D+ internal pull-up will be activate with the ATTACH bit will be set). Set to select low speed mode (D- internal pull-up will be activate with the ATTACH bit will be set). This bit has no effect when the USB interface is configured in HOST mode. • Bit 1- RMWKUP - Remote Wake-up Bit Set to send an “upstream-resume” to the host for a remote wake-up (the SUSPI bit must be set). Cleared by hardware when signalling finished. Clearing by software has no effect. See Section22.10, page273 for more details. • Bit 0 - DETACH - Detach Bit Set to physically detach de device (disconnect internal pull-up on D+ or D-). Clear to reconnect the device. See Section22.9, page272 for more details ATmega16U4/32U4 [DATASHEET] 281 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Bit 7 6 5 4 3 2 1 0 - UPRSMI EORSMI WAKEUPI EORSTI SOFI - SUSPI UDINT Read/Write Initial Value 0 0 0 0 0 0 0 0 • Bit 7 - Reserved The value read from this bits is always 0. Do not set this bit. • Bit 6 - UPRSMI - Upstream Resume Interrupt Flag Set by hardware when the USB controller is sending a resume signal called “Upstream Resume”. This triggers an USB interrupt if UPRSME is set. Shall be cleared by software (USB clocks must be enabled before). Setting by software has no effect. • Bit 5 - EORSMI - End Of Resume Interrupt Flag Set by hardware when the USB controller detects a good “End Of Resume” signal initiated by the host. This triggers an USB interrupt if EORSME is set. Shall be cleared by software. Setting by software has no effect. • Bit 4 - WAKEUPI - Wake-up CPU Interrupt Flag Set by hardware when the USB controller is re-activated by a filtered non-idle signal from the lines (not by an upstream resume). This triggers an interrupt if WAKEUPE is set. Shall be cleared by software (USB clock inputs must be enabled before). Setting by software has no effect. See “Suspend, Wake-up and Resume” on page272 for more details. • Bit 3 - EORSTI - End Of Reset Interrupt Flag Set by hardware when an “End Of Reset” has been detected by the USB controller. This triggers an USB interrupt if EORSTE is set. Shall be cleared by software. Setting by software has no effect. • Bit 2 - SOFI - Start Of Frame Interrupt Flag Set by hardware when an USB “Start Of Frame” PID (SOF) has been detected (every 1ms). This triggers an USB interrupt if SOFE is set. • Bit 1 - Reserved The value read from this bits is always 0. Do not set this bit • Bit 0 - SUSPI - Suspend Interrupt Flag Shall be cleared by software. Setting by software has no effect. The interrupt bits are set even if their corresponding ‘Enable’ bits is not set. See “Suspend, Wake-up and Resume” on page272 for more details. See for more details. ATmega16U4/32U4 [DATASHEET] 282 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Bit 7 6 5 4 3 2 1 0 - UPRSME EORSME WAKEUPE EORSTE SOFE - SUSPE UDIEN Read/Write Initial Value 0 0 0 0 0 0 0 0 • Bit 7 - Reserved The value read from this bits is always 0. Do not set this bit. • Bit 6 - UPRSME - Upstream Resume Interrupt Enable Bit Set to enable the UPRSMI interrupt. Clear to disable the UPRSMI interrupt. • Bit 5 - EORSME - End Of Resume Interrupt Enable Bit Set to enable the EORSMI interrupt. Clear to disable the EORSMI interrupt. • Bit 4 - WAKEUPE - Wake-up CPU Interrupt Enable Bit Set to enable the WAKEUPI interrupt. Clear to disable the WAKEUPI interrupt. • Bit 3 - EORSTE - End Of Reset Interrupt Enable Bit Set to enable the EORSTI interrupt. This bit is set after a reset. Clear to disable the EORSTI interrupt. • Bit 2 - SOFE - Start Of Frame Interrupt Enable Bit Set to enable the SOFI interrupt. Clear to disable the SOFI interrupt. • Bit 1 - Reserved The value read from this bits is always 0. Do not set this bit. • Bit 0 - SUSPE - Suspend Interrupt Enable Bit Set to enable the SUSPI interrupt. Clear to disable the SUSPI interrupt. ATmega16U4/32U4 [DATASHEET] 283 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Bit 7 6 5 4 3 2 1 0 ADDEN UADD6:0 UDADDR Read/Write W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 7 - ADDEN - Address Enable Bit Set to activate the UADD (USB address). Cleared by hardware. Clearing by software has no effect. See “Address Setup” on page272 for more details. • Bits 6-0 - UADD6:0 - USB Address Bits Load by software to configure the device address Bit 7 6 5 4 3 2 1 0 - - - - - FNUM10:8 UDFNUMH Read/Write R R R R R R R R Initial Value 0 0 0 0 0 0 0 0 • 7-3 - Reserved The value read from these bits is always 0. Do not set these bits. • 2-0 - FNUM10:8 - Frame Number Upper Value Set by hardware. These bits are the three MSB of the 11-bits Frame Number information. They are provided in the last received SOF packet. FNUM is updated if a corrupted SOF is received. Bit 7 6 5 4 3 2 1 0 FNUM7:0 UDFNUML Read/Write R R R R R R R R Initial Value 0 0 0 0 0 0 0 0 • Bits 7:0 - FNUM7:0 - Frame Number Lower Value Set by hardware. These bits are the 8 LSB of the 11-bits Frame Number information ATmega16U4/32U4 [DATASHEET] 284 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Bit 7 6 5 4 3 2 1 0 - - - FNCERR - - - - UDMFN Read/Wri R te Initial 0 0 0 0 0 0 0 0 Value • Bits 7:5 - Reserved The value read from these bits is always 0. Do not set these bits. • Bit 4 - FNCERR -Frame Number CRC Error Flag Set by hardware when a corrupted Frame Number in start of frame packet is received. This bit and the SOFI interrupt are updated at the same time. • Bits 3:0 - Reserved The value read from these bits is always 0. Do not set these bits. 22.18.2USB Device Endpoint Registers Bit 7 6 5 4 3 2 1 0 - - - - - EPNUM2:0 UENUM Read/Write R R R R R R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bits 7:3 - Reserved The value read from these bits is always 0. Do not set these bits. • Bits 2-0 - EPNUM2:0 Endpoint Number Bits Load by software to select the number of the endpoint which shall be accessed by the CPU. See “Endpoint Reset” on page270 for more details. EPNUM = 111b is forbidden Bit 7 6 5 4 3 2 1 0 - EPRST6 EPRST5 EPRST4 EPRST3 EPRST2 EPRST1 EPRST0 UERST Read/Write R R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 7 - Reserved The value read from these bits is always 0. Do not set these bits. • Bits 6-0 - EPRST6:0 - Endpoint FIFO Reset Bits Set to reset the selected endpoint FIFO prior to any other operation, upon hardware reset or when an USB bus reset has been received. See “Endpoint Reset” on page270 for more information Then, clear by software to complete the reset operation and start using the endpoint. ATmega16U4/32U4 [DATASHEET] 285 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Bit 7 6 5 4 3 2 1 0 - - STALLRQ STALLRQC RSTDT - - EPEN UECONX Read/Write R R W W W R R R/W Initial Value 0 0 0 0 0 0 0 0 • Bits 7:6 - Reserved The value read from these bits is always 0. Do not set these bits. • Bit 5 - STALLRQ - STALL Request Handshake Bit Set to request a STALL answer to the host for the next handshake. Cleared by hardware when a new SETUP is received. Clearing by software has no effect. See “STALL Request” on page273 for more details. • Bit 4 - STALLRQC - STALL Request Clear Handshake Bit Set to disable the STALL handshake mechanism. Cleared by hardware immediately after the set. Clearing by software has no effect. See “STALL Request” on page273 for more details. • Bit 3 - RSTDT - Reset Data Toggle Bit Set to automatically clear the data toggle sequence: For OUT endpoint: the next received packet will have the data toggle 0. For IN endpoint: the next packet to be sent will have the data toggle 0. Cleared by hardware instantaneously. The firmware does not have to wait that the bit is cleared. Clearing by software has no effect. • 2 - Reserved The value read from these bits is always 0. Do not set these bits. • Bit 1 - Reserved The value read from these bits is always 0. Do not set these bits. • Bit 0 - EPEN - Endpoint Enable Bit Set to enable the endpoint according to the device configuration. Endpoint 0 shall always be enabled after a hardware or USB reset and participate in the device configuration. Clear this bit to disable the endpoint. See “Endpoint Activation” on page271 for more details. Bit 7 6 5 4 3 2 1 0 EPTYPE1:0 - - - - - EPDIR UECFG0X Read/Write R/W R/W R R R R R R/W Initial Value 0 0 0 0 0 0 0 0 • Bits 7:6 - EPTYPE1:0 - Endpoint Type Bits Set this bit according to the endpoint configuration: 00b: Control10b: Bulk 01b: Isochronous11b: Interrupt ATmega16U4/32U4 [DATASHEET] 286 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bits 5:1 - Reserved The value read from these bits is always 0. Do not set these bits. • Bit 0 - EPDIR - Endpoint Direction Bit Set to configure an IN direction for bulk, interrupt or isochronous endpoints. Clear to configure an OUT direction for bulk, interrupt, isochronous or control endpoints. Bit 7 6 5 4 3 2 1 0 - EPSIZE2:0 EPBK1:0 ALLOC - UECFG1X Read/Write R R/W R/W R/W R/W R/W R/W R Initial Value 0 0 0 0 0 0 0 0 • Bit 7 - Reserved The value read from these bits is always 0. Do not set these bits. • Bits 6-4 - EPSIZE2:0 - Endpoint Size Bits Set this bit according to the endpoint size: 000b: 8 bytes100b: 128 bytes 001b: 16 bytes101b: 256 bytes 010b: 32 bytes110b: 512 bytes 011b: 64 bytes111b: Reserved. Do not use this configuration. • Bits 3:2 - EPBK1:0 - Endpoint Bank Bits Set this field according to the endpoint size: 00b: One bank 01b: Double bank 1xb: Reserved. Do not use this configuration. • Bit 1 - ALLOC - Endpoint Allocation Bit Set this bit to allocate the endpoint memory. Clear to free the endpoint memory. See “Endpoint Activation” on page271 for more details. • Bit 0 - Reserved The value read from these bits is always 0. Do not set these bits. Bit 7 6 5 4 3 2 1 0 CFGOK OVERFI UNDERFI - DTSEQ1:0 NBUSYBK1:0 UESTA0X Read/Write R R/W R/W R/W R R R R Initial Value 0 0 0 0 0 0 0 0 • Bit 7 - CFGOK - Configuration Status Flag Set by hardware when the endpoint X size parameter (EPSIZE) and the bank parametrization (EPBK) are correct compared to the maximum FIFO capacity and the maximum number of allowed bank. This bit is updated when the bit ALLOC is set. If this bit is cleared, the user should reprogram the UECFG1X register with correct EPSIZE and EPBK values. ATmega16U4/32U4 [DATASHEET] 287 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 6 - OVERFI - Overflow Error Interrupt Flag Set by hardware when an overflow error occurs in an isochronous endpoint. An interrupt (EPINTx) is triggered (if enabled). See Section22.15, page278 for more details. Shall be cleared by software. Setting by software has no effect. • Bit 5 - UNDERFI - Flow Error Interrupt Flag Set by hardware when an underflow error occurs in an isochronous endpoint. An interrupt (EPINTx) is triggered (if enabled). See Section22.15, page278 for more details. Shall be cleared by software. Setting by software has no effect. • Bit 4 - Reserved The value read from these bits is always 0. Do not set these bits. • Bits 3-2 - DTSEQ1:0 - Data Toggle Sequencing Flag Set by hardware to indicate the PID data of the current bank: 00bData0 01bData1 1xbReserved For OUT transfer, this value indicates the last data toggle received on the current bank. For IN transfer, it indicates the Toggle that will be used for the next packet to be sent. This is not relative to the current bank. • Bits 1:0 - NBUSYBK1:0 - Busy Bank Flag Set by hardware to indicate the number of busy bank. For IN endpoint, it indicates the number of busy bank(s), filled by the user, ready for IN transfer. For OUT endpoint, it indicates the number of busy bank(s) filled by OUT transaction from the host. 00bAll banks are free 01b1 busy bank 10b2 busy banks 11bReserved Bit 7 6 5 4 3 2 1 0 - - - - - CTRLDIR CURRBK1:0 UESTA1X Read/Write R R R R R R R R Initial Value 0 0 0 0 0 0 0 0 • Bits 7:3 - Reserved The value read from these bits is always 0. Do not set these bits. • Bit 2 - CTRLDIR - Control Direction (Flag, and bit for debug purpose) Set by hardware after a SETUP packet, and gives the direction of the following packet: ATmega16U4/32U4 [DATASHEET] 288 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

- 1 for IN endpoint - 0 for OUT endpoint Can not be set or cleared by software. • Bits 1:0 - CURRBK1:0 - Current Bank (all endpoints except Control endpoint) Flag Set by hardware to indicate the number of the current bank: 00bBank0 01bBank1 1xbReserved Can not be set or cleared by software. Bit 7 6 5 4 3 2 1 0 FIFOCON NAKINI RWAL NAKOUTI RXSTPI RXOUTI STALLEDI TXINI UEINTX Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 7 - FIFOCON - FIFO Control Bit For OUT and SETUP Endpoint: Set by hardware when a new OUT message is stored in the current bank, at the same time than RXOUT or RXSTP. Clear to free the current bank and to switch to the following bank. Setting by software has no effect. For IN Endpoint: Set by hardware when the current bank is free, at the same time than TXIN. Clear to send the FIFO data and to switch the bank. Setting by software has no effect. • Bit 6 - NAKINI - NAK IN Received Interrupt Flag Set by hardware when a NAK handshake has been sent in response of a IN request from the host. This triggers an USB interrupt if NAKINE is sent. Shall be cleared by software. Setting by software has no effect. • Bit 5 - RWAL - Read/Write Allowed Flag Set by hardware to signal: - for an IN endpoint: the current bank is not full i.e. the firmware can push data into the FIFO - for an OUT endpoint: the current bank is not empty, i.e. the firmware can read data from the FIFO The bit is never set if STALLRQ is set, or in case of error. Cleared by hardware otherwise. This bit shall not be used for the control endpoint. • Bit 4 - NAKOUTI - NAK OUT Received Interrupt Flag Set by hardware when a NAK handshake has been sent in response of a OUT/PING request from the host. This triggers an USB interrupt if NAKOUTE is sent. Shall be cleared by software. Setting by software has no effect. ATmega16U4/32U4 [DATASHEET] 289 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 3 - RXSTPI - Received SETUP Interrupt Flag Set by hardware to signal that the current bank contains a new valid SETUP packet. An interrupt (EPINTx) is triggered (if enabled). Shall be cleared by software to handshake the interrupt. Setting by software has no effect. This bit is inactive (cleared) if the endpoint is an IN endpoint. • Bit 2 - RXOUTI / KILLBK - Received OUT Data Interrupt Flag Set by hardware to signal that the current bank contains a new packet. An interrupt (EPINTx) is triggered (if enabled). Shall be cleared by software to handshake the interrupt. Setting by software has no effect. Kill Bank IN Bit Set this bit to kill the last written bank. Cleared by hardware when the bank is killed. Clearing by software has no effect. See “Abort” on page277 for more details on the Abort. • Bit 1 - STALLEDI - STALLEDI Interrupt Flag Set by hardware to signal that a STALL handshake has been sent, or that a CRC error has been detected in a OUT isochronous endpoint. Shall be cleared by software. Setting by software has no effect. • Bit 0 - TXINI - Transmitter Ready Interrupt Flag Set by hardware to signal that the current bank is free and can be filled. An interrupt (EPINTx) is triggered (if enabled). Shall be cleared by software to handshake the interrupt. Setting by software has no effect. This bit is inactive (cleared) if the endpoint is an OUT endpoint. Bit 7 6 5 4 3 2 1 0 FLERRE NAKINE - NAKOUTE RXSTPE RXOUTE STALLEDE TXINE UEIENX Read/Write R/W R/W R R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 7 - FLERRE - Flow Error Interrupt Enable Flag Set to enable an endpoint interrupt (EPINTx) when OVERFI or UNDERFI are sent. Clear to disable an endpoint interrupt (EPINTx) when OVERFI or UNDERFI are sent. • Bit 6 - NAKINE - NAK IN Interrupt Enable Bit Set to enable an endpoint interrupt (EPINTx) when NAKINI is set. Clear to disable an endpoint interrupt (EPINTx) when NAKINI is set. • Bit 5 - Reserved The value read from these bits is always 0. Do not set these bits. • Bit 4 - NAKOUTE - NAK OUT Interrupt Enable Bit Set to enable an endpoint interrupt (EPINTx) when NAKOUTI is set. Clear to disable an endpoint interrupt (EPINTx) when NAKOUTI is set. ATmega16U4/32U4 [DATASHEET] 290 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 3 - RXSTPE - Received SETUP Interrupt Enable Flag Set to enable an endpoint interrupt (EPINTx) when RXSTPI is sent. Clear to disable an endpoint interrupt (EPINTx) when RXSTPI is sent. • Bit 2 - RXOUTE - Received OUT Data Interrupt Enable Flag Set to enable an endpoint interrupt (EPINTx) when RXOUTI is sent. Clear to disable an endpoint interrupt (EPINTx) when RXOUTI is sent. • Bit 1 - STALLEDE - Stalled Interrupt Enable Flag Set to enable an endpoint interrupt (EPINTx) when STALLEDI is sent. Clear to disable an endpoint interrupt (EPINTx) when STALLEDI is sent. • Bit 0 - TXINE - Transmitter Ready Interrupt Enable Flag Set to enable an endpoint interrupt (EPINTx) when TXINI is sent. Clear to disable an endpoint interrupt (EPINTx) when TXINI is sent. Bit 7 6 5 4 3 2 1 0 DAT D7 DAT D6 DAT D5 DAT D4 DAT D3 DAT D2 DAT D1 DAT D0 UEDATX Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bits 7:0 - DAT7:0 -Data Bits Set by the software to read/write a byte from/to the endpoint FIFO selected by EPNUM Bit 7 6 5 4 3 2 1 0 - - - - - BYCT D10 BYCT D9 BYCT D8 UEBCHX Read/Write R R R R R R R R Initial Value 0 0 0 0 0 0 0 0 • Bits 7:3 - Reserved The value read from these bits is always 0. Do not set these bits. • Bits 2:0 - BYCT10:8 - Byte count (high) Bits Set by hardware. This field is the MSB of the byte count of the FIFO endpoint. The LSB part is provided by the UEBCLX register Bit 7 6 5 4 3 2 1 0 BYCT D7 BYCT D6 BYCT D5 BYCT D4 BYCT D3 BYCT D2 BYCT D1 BYCT D0 UEBCLX Read/Write R R R R R R R R Initial Value 0 0 0 0 0 0 0 0 • Bits 7-0 - BYCT7:0 - Byte Count (low) Bits Set by the hardware. BYCT10:0 is:  (for IN endpoint) increased after each writing into the endpoint and decremented after each byte sent,  (for OUT endpoint) increased after each byte sent by the host, and decremented after each byte read by the software. ATmega16U4/32U4 [DATASHEET] 291 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Bit 7 6 5 4 3 2 1 0 - EPINT D6 EPINT D5 EPINT D4 EPINT D3 EPINT D2 EPINT D1 EPINT D0 UEINT Read/Write R R R R R R R R Initial Value 0 0 0 0 0 0 0 0 • Bit 7 - Reserved The value read from these bits is always 0. Do not set these bits. • Bits 6:0 - EPINT6:0 - Endpoint Interrupts Bits Set by hardware when an interrupt is triggered by the UEINTX register and if the corresponding endpoint interrupt enable bit is set. Cleared by hardware when the interrupt source is served. ATmega16U4/32U4 [DATASHEET] 292 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

23. Analog Comparator The Analog Comparator compares the input values on the positive pin AIN+ and negative pin AIN-. When the voltage on the positive pin AIN+ is higher than the voltage on the negative pin AIN-, the Analog Comparator output, ACO, is set. The comparator’s output can be set to trigger the Timer/Counter1 Input Capture function. In addition, the comparator can trigger a separate interrupt, exclusive to the Analog Comparator. The user can select Interrupt triggering on comparator output rise, fall or toggle. A block diagram of the comparator and its surrounding logic is shown in Figure 23-1. AIN+ can be connected either to the AIN0 (PE6) pin, or to the internal Bandgap reference. AIN- can only be connected to the ADC multiplexer. The Power Reduction ADC bit, PRADC, in “Power Reduction Register 0 - PRR0” on page47 must be disabled by writing a logical zero to be able to use the ADC input MUX. Figure 23-1. Analog Comparator Block Diagram(2) BANDGAP REFERENCE ACBG AIN+ AIN- BANDGAP REFERENCE ACME ADEN ADC MULTIPLEXER OUTPUT(1) Notes: 1. See Table23-2 on page295. 2. Refer to “Pinout” on page3 and Table10-3 on page74 for Analog Comparator pin placement. ATmega16U4/32U4 [DATASHEET] 293 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

23.1 Register Description 23.1.1 ADC Control and Status Register B – ADCSRB Bit 7 6 5 4 3 2 1 0 ADHSM ACME MUX5 – ADTS3 ADTS2 ADTS1 ADTS0 ADCSRB Read/Write R R/W R/W R R R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 6 – ACME: Analog Comparator Multiplexer Enable When this bit is written logic one and the ADC is switched off (ADEN in ADCSRA is zero), the ADC multiplexer is connected to the negative input to the Analog Comparator. When this bit is written logic zero, the Bandgap reference is connected to the negative input of the Analog Comparator (See “Internal Voltage Reference” on page54.) For a detailed description of this bit, see “Analog Comparator Multiplexed Input” on page295. 23.1.2 Analog Comparator Control and Status Register – ACSR Bit 7 6 5 4 3 2 1 0 ACD ACBG ACO ACI ACIE ACIC ACIS1 ACIS0 ACSR Read/Write R/W R/W R R/W R/W R/W R/W R/W Initial Value 0 0 N/A 0 0 0 0 0 • Bit 7 – ACD: Analog Comparator Disable When this bit is written logic one, the power to the Analog Comparator is switched off. This bit can be set at any time to turn off the Analog Comparator. This will reduce power consumption in Active and Idle mode. When changing the ACD bit, the Analog Comparator Interrupt must be disabled by clearing the ACIE bit in ACSR. Otherwise an interrupt can occur when the bit is changed. • Bit 6 – ACBG: Analog Comparator Bandgap Select When this bit is set, a fixed bandgap reference voltage replaces the positive input to the Analog Comparator. When this bit is cleared, AIN0 is applied to the positive input of the Analog Comparator. See “Internal Voltage Reference” on page54. • Bit 5 – ACO: Analog Comparator Output The output of the Analog Comparator is synchronized and then directly connected to ACO. The synchronization introduces a delay of 1 - 2 clock cycles. • Bit 4 – ACI: Analog Comparator Interrupt Flag This bit is set by hardware when a comparator output event triggers the interrupt mode defined by ACIS1 and ACIS0. The Analog Comparator interrupt routine is executed if the ACIE bit is set and the I-bit in SREG is set. ACI is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, ACI is cleared by writing a logic one to the flag. • Bit 3 – ACIE: Analog Comparator Interrupt Enable When the ACIE bit is written logic one and the I-bit in the Status Register is set, the Analog Comparator interrupt is activated. When written logic zero, the interrupt is disabled. ATmega16U4/32U4 [DATASHEET] 294 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 2 – ACIC: Analog Comparator Input Capture Enable When written logic one, this bit enables the input capture function in Timer/Counter1 to be triggered by the Analog Comparator. The comparator output is in this case directly connected to the input capture front-end logic, making the comparator utilize the noise canceler and edge select features of the Timer/Counter1 Input Capture interrupt. When written logic zero, no connection between the Analog Comparator and the input capture function exists. To make the comparator trigger the Timer/Counter1 Input Capture interrupt, the ICIE1 bit in the Timer Interrupt Mask Register (TIMSK1) must be set. • Bits 1, 0 – ACIS1, ACIS0: Analog Comparator Interrupt Mode Select These bits determine which comparator events that trigger the Analog Comparator interrupt. The different settings are shown in the table below. Table 23-1. ACIS1/ACIS0 Settings ACIS1 ACIS0 Interrupt Mode 0 0 Comparator Interrupt on Output Toggle 0 1 Reserved 1 0 Comparator Interrupt on Falling Output Edge 1 1 Comparator Interrupt on Rising Output Edge When changing the ACIS1/ACIS0 bits, the Analog Comparator Interrupt must be disabled by clearing its Interrupt Enable bit in the ACSR Register. Otherwise an interrupt can occur when the bits are changed. 23.2 Analog Comparator Multiplexed Input It is possible to select any of the ADC13..0 pins to replace the negative input to the Analog Comparator. The ADC multiplexer is used to select this input, and consequently, the ADC must be switched off to utilize this feature. If the Analog Comparator Multiplexer Enable bit (ACME in ADCSRB) is set and the ADC is switched off (ADEN in ADCSRA is zero), and MUX2..0 in ADMUX select the input pin to replace the negative input to the Analog Comparator, as shown in the table. If ACME is cleared or ADEN is set, the Bandgap reference is applied to the negative input to the Analog Comparator. Table 23-2. Analog Comparator Multiplexed Input ACME ADEN MUX2..0 Analog Comparator Negative Input 0 x xxx Bandgap Ref. 1 1 xxx Bandgap Ref. 1 0 000 ADC0 1 0 001 ADC1 1 0 010 N/A 1 0 011 1 0 100 ADC4 1 0 101 ADC5 1 0 110 ADC6 1 0 111 ADC7 ATmega16U4/32U4 [DATASHEET] 295 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

23.2.1 Digital Input Disable Register 1 – DIDR1 Bit 7 6 5 4 3 2 1 0 – – – – – – – AIN0D DIDR1 Read/Write R R R R R R R R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 0 – AIN0D: AIN0 Digital Input Disable When this bit is written logic one, the digital input buffer on the AIN0 pin is disabled. The corresponding PIN Register bit will always read as zero when this bit is set. When an analog signal is applied to the AIN0 pin and the digital input from this pin is not needed, this bit should be written logic one to reduce power consumption in the digital input buffer. ATmega16U4/32U4 [DATASHEET] 296 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

24. Analog to Digital Converter - ADC 24.1 Features • 10/8-bit Resolution • 0.5LSB Integral Non-linearity • ±2LSB Absolute Accuracy • 65 - 260µs Conversion Time • Up to 15kSPS at Maximum Resolution • Twelve Multiplexed Single-Ended Input Channels • One Differential amplifier providing gain of 1x - 10x - 40x - 200x • Temperature sensor • Optional Left Adjustment for ADC Result Readout • 0 - V ADC Input Voltage Range CC • Selectable 2.56V ADC Reference Voltage • Free Running or Single Conversion Mode • ADC Start Conversion by Auto Triggering on Interrupt Sources • Interrupt on ADC Conversion Complete • Sleep Mode Noise Canceler The ATmega16U4/ATmega32U4 features a 10-bit successive approximation ADC. The ADC is connected to an 12-channel Analog Multiplexer which allows six single-ended voltage inputs constructed from several pins of Port B, D, and F. The single-ended voltage inputs refer to 0V(GND). The device also supports 32 differential voltage input combinations, thanks to a differential amplifier equipped with a programmable gain stage, providing amplification steps of 0 dB (1x), 10 dB (10x), 16dB (40x), or 23dB (200x) on the differential input voltage before the A/D conversion. Two differential analog input channels share a common negative terminal (ADC0/ADC1), while any other ADC input can be selected as the positive input terminal. If 1x, 10x, or 40x gain is used, 8-bit resolution can be expected. If 200x gain is used, 7-bit resolution can be expected. The ADC contains a Sample and Hold circuit which ensures that the input voltage to the ADC is held at a constant level during conversion. A block diagram of the ADC is shown in Figure 24-1. The ADC has a separate analog supply voltage pin, AV . AV must not differ more than ± 0.3V from V . See CC CC CC the paragraph “ADC Noise Canceler” on page305 on how to connect this pin. Internal reference voltages of nominally 2.56V or AV are provided On-chip. The voltage reference may be CC externally decoupled at the AREF pin by a capacitor for better noise performance. ATmega16U4/32U4 [DATASHEET] 297 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 24-1. Analog to Digital Converter Block Schematic ADC CONVERSION COMPLETE IRQ INTERRUPT FLAGS ADTS[3:0] 8-BIT DATA BUS ADIF ADIE 15 0 ADC MULTIPLEXER ADC CTRL. & STATUS ADC DATA REGISTER SELECT (ADMUX) REGISTER (ADCSRA) (ADCH/ADCL) REFS1 REFS0 ADLAR MUX5 MUX4 MUX3 MUX2 MUX1 MUX0 ADEN ADSC ADATE ADIF ADPS2 ADPS1 ADPS0 TRIGGER C[9:0] SELECT AD MUX DECODER PRESCALER AVCC HANNEL SELECTION GAIN SELECTION CONVERSION LOSGTIACRT C INTERNAL REFERENCE SAMPLE & HOLD COMPARATOR AREF 10-BIT DAC - + ADHSM GND BANDGAP REFERENCE TEMPERATURE SENSOR SINGLE ENDED / DIFFERENTIAL SELECTION ADC13 POS. ADC MULTIPLEXER INPUT OUTPUT ADC12 MUX ADC11 DIFFERENTIAL ADC10 + AMPLIFIER ADC9 - ADC8 ADC7 ADC6 ADC5 ADC4 ADC1 ADC0 NEG. INPUT MUX ATmega16U4/32U4 [DATASHEET] 298 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

24.2 Operation The ADC converts an analog input voltage to a 10-bit digital value through successive approximation. The minimum value represents GND and the maximum value represents the voltage on the AREF pin minus 1 LSB. Optionally, AV or an internal 2.56V reference voltage may be connected to the AREF pin by writing to the CC REFSn bits in the ADMUX Register. The internal voltage reference may thus be decoupled by an external capacitor at the AREF pin to improve noise immunity. The analog input channel and differential gain are selected by writing to the MUX bits in ADMUX. Any of the ADC input pins, as well as GND and a fixed bandgap voltage reference, can be selected as single ended inputs to the ADC. A selection of ADC input pins can be selected as positive and negative inputs to the differential amplifier. The ADC is enabled by setting the ADC Enable bit, ADEN in ADCSRA. Voltage reference and input channel selections will not go into effect until ADEN is set. The ADC does not consume power when ADEN is cleared, so it is recommended to switch off the ADC before entering power saving sleep modes. The ADC generates a 10-bit result which is presented in the ADC Data Registers, ADCH and ADCL. By default, the result is presented right adjusted, but can optionally be presented left adjusted by setting the ADLAR bit in ADMUX. If the result is left adjusted and no more than 8-bit precision is required, it is sufficient to read ADCH. Otherwise, ADCL must be read first, then ADCH, to ensure that the content of the Data Registers belongs to the same conversion. Once ADCL is read, ADC access to Data Registers is blocked. This means that if ADCL has been read, and a conversion completes before ADCH is read, neither register is updated and the result from the conversion is lost. When ADCH is read, ADC access to the ADCH and ADCL Registers is re-enabled. The ADC has its own interrupt which can be triggered when a conversion completes. The ADC access to the Data Registers is prohibited between reading of ADCH and ADCL, the interrupt will trigger even if the result is lost. 24.3 Starting a Conversion A single conversion is started by writing a logical one to the ADC Start Conversion bit, ADSC. This bit stays high as long as the conversion is in progress and will be cleared by hardware when the conversion is completed. If a different data channel is selected while a conversion is in progress, the ADC will finish the current conversion before performing the channel change. Alternatively, a conversion can be triggered automatically by various sources. Auto Triggering is enabled by setting the ADC Auto Trigger Enable bit, ADATE in ADCSRA. The trigger source is selected by setting the ADC Trigger Select bits, ADTS in ADCSRB (See description of the ADTS bits for a list of the trigger sources). When a positive edge occurs on the selected trigger signal, the ADC prescaler is reset and a conversion is started. This provides a method of starting conversions at fixed intervals. If the trigger signal is still set when the conversion completes, a new conversion will not be started. If another positive edge occurs on the trigger signal during conversion, the edge will be ignored. Note that an interrupt flag will be set even if the specific interrupt is disabled or the Global Interrupt Enable bit in SREG is cleared. A conversion can thus be triggered without causing an interrupt. However, the interrupt flag must be cleared in order to trigger a new conversion at the next interrupt event. ATmega16U4/32U4 [DATASHEET] 299 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 24-2. ADC Auto Trigger Logic ADTS[2:0] PRESCALER START CLK ADC ADIF ADATE SOURCE 1 . CONVERSION . LOGIC . . EDGE SOURCE n DETECTOR ADSC Using the ADC Interrupt Flag as a trigger source makes the ADC start a new conversion as soon as the ongoing conversion has finished. The ADC then operates in Free Running mode, constantly sampling and updating the ADC Data Register. The first conversion must be started by writing a logical one to the ADSC bit in ADCSRA. In this mode the ADC will perform successive conversions independently of whether the ADC Interrupt Flag, ADIF is cleared or not. If Auto Triggering is enabled, single conversions can be started by writing ADSC in ADCSRA to one. ADSC can also be used to determine if a conversion is in progress. The ADSC bit will be read as one during a conversion, independently of how the conversion was started. 24.4 Prescaling and Conversion Timing Figure 24-3. ADC Prescaler ADEN START Reset 7-BIT ADC PRESCALER CK 8 6 2 4 2 2 4 8 1 3 6 1 K/ K/ K/ K/ K/ K/ K/ C C C C C C C ADPS0 ADPS1 ADPS2 ADC CLOCK SOURCE By default, the successive approximation circuitry requires an input clock frequency between 50kHz and 200kHz to get maximum resolution. If a lower resolution than 10 bits is needed, the input clock frequency to the ADC can be higher than 200kHz to get a higher sample rate. Alternatively, setting the ADHSM bit in ADCSRB allows an increased ADC clock frequency at the expense of higher power consumption. The ADC module contains a prescaler, which generates an acceptable ADC clock frequency from any CPU frequency above 100kHz. The prescaling is set by the ADPS bits in ADCSRA. The prescaler starts counting from the moment the ADC is switched on by setting the ADEN bit in ADCSRA. The prescaler keeps running for as long as the ADEN bit is set, and is continuously reset when ADEN is low. When initiating a single ended conversion by setting the ADSC bit in ADCSRA, the conversion starts at the following rising edge of the ADC clock cycle. See “Differential Channels” on page302 for details on differential conversion timing. ATmega16U4/32U4 [DATASHEET] 300 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

A normal conversion takes 13 ADC clock cycles. The first conversion after the ADC is switched on (ADEN in ADCSRA is set) takes 25 ADC clock cycles in order to initialize the analog circuitry. The actual sample-and-hold takes place 1.5 ADC clock cycles after the start of a normal conversion and 13.5 ADC clock cycles after the start of an first conversion. When a conversion is complete, the result is written to the ADC Data Registers, and ADIF is set. In Single Conversion mode, ADSC is cleared simultaneously. The software may then set ADSC again, and a new conversion will be initiated on the first rising ADC clock edge. When Auto Triggering is used, the prescaler is reset when the trigger event occurs. This assures a fixed delay from the trigger event to the start of conversion. In this mode, the sample-and-hold takes place two ADC clock cycles after the rising edge on the trigger source signal. Three additional CPU clock cycles are used for synchronization logic. In Free Running mode, a new conversion will be started immediately after the conversion completes, while ADSC remains high. For a summary of conversion times, see the table below. Figure 24-4. ADC Timing Diagram, First Conversion (Single Conversion Mode) Next First Conversion Conversion Cycle Number 1 2 12 13 14 15 16 17 18 19 20 21 22 23 24 25 1 2 3 ADC Clock ADEN ADSC ADIF ADCH Sign and MSB of Result ADCL LSB of Result MUX MUX and REFS Conversion and REFS Update Sample & Hold Complete Update Figure 24-5. ADC Timing Diagram, Single Conversion One Conversion Next Conversion Cycle Number 1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 ADC Clock ADSC ADIF ADCH Sign and MSB of Result ADCL LSB of Result Sample & Hold Conversion MUX and REFS MUX and REFS Complete Update Update ATmega16U4/32U4 [DATASHEET] 301 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 24-6. ADC Timing Diagram, Auto Triggered Conversion One Conversion Next Conversion Cycle Number 1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 ADC Clock Trigger Source ADATE ADIF ADCH Sign and MSB of Result ADCL LSB of Result Sample & Conversion Prescaler Prescaler Hold Complete Reset Reset MUX and REFS Update Figure 24-7. ADC Timing Diagram, Free Running Conversion One Conversion Next Conversion Cycle Number 11 12 13 1 2 3 4 ADC Clock ADSC ADIF ADCH Sign and MSB of Result ADCL LSB of Result Conversion Sample & Hold Complete MUX and REFS Update Table 24-1. ADC Conversion Time Normal Conversion, Auto Triggered Condition First Conversion Single Ended Conversion Sample and Hold 14.5 1.5 2 (Cycles from Start of Convention) Conversion Time 25 13 13.5 (Cycles) 24.4.1 Differential Channels When using differential channels, certain aspects of the conversion need to be taken into consideration. Differential conversions are synchronized to the internal clock CK equal to half the ADC clock frequency. ADC2 This synchronization is done automatically by the ADC interface in such a way that the sample-and-hold occurs at a specific phase of CK . A conversion initiated by the user (i.e., all single conversions, and the first free ADC2 running conversion) when CK is low will take the same amount of time as a single ended conversion (13 ADC2 ADC clock cycles from the next prescaled clock cycle). A conversion initiated by the user when CK is high ADC2 will take 14 ADC clock cycles due to the synchronization mechanism. In Free Running mode, a new conversion is initiated immediately after the previous conversion completes, and since CK is high at this time, all ADC2 automatically started (i.e., all but the first) Free Running conversions will take 14 ADC clock cycles. If differential channels are used and conversions are started by Auto Triggering, the ADC must be switched off between conversions. When Auto Triggering is used, the ADC prescaler is reset before the conversion is started. Since the stage is dependent of a stable ADC clock prior to the conversion, this conversion will not be valid. By disabling and then re-enabling the ADC between each conversion (writing ADEN in ADCSRA to “0” ATmega16U4/32U4 [DATASHEET] 302 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

then to “1”), only extended conversions are performed. The result from the extended conversions will be valid. See “Prescaling and Conversion Timing” on page300 for timing details. The gain stage is optimized for a bandwidth of 4kHz at all gain settings. Higher frequencies may be subjected to non-linear amplification. An external low-pass filter should be used if the input signal contains higher frequency components than the gain stage bandwidth. Note that the ADC clock frequency is independent of the gain stage bandwidth limitation. E.g. the ADC clock period may be 6µs, allowing a channel to be sampled at 12kSPS, regardless of the bandwidth of this channel. 24.5 Changing Channel or Reference Selection The MUXn and REFS1:0 bits in the ADMUX Register are single buffered through a temporary register to which the CPU has random access. This ensures that the channels and reference selection only takes place at a safe point during the conversion. The channel and reference selection is continuously updated until a conversion is started. Once the conversion starts, the channel and reference selection is locked to ensure a sufficient sampling time for the ADC. Continuous updating resumes in the last ADC clock cycle before the conversion completes (ADIF in ADCSRA is set). Note that the conversion starts on the following rising ADC clock edge after ADSC is written. The user is thus advised not to write new channel or reference selection values to ADMUX until one ADC clock cycle after ADSC is written. If Auto Triggering is used, the exact time of the triggering event can be indeterministic. Special care must be taken when updating the ADMUX Register, in order to control which conversion will be affected by the new settings. If both ADATE and ADEN is written to one, an interrupt event can occur at any time. If the ADMUX Register is changed in this period, the user cannot tell if the next conversion is based on the old or the new settings. ADMUX can be safely updated in the following ways: a. When ADATE or ADEN is cleared. 3. During conversion, minimum one ADC clock cycle after the trigger event. 4. After a conversion, before the interrupt flag used as trigger source is cleared. When updating ADMUX in one of these conditions, the new settings will affect the next ADC conversion. Special care should be taken when changing differential channels. Once a differential channel has been selected, the stage may take as much as 125µs to stabilize to the new value. Thus conversions should not be started within the first 125µs after selecting a new differential channel. Alternatively, conversion results obtained within this period should be discarded. The same settling time should be observed for the first differential conversion after changing ADC reference (by changing the REFS1:0 bits in ADMUX). The settling time and gain stage bandwidth is independent of the ADHSM bit setting. 24.5.1 ADC Input Channels When changing channel selections, the user should observe the following guidelines to ensure that the correct channel is selected:  In Single Conversion mode, always select the channel before starting the conversion. The channel selection may be changed one ADC clock cycle after writing one to ADSC. However, the simplest method is to wait for the conversion to complete before changing the channel selection.  In Free Running mode, always select the channel before starting the first conversion. The channel selection may be changed one ADC clock cycle after writing one to ADSC. However, the simplest method is to wait for the first conversion to complete, and then change the channel selection. Since the next conversion has already started automatically, the next result will reflect the previous channel selection. Subsequent conversions will reflect the new channel selection. ATmega16U4/32U4 [DATASHEET] 303 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

When switching to a differential gain channel, the first conversion result may have a poor accuracy due to the required settling time for the automatic offset cancellation circuitry. The user should preferably disregard the first conversion result. 24.5.2 ADC Voltage Reference The reference voltage for the ADC (V ) indicates the conversion range for the ADC. Single ended channels REF that exceed V will result in codes close to 0x3FF. V can be selected as either AV , internal 2.56V REF REF CC reference, or external AREF pin. AV is connected to the ADC through a passive switch. The internal 2.56V reference is generated from the CC internal bandgap reference (V ) through an internal amplifier. In either case, the external AREF pin is directly BG connected to the ADC, and the reference voltage can be made more immune to noise by connecting a capacitor between the AREF pin and ground. V can also be measured at the AREF pin with a high impedance REF voltmeter. Note that V is a high impudent source, and only a capacitive load should be connected in a REF system. If the user has a fixed voltage source connected to the AREF pin, the user may not use the other reference voltage options in the application, as they will be shorted to the external voltage. If no external voltage is applied to the AREF pin, the user may switch between AV and 2.56V as reference selection. The first ADC CC conversion result after switching reference voltage source may be inaccurate, and the user is advised to discard this result. If differential channels are used, the selected reference should not be closer to AV than indicated in Table29- CC 7 on page390. 24.6 Temperature Sensor The ATmega16U4/ATmega32U4 includes an on-chip temperature sensor, whose the value can be read through the A/D Converter. The temperature measurement is based on an on-chip temperature sensor that is coupled to a single ended ADC input. MUX[5..0] bits in ADMUX register enables the temperature sensor. The internal 2.56V voltage reference must also be selected for the ADC voltage reference source in he temperature sensor measurement. When the temperature sensor is enabled, the ADC converter can be used in single conversion mode to measure the voltage over the temperature sensor. The temperature sensor and its internal driver are enabled when ADMUX value selects the temperature sensor as ADC input. The propagation delay of this driver is approximately 2µS. Therefore two successive conversions are required. The correct temperature measurement will be the second one. One can also reduce this timing to one conversion by setting the ADMUX during the previous conversion. Indeed the ADMUX can be programmed to select the temperature sensor just after the beginning of the previous conversion start event and then the driver will be enabled 2µS before sampling and hold phase of temperature sensor measurement. 24.6.1 Sensor Calibration The sensor initial tolerance is large (±10°C), but its characteristic is linear. Thus, if the application requires accuracy, the firmware must include a calibration stage to use the sensor for direct temperature measurement. Another application of this sensor may concern the Internal Calibrated RC Oscillator, whose the frequency can be adjusted by the user through the OSCCAL register (see “Oscillator Calibration Register – OSCCAL” on page32). During the production, a calibration is done at two temperatures (+25°C and +85°C, with a tolerance of ±10°C(1)). At each temperature, the temperature sensor value T is measured and stored in EEPROM i memory(2), and the OSCCAL calibration value O (i.e. the value that should be set in OSCCAL register at this i temperature to have an accurate 8MHz output) is stored in another memory zone. ATmega16U4/32U4 [DATASHEET] 304 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Thanks to these four values and the linear characteristics of the temperature sensor and Internal RC Oscillator, firmware can easily recalibrate the RC Oscillator on-the-go in function of the temperature sensor measure(3) (an application note describes the operation): Figure 24-8. Linear Characterization of OSCCAL in Function of T° Measurement from ADC OSCCAL O2 O1 T1 T2 T(cid:31)(ADC Notes: 1. The temperature sensor calibration values cannot be used to do accurate temperature measurements since the calibration temperature during production is not accurate (±10°C) 2. Be aware that if EESAVE fuse is left unprogrammed, any chip erase operation will clear the temperature sensor calibration values contained in EEPROM memory. 3. Accuracy results after a software recalibration of OSCCAL in function of Temperature is given by characterization. 24.7 ADC Noise Canceler The ADC features a noise canceler that enables conversion during sleep mode to reduce noise induced from the CPU core and other I/O peripherals. The noise canceler can be used with ADC Noise Reduction and Idle mode. To make use of this feature, the following procedure should be used: a. Make sure that the ADC is enabled and is not busy converting. Single Conversion mode must be selected and the ADC conversion complete interrupt must be enabled. 5. Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion once the CPU has been halted. 6. If no other interrupts occur before the ADC conversion completes, the ADC interrupt will wake up the CPU and execute the ADC Conversion Complete interrupt routine. If another interrupt wakes up the CPU before the ADC conversion is complete, that interrupt will be executed, and an ADC Conversion Complete interrupt request will be generated when the ADC conversion completes. The CPU will remain in active mode until a new sleep command is executed. Note that the ADC will not be automatically turned off when entering other sleep modes than Idle mode and ADC Noise Reduction mode. The user is advised to write zero to ADEN before entering such sleep modes to avoid excessive power consumption. If the ADC is enabled in such sleep modes and the user wants to perform differential conversions, the user is advised to switch the ADC off and on after waking up from sleep to prompt an extended conversion to get a valid result. 24.7.1 Analog Input Circuitry The analog input circuitry for single ended channels is illustrated in Figure 24-9. An analog source applied to ADCn is subjected to the pin capacitance and input leakage of that pin, regardless of whether that channel is selected as input for the ADC. When the channel is selected, the source must drive the S/H capacitor through the series resistance (combined resistance in the input path). ATmega16U4/32U4 [DATASHEET] 305 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The ADC is optimized for analog signals with an output impedance of approximately 10k or less. If such a source is used, the sampling time will be negligible. If a source with higher impedance is used, the sampling time will depend on how long time the source needs to charge the S/H capacitor, with can vary widely. The user is recommended to only use low impedance sources with slowly varying signals, since this minimizes the required charge transfer to the S/H capacitor. If differential gain channels are used, the input circuitry looks somewhat different, although source impedances of a few hundred k or less is recommended. Signal components higher than the Nyquist frequency (f /2) should not be present for either kind of channels, ADC to avoid distortion from unpredictable signal convolution. The user is advised to remove high frequency components with a low-pass filter before applying the signals as inputs to the ADC. Figure 24-9. Analog Input Circuitry I IH ADCn 1..100 kΩ C = 14 pF S/H I IL V /2 CC 24.7.2 Analog Noise Canceling Techniques Digital circuitry inside and outside the device generates EMI which might affect the accuracy of analog measurements. If conversion accuracy is critical, the noise level can be reduced by applying the following techniques: a. Keep analog signal paths as short as possible. Make sure analog tracks run over the analog ground plane, and keep them well away from high-speed switching digital tracks. 7. The AV pin on the device should be connected to the digital V supply voltage via an LC network CC CC as shown in Figure 24-10. 8. Use the ADC noise canceler function to reduce induced noise from the CPU. 9. If any ADC port pins are used as digital outputs, it is essential that these do not switch while a conversion is in progress. ATmega16U4/32U4 [DATASHEET] 306 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 24-10. ADC Power Connections 34 VCC GND 35 (ADC7) PF7 36 (ADC6) PF6 37 (ADC5) PF5 38 (ADC4) PF4 39 (ADC1) PF1 40 (ADC0) PF0 41 10µH AREF 42 GND 43 AVCC 44 100nF 1 Analog Ground Plane Note: The same circuitry should be used for AVCC filtering on the ADC8-ADC13 side. 24.7.3 Offset Compensation Schemes The gain stage has a built-in offset cancellation circuitry that nulls the offset of differential measurements as much as possible. The remaining offset in the analog path can be measured directly by selecting the same channel for both differential inputs. This offset residue can be then subtracted in software from the measurement results. Using this kind of software based offset correction, offset on any channel can be reduced below one LSB. 24.7.4 ADC Accuracy Definitions An n-bit single-ended ADC converts a voltage linearly between GND and V in 2n steps (LSBs). The lowest REF code is read as 0, and the highest code is read as 2n-1. Several parameters describe the deviation from the ideal behavior:  Offset: The deviation of the first transition (0x000 to 0x001) compared to the ideal transition (at 0.5 LSB). Ideal value: 0 LSB. ATmega16U4/32U4 [DATASHEET] 307 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 24-11. Offset Error Output Code Ideal ADC Actual ADC Offset Error V Input Voltage REF  Gain Error: After adjusting for offset, the Gain Error is found as the deviation of the last transition (0x3FE to 0x3FF) compared to the ideal transition (at 1.5 LSB below maximum). Ideal value: 0 LSB. Figure 24-12. Gain Error Output Code Gain Error Ideal ADC Actual ADC V Input Voltage REF  Integral Non-linearity (INL): After adjusting for offset and gain error, the INL is the maximum deviation of an actual transition compared to an ideal transition for any code. Ideal value: 0 LSB. ATmega16U4/32U4 [DATASHEET] 308 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 24-13. Integral Non-linearity (INL) Output Code IN L Ideal ADC Actual ADC V Input Voltage REF  Differential Non-linearity (DNL): The maximum deviation of the actual code width (the interval between two adjacent transitions) from the ideal code width (1 LSB). Ideal value: 0 LSB. Figure 24-14. Differential Non-linearity (DNL) Output Code 0x3FF 1 LSB DNL 0x000 0 V Input Voltage REF  Quantization Error: Due to the quantization of the input voltage into a finite number of codes, a range of input voltages (1 LSB wide) will code to the same value. Always ±0.5 LSB.  Absolute Accuracy: The maximum deviation of an actual (unadjusted) transition compared to an ideal transition for any code. This is the compound effect of offset, gain error, differential error, non-linearity, and quantization error. Ideal value: ±0.5 LSB. 24.8 ADC Conversion Result After the conversion is complete (ADIF is high), the conversion result can be found in the ADC Result Registers (ADCL, ADCH). For single ended conversion, the result is: ATmega16U4/32U4 [DATASHEET] 309 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

V 1023 IN ADC = -------------------------- V REF where V is the voltage on the selected input pin and V the selected voltage reference (see Table24-3 on IN REF page313 and Table24-4 on page313). 0x000 represents analog ground, and 0x3FF represents the selected reference voltage minus one LSB. If differential channels are used, the result is: V –V GAIN512 ADC = -------P---O----S------------N----E---G--------------------------------------- V REF where V is the voltage on the positive input pin, V the voltage on the negative input pin, GAIN the POS NEG selected gain factor and V the selected voltage reference. The result is presented in two’s complement form, REF from 0x200 (-512d) through 0x1FF (+511d). Note that if the user wants to perform a quick polarity check of the result, it is sufficient to read the MSB of the result (ADC9 in ADCH). If the bit is one, the result is negative, and if this bit is zero, the result is positive. Figure 24-15 on page 311 shows the decoding of the differential input range. Table 82 shows the resulting output codes if the differential input channel pair (ADCn - ADCm) is selected with a reference voltage of V . REF ATmega16U4/32U4 [DATASHEET] 310 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 24-15. Differential Measurement Range Output Code 0x1FF 0x000 - VREF 0x3FF 0 VREF DVoiflfteargeen t(iVaol Iltnsp)ut 0x200 Table 24-2. Correlation Between Input Voltage and Output Codes V Read code Corresponding decimal value ADCn V + V /GAIN 0x1FF 511 ADCm REF V + 0.999 V /GAIN 0x1FF 511 ADCm REF V + 0.998 V /GAIN 0x1FE 510 ADCm REF ... ... ... V + 0.001 V /GAIN 0x001 1 ADCm REF V 0x000 0 ADCm V - 0.001 V /GAIN 0x3FF -1 ADCm REF ... ... ... V - 0.999 V /GAIN 0x201 -511 ADCm REF V - V /GAIN 0x200 -512 ADCm REF Example 1: ̶ ADMUX = 0xE9, MUX5 = 0 (ADC1 - ADC0, 10x gain, 2.56V reference, left adjusted result) ̶ Voltage on ADC1 is 300mV, voltage on ADC0 is 500mV ̶ ADCR = 512 * 10 * (300 - 500) / 2560 = -400 = 0x270 ATmega16U4/32U4 [DATASHEET] 311 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

̶ ADCL will thus read 0x00, and ADCH will read 0x9C. Writing zero to ADLAR right adjusts the result: ADCL = 0x70, ADCH = 0x02. Example 2: ̶ ADMUX = 0xF0, MUX5 = 0 (ADC0 - ADC1, 1x gain, 2.56V reference, left adjusted result) ̶ Voltage on ADC0 is 300mV, voltage on ADC1 is 500mV ̶ ADCR = 512 * 1 * (300 - 500) / 2560 = -41 = 0x029 ̶ ADCL will thus read 0x40, and ADCH will read 0x0A. Writing zero to ADLAR right adjusts the result: ADCL = 0x00, ADCH = 0x29. ATmega16U4/32U4 [DATASHEET] 312 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

24.9 ADC Register Description 24.9.1 ADC Multiplexer Selection Register – ADMUX Bit 7 6 5 4 3 2 1 0 REFS1 REFS0 ADLAR MUX4 MUX3 MUX2 MUX1 MUX0 ADMUX Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 7:6 – REFS1:0: Reference Selection Bits These bits select the voltage reference for the ADC, as shown in the table. If these bits are changed during a conversion, the change will not go in effect until this conversion is complete (ADIF in ADCSRA is set). The internal voltage reference options may not be used if an external reference voltage is being applied to the AREF pin. Table 24-3. Voltage Reference Selections for ADC REFS1 REFS0 Voltage Reference Selection 0 0 AREF, Internal V turned off REF 0 1 AV with external capacitor on AREF pin CC 1 0 Reserved 1 1 Internal 2.56V Voltage Reference with external capacitor on AREF pin • Bit 5 – ADLAR: ADC Left Adjust Result The ADLAR bit affects the presentation of the ADC conversion result in the ADC Data Register. Write one to ADLAR to left adjust the result. Otherwise, the result is right adjusted. Changing the ADLAR bit will affect the ADC Data Register immediately, regardless of any ongoing conversions. For a complete description of this bit, see “The ADC Data Register – ADCL and ADCH” on page316. • Bits 4:0 – MUX4:0: Analog Channel Selection Bits The value of these bits selects which combination of analog inputs are connected to the ADC. These bits also select the gain for the differential channels as shown in the table. If these bits are changed during a conversion, the change will not go in effect until this conversion is complete (ADIF in ADCSRA is set). Table 24-4. Input Channel and Gain Selections MUX5..0(1) Single Ended Input Positive Differential Input Negative Differential Input Gain 000000 ADC0 000001 ADC1 000010 N/A 000011 N/A 000100 ADC4 000101 ADC5 000110 ADC6 000111 ADC7 ATmega16U4/32U4 [DATASHEET] 313 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 24-4. Input Channel and Gain Selections MUX5..0(1) Single Ended Input Positive Differential Input Negative Differential Input Gain 001000 N/A N/A N/A 001001 ADC1 ADC0 10x 001010 N/A N/A N/A 001011 ADC1 ADC0 200x 001100 N/A 001101 N/A 001110 001111 010000 ADC0 ADC1 1x 010001 010010 N/A 010011 010100 ADC4 ADC1 1x 010101 ADC5 ADC1 1x 010110 ADC6 ADC1 1x 010111 N/A ADC7 ADC1 1x 011000 011001 011010 011011 011100 011101 011110 1.1V (V ) Band Gap N/A 011111 0V (GND) 100000 ADC8 100001 ADC9 100010 ADC10 100011 ADC11 100100 ADC12 100101 ADC13 100110 N/A ADC1 ADC0 40x 100111 Temperature Sensor ATmega16U4/32U4 [DATASHEET] 314 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 24-4. Input Channel and Gain Selections MUX5..0(1) Single Ended Input Positive Differential Input Negative Differential Input Gain 101000 ADC4 ADC0 10x 101001 ADC5 ADC0 10x 101010 ADC6 ADC0 10x 101011 ADC7 ADC0 10x 101100 ADC4 ADC1 10x 101101 ADC5 ADC1 10x N/A 101110 ADC6 ADC1 10x 101111 ADC7 ADC1 10x 110000 ADC4 ADC0 40x 110001 ADC5 ADC0 40x 110010 ADC6 ADC0 40x 110011 ADC7 ADC0 40x 110100 ADC4 ADC1 40x 110101 ADC5 ADC1 40x 110110 ADC6 ADC1 40x 110111 ADC7 ADC1 40x 111000 ADC4 ADC0 200x 111001 ADC5 ADC0 200x N/A 111010 ADC6 ADC0 200x 111011 ADC7 ADC0 200x 111100 ADC4 ADC1 200x 111101 ADC5 ADC1 200x 111110 ADC6 ADC1 200x 111111 ADC7 ADC1 200x Note: 1. MUX5 bit make part of ADCSRB register. 24.9.2 ADC Control and Status Register A – ADCSRA Bit 7 6 5 4 3 2 1 0 ADEN ADSC ADATE ADIF ADIE ADPS2 ADPS1 ADPS0 ADCSRA Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 7 – ADEN: ADC Enable Writing this bit to one enables the ADC. By writing it to zero, the ADC is turned off. Turning the ADC off while a conversion is in progress, will terminate this conversion. ATmega16U4/32U4 [DATASHEET] 315 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 6 – ADSC: ADC Start Conversion In Single Conversion mode, write this bit to one to start each conversion. In Free Running mode, write this bit to one to start the first conversion. The first conversion after ADSC has been written after the ADC has been enabled, or if ADSC is written at the same time as the ADC is enabled, will take 25 ADC clock cycles instead of the normal 13. This first conversion performs initialization of the ADC. ADSC will read as one as long as a conversion is in progress. When the conversion is complete, it returns to zero. Writing zero to this bit has no effect. • Bit 5 – ADATE: ADC Auto Trigger Enable When this bit is written to one, Auto Triggering of the ADC is enabled. The ADC will start a conversion on a positive edge of the selected trigger signal. The trigger source is selected by setting the ADC Trigger Select bits, ADTS in ADCSRB. • Bit 4 – ADIF: ADC Interrupt Flag This bit is set when an ADC conversion completes and the Data Registers are updated. The ADC Conversion Complete Interrupt is executed if the ADIE bit and the I-bit in SREG are set. ADIF is cleared by hardware when executing the corresponding interrupt handling vector. Alternatively, ADIF is cleared by writing a logical one to the flag. Beware that if doing a Read-Modify-Write on ADCSRA, a pending interrupt can be disabled. This also applies if the SBI and CBI instructions are used. • Bit 3 – ADIE: ADC Interrupt Enable When this bit is written to one and the I-bit in SREG is set, the ADC Conversion Complete Interrupt is activated. • Bits 2:0 – ADPS2:0: ADC Prescaler Select Bits These bits determine the division factor between the XTAL frequency and the input clock to the ADC. Table 24-5. ADC Prescaler Selections ADPS2 ADPS1 ADPS0 Division Factor 0 0 0 2 0 0 1 2 0 1 0 4 0 1 1 8 1 0 0 16 1 0 1 32 1 1 0 64 1 1 1 128 24.9.3 The ADC Data Register – ADCL and ADCH 24.9.3.1ADLAR = 0 Bit 15 14 13 12 11 10 9 8 – – – – – – ADC9 ADC8 ADCH ADC7 ADC6 ADC5 ADC4 ADC3 ADC2 ADC1 ADC0 ADCL Bit 7 6 5 4 3 2 1 0 Read/Write R R R R R R R R R R R R R R R R Initial Value 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ATmega16U4/32U4 [DATASHEET] 316 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

24.9.3.2ADLAR = 1 Bit 15 14 13 12 11 10 9 8 ADC9 ADC8 ADC7 ADC6 ADC5 ADC4 ADC3 ADC2 ADCH ADC1 ADC0 – – – – – – ADCL Bit 7 6 5 4 3 2 1 0 Read/Write R R R R R R R R R R R R R R R R Initial Value 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 When an ADC conversion is complete, the result is found in these two registers. If differential channels are used, the result is presented in two’s complement form. When ADCL is read, the ADC Data Register is not updated until ADCH is read. Consequently, if the result is left adjusted and no more than 8-bit precision (7 bit + sign bit for differential input channels) is required, it is sufficient to read ADCH. Otherwise, ADCL must be read first, then ADCH. The ADLAR bit in ADMUX, and the MUXn bits in ADMUX affect the way the result is read from the registers. If ADLAR is set, the result is left adjusted. If ADLAR is cleared (default), the result is right adjusted. • ADC9:0: ADC Conversion Result These bits represent the result from the conversion, as detailed in “ADC Conversion Result” on page309. 24.9.4 ADC Control and Status Register B – ADCSRB Bit 7 6 5 4 3 2 1 0 ADHSM ACME MUX5 – ADTS3 ADTS2 ADTS1 ADTS0 ADCSRB Read/Write R/W R/W R R R R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 7 – ADHSM: ADC High Speed Mode Writing this bit to one enables the ADC High Speed mode. This mode enables higher conversion rate at the expense of higher power consumption. • Bit 5 – MUX5: Analog Channel Additional Selection Bits This bit make part of MUX5:0 bits of ADRCSRB and ADMUX register, that select the combination of analog inputs connected to the ADC (including differential amplifier configuration). • Bit 3:0 – ADTS3:0: ADC Auto Trigger Source If ADATE in ADCSRA is written to one, the value of these bits selects which source will trigger an ADC conversion. If ADATE is cleared, the ADTS3:0 settings will have no effect. A conversion will be triggered by the rising edge of the selected interrupt flag. Note that switching from a trigger source that is cleared to a trigger source that is set, will generate a positive edge on the trigger signal. If ADEN in ADCSRA is set, this will start a conversion. Switching to Free Running mode (ADTS[3:0]=0) will not cause a trigger event, even if the ADC Interrupt Flag is set. ATmega16U4/32U4 [DATASHEET] 317 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 24-6. ADC Auto Trigger Source Selections ADTS3 ADTS2 ADTS1 ADTS0 Trigger Source 0 0 0 0 Free Running mode 0 0 0 1 Analog Comparator 0 0 1 0 External Interrupt Request 0 0 0 1 1 Timer/Counter0 Compare Match A 0 1 0 0 Timer/Counter0 Overflow 0 1 0 1 Timer/Counter1 Compare Match B 0 1 1 0 Timer/Counter1 Overflow 0 1 1 1 Timer/Counter1 Capture Event 1 0 0 0 Timer/Counter4 Overflow 1 0 0 1 Timer/Counter4 Compare Match A 1 0 1 0 Timer/Counter4 Compare Match B 1 0 1 1 Timer/Counter4 Compare Match D 24.9.5 Digital Input Disable Register0 – DIDR0 Bit 7 6 5 4 3 2 1 0 ADC7D ADC6D ADC5D ADC4D - - ADC1D ADC0D DIDR0 Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 7:4, 1:0 – ADC7D..4D - ADC1D..0D: ADC7:4 - ADC1:0 Digital Input Disable When this bit is written logic one, the digital input buffer on the corresponding ADC pin is disabled. The corresponding PIN Register bit will always read as zero when this bit is set. When an analog signal is applied to the ADC7..4 / ADC1..0 pin and the digital input from this pin is not needed, this bit should be written logic one to reduce power consumption in the digital input buffer. 24.9.6 Digital Input Disable Register2 – DIDR2 Bit 7 6 5 4 3 2 1 0 - - ADC13D ADC12D ADC11D ADC10D ADC9D ADC8D DIDR2 Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 5:0 – ADC13D..ADC8D: ADC13:8 Digital Input Disable When this bit is written logic one, the digital input buffer on the corresponding ADC pin is disabled. The corresponding PIN Register bit will always read as zero when this bit is set. When an analog signal is applied to the ADC13..8 pin and the digital input from this pin is not needed, this bit should be written logic one to reduce power consumption in the digital input buffer. ATmega16U4/32U4 [DATASHEET] 318 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

25. JTAG Interface and On-chip Debug System 25.0.1 Features • JTAG (IEEE std. 1149.1 Compliant) Interface • Boundary-scan Capabilities According to the IEEE std. 1149.1 (JTAG) Standard • Debugger Access to: – All Internal Peripheral Units – Internal and External RAM – The Internal Register File – Program Counter – EEPROM and Flash Memories • Extensive On-chip Debug Support for Break Conditions, Including – AVR Break Instruction – Break on Change of Program Memory Flow – Single Step Break – Program Memory Break Points on Single Address or Address Range – Data Memory Break Points on Single Address or Address Range • Programming of Flash, EEPROM, Fuses, and Lock Bits through the JTAG Interface • On-chip Debugging Supported by AVR Studio® 25.1 Overview The AVR IEEE std. 1149.1 compliant JTAG interface can be used for:  Testing PCBs by using the JTAG Boundary-scan capability  Programming the non-volatile memories, Fuses and Lock bits  On-chip debugging A brief description is given in the following sections. Detailed descriptions for Programming via the JTAG interface, and using the Boundary-scan Chain can be found in the sections “Programming via the JTAG Interface” on page371 and “IEEE 1149.1 (JTAG) Boundary-scan” on page325, respectively. The On-chip Debug support is considered being private JTAG instructions, and distributed within Atmel and to selected third party vendors only. Figure 25-1 shows a block diagram of the JTAG interface and the On-chip Debug system. The TAP Controller is a state machine controlled by the TCK and TMS signals. The TAP Controller selects either the JTAG Instruction Register or one of several Data Registers as the scan chain (Shift Register) between the TDI – input and TDO – output. The Instruction Register holds JTAG instructions controlling the behavior of a Data Register. The ID-Register, Bypass Register, and the Boundary-scan Chain are the Data Registers used for board-level testing. The JTAG Programming Interface (actually consisting of several physical and virtual Data Registers) is used for serial programming via the JTAG interface. The Internal Scan Chain and Break Point Scan Chain are used for On-chip debugging only. 25.2 Test Access Port – TAP The JTAG interface is accessed through four of the AVR’s pins. In JTAG terminology, these pins constitute the Test Access Port – TAP. These pins are:  TMS: Test mode select. This pin is used for navigating through the TAP-controller state machine.  TCK: Test Clock. JTAG operation is synchronous to TCK.  TDI: Test Data In. Serial input data to be shifted in to the Instruction Register or Data Register (Scan Chains).  TDO: Test Data Out. Serial output data from Instruction Register or Data Register. ATmega16U4/32U4 [DATASHEET] 319 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The IEEE std. 1149.1 also specifies an optional TAP signal; TRST – Test ReSeT – which is not provided. When the JTAGEN Fuse is unprogrammed, these four TAP pins are normal port pins, and the TAP controller is in reset. When programmed, the input TAP signals are internally pulled high and the JTAG is enabled for Boundary-scan and programming. The device is shipped with this fuse programmed. For the On-chip Debug system, in addition to the JTAG interface pins, the RESET pin is monitored by the debugger to be able to detect external reset sources. The debugger can also pull the RESET pin low to reset the whole system, assuming only open collectors on the reset line are used in the application. Figure 25-1. Block Diagram I/O PORT 0 DEVICE BOUNDARY BOUNDARY SCAN CHAIN TDI TDO TAP JTAGI NPTREORGFRAACMEMING TCK CONTROLLER TMS AVR CPU INTERNAL INRSETGRUISCTTEIRON MFELMAOSRHY AddDreastas CSHCAAINN PInCstruction ID REGISTER BREAKPOINT UNIT MUX RBEYGPISATSESR FLOWU CNOITNTROL PEDRUIIGPNIHITTEASRLAL PEARUNIPNAHILTEOSRGIAL Analog inputs BREAKPOINT SCAN CHAIN JTAG / AVR CORE COMMUNICATION DAEDCDORDESESR OCD STATUS INTERFACE AND CONTROL Control & Clock lines I/O PORT n ATmega16U4/32U4 [DATASHEET] 320 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 25-2. TAP Controller State Diagram 1 Test-Logic-Reset 0 0 Run-Test/Idle 1 Select-DR Scan 1 Select-IR Scan 1 0 0 1 1 Capture-DR Capture-IR 0 0 Shift-DR 0 Shift-IR 0 1 1 1 1 Exit1-DR Exit1-IR 0 0 Pause-DR 0 Pause-IR 0 1 1 0 0 Exit2-DR Exit2-IR 1 1 Update-DR Update-IR 1 0 1 0 25.3 TAP Controller The TAP controller is a 16-state finite state machine that controls the operation of the Boundary-scan circuitry, JTAG programming circuitry, or On-chip Debug system. The state transitions depicted in Figure 25-2 depend on the signal present on TMS (shown adjacent to each state transition) at the time of the rising edge at TCK. The initial state after a Power-on Reset is Test-Logic-Reset. As a definition in this document, the LSB is shifted in and out first for all Shift Registers. Assuming Run-Test/Idle is the present state, a typical scenario for using the JTAG interface is:  At the TMS input, apply the sequence 1, 1, 0, 0 at the rising edges of TCK to enter the Shift Instruction Register – Shift-IR state. While in this state, shift the four bits of the JTAG instructions into the JTAG Instruction Register from the TDI input at the rising edge of TCK. The TMS input must be held low during input of the three LSBs in order to remain in the Shift-IR state. The MSB of the instruction is shifted in when this state is left by setting TMS high. While the instruction is shifted in from the TDI pin, the captured IR-state 0x01 is shifted out on the TDO pin. The JTAG Instruction selects a particular Data Register as path between TDI and TDO and controls the circuitry surrounding the selected Data Register.  Apply the TMS sequence 1, 1, 0 to re-enter the Run-Test/Idle state. The instruction is latched onto the parallel output from the Shift Register path in the Update-IR state. The Exit-IR, Pause-IR, and Exit2-IR states are only used for navigating the state machine. ATmega16U4/32U4 [DATASHEET] 321 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

 At the TMS input, apply the sequence 1, 0, 0 at the rising edges of TCK to enter the Shift Data Register – Shift-DR state. While in this state, upload the selected Data Register (selected by the present JTAG instruction in the JTAG Instruction Register) from the TDI input at the rising edge of TCK. In order to remain in the Shift-DR state, the TMS input must be held low during input of all bits except the MSB. The MSB of the data is shifted in when this state is left by setting TMS high. While the Data Register is shifted in from the TDI pin, the parallel inputs to the Data Register captured in the Capture-DR state is shifted out on the TDO pin.  Apply the TMS sequence 1, 1, 0 to re-enter the Run-Test/Idle state. If the selected Data Register has a latched parallel-output, the latching takes place in the Update-DR state. The Exit-DR, Pause-DR, and Exit2-DR states are only used for navigating the state machine. As shown in the state diagram, the Run-Test/Idle state need not be entered between selecting JTAG instruction and using Data Registers, and some JTAG instructions may select certain functions to be performed in the Run- Test/Idle, making it unsuitable as an Idle state. Note: Independent of the initial state of the TAP Controller, the Test-Logic-Reset state can always be entered by holding TMS high for five TCK clock periods. For detailed information on the JTAG specification, refer to the literature listed in “Bibliography” on page324. 25.4 Using the Boundary-scan Chain A complete description of the Boundary-scan capabilities are given in the section “IEEE 1149.1 (JTAG) Boundary-scan” on page325. 25.5 Using the On-chip Debug System As shown in Figure 25-1 on page 320, the hardware support for On-chip Debugging consists mainly of  A scan chain on the interface between the internal AVR CPU and the internal peripheral units  Break Point unit  Communication interface between the CPU and JTAG system All read or modify/write operations needed for implementing the Debugger are done by applying AVR instructions via the internal AVR CPU Scan Chain. The CPU sends the result to an I/O memory mapped location which is part of the communication interface between the CPU and the JTAG system. The Break Point Unit implements Break on Change of Program Flow, Single Step Break, two Program Memory Break Points, and two combined Break Points. Together, the four Break Points can be configured as either:  Four single Program Memory Break Points  Three Single Program Memory Break Point + one single Data Memory Break Point  Two Single Program Memory Break Points + two single Data Memory Break Points  Two Single Program Memory Break Points + one Program Memory Break Point with mask (“range Break Point”)  Two Single Program Memory Break Points + 1 Data Memory Break Point with mask (“range Break Point”). A debugger, like the AVR Studio, may however use one or more of these resources for its internal purpose, leaving less flexibility to the end-user. A list of the On-chip Debug specific JTAG instructions is given in “On-chip Debug Specific JTAG Instructions” on page323. The JTAGEN Fuse must be programmed to enable the JTAG Test Access Port. In addition, the OCDEN Fuse must be programmed and no Lock bits must be set for the On-chip debug system to work. As a security feature, the On-chip debug system is disabled when either of the LB1 or LB2 Lock bits are set. Otherwise, the On-chip debug system would have provided a back-door into a secured device. ATmega16U4/32U4 [DATASHEET] 322 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The AVR Studio enables the user to fully control execution of programs on an AVR device with On-chip Debug capability, AVR In-Circuit Emulator, or the built-in AVR Instruction Set Simulator. AVR Studio® supports source level execution of Assembly programs assembled with Atmel Corporation’s AVR Assembler and C programs compiled with third party vendors’ compilers. AVR Studio runs under Microsoft® Windows® 95/98/2000 and Microsoft Windows NT. For a full description of the AVR Studio, refer to the AVR Studio User Guide. Only highlights are presented in this document. All necessary execution commands are available in AVR Studio, both on source level and on disassembly level. The user can execute the program, single step through the code either by tracing into or stepping over functions, step out of functions, place the cursor on a statement and execute until the statement is reached, stop the execution, and reset the execution target. In addition, the user can have an unlimited number of code Break Points (using the BREAK instruction) and up to two data memory Break Points, alternatively combined as a mask (range) Break Point. 25.6 On-chip Debug Specific JTAG Instructions The On-chip debug support is considered being private JTAG instructions, and distributed within Atmel and to selected third party vendors only. Instruction opcodes are listed for reference. 25.6.1 PRIVATE0; 0x8 Private JTAG instruction for accessing On-chip debug system. 25.6.2 PRIVATE1; 0x9 Private JTAG instruction for accessing On-chip debug system. 25.6.3 PRIVATE2; 0xA Private JTAG instruction for accessing On-chip debug system. 25.6.4 PRIVATE3; 0xB Private JTAG instruction for accessing On-chip debug system. 25.7 On-chip Debug Related Register in I/O Memory 25.7.1 On-chip Debug Register – OCDR Bit 7 6 5 4 3 2 1 0 MSB/IDRD LSB OCDR Read/Write R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 The OCDR Register provides a communication channel from the running program in the microcontroller to the debugger. The CPU can transfer a byte to the debugger by writing to this location. At the same time, an internal flag; I/O Debug Register Dirty – IDRD – is set to indicate to the debugger that the register has been written. When the CPU reads the OCDR Register the seven LSB will be from the OCDR Register, while the MSB is the IDRD bit. The debugger clears the IDRD bit when it has read the information. In some AVR devices, this register is shared with a standard I/O location. In this case, the OCDR Register can only be accessed if the OCDEN Fuse is programmed, and the debugger enables access to the OCDR Register. In all other cases, the standard I/O location is accessed. ATmega16U4/32U4 [DATASHEET] 323 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Refer to the debugger documentation for further information on how to use this register. 25.8 Using the JTAG Programming Capabilities Programming of AVR parts via JTAG is performed via the 4-pin JTAG port, TCK, TMS, TDI, and TDO. These are the only pins that need to be controlled/observed to perform JTAG programming (in addition to power pins). It is not required to apply 12V externally. The JTAGEN Fuse must be programmed and the JTD bit in the MCUCR Register must be cleared to enable the JTAG Test Access Port. The JTAG programming capability supports:  Flash programming and verifying  EEPROM programming and verifying  Fuse programming and verifying  Lock bit programming and verifying The Lock bit security is exactly as in parallel programming mode. If the Lock bits LB1 or LB2 are programmed, the OCDEN Fuse cannot be programmed unless first doing a chip erase. This is a security feature that ensures no back-door exists for reading out the content of a secured device. The details on programming through the JTAG interface and programming specific JTAG instructions are given in the section “Programming via the JTAG Interface” on page371. 25.9 Bibliography For more information about general Boundary-scan, the following literature can be consulted:  IEEE: IEEE Std. 1149.1-1990. IEEE Standard Test Access Port and Boundary-scan Architecture, IEEE, 1993.  Colin Maunder: The Board Designers Guide to Testable Logic Circuits, Addison-Wesley, 1992. ATmega16U4/32U4 [DATASHEET] 324 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

26. IEEE 1149.1 (JTAG) Boundary-scan 26.1 Features • JTAG (IEEE std. 1149.1 compliant) Interface • Boundary-scan Capabilities According to the JTAG Standard • Full Scan of all Port Functions as well as Analog Circuitry having Off-chip Connections • Supports the Optional IDCODE Instruction • Additional Public AVR_RESET Instruction to Reset the AVR 26.2 System Overview The Boundary-scan chain has the capability of driving and observing the logic levels on the digital I/O pins, as well as the boundary between digital and analog logic for analog circuitry having off-chip connections. At system level, all ICs having JTAG capabilities are connected serially by the TDI/TDO signals to form a long Shift Register. An external controller sets up the devices to drive values at their output pins, and observe the input values received from other devices. The controller compares the received data with the expected result. In this way, Boundary-scan provides a mechanism for testing interconnections and integrity of components on Printed Circuits Boards by using the four TAP signals only. The four IEEE 1149.1 defined mandatory JTAG instructions IDCODE, BYPASS, SAMPLE/PRELOAD, and EXTEST, as well as the AVR specific public JTAG instruction AVR_RESET can be used for testing the Printed Circuit Board. Initial scanning of the Data Register path will show the ID-Code of the device, since IDCODE is the default JTAG instruction. It may be desirable to have the AVR device in reset during test mode. If not reset, inputs to the device may be determined by the scan operations, and the internal software may be in an undetermined state when exiting the test mode. Entering reset, the outputs of any port pin will instantly enter the high impedance state, making the HIGHZ instruction redundant. If needed, the BYPASS instruction can be issued to make the shortest possible scan chain through the device. The device can be set in the reset state either by pulling the external RESET pin low, or issuing the AVR_RESET instruction with appropriate setting of the Reset Data Register. The EXTEST instruction is used for sampling external pins and loading output pins with data. The data from the output latch will be driven out on the pins as soon as the EXTEST instruction is loaded into the JTAG IR- Register. Therefore, the SAMPLE/PRELOAD should also be used for setting initial values to the scan ring, to avoid damaging the board when issuing the EXTEST instruction for the first time. SAMPLE/PRELOAD can also be used for taking a snapshot of the external pins during normal operation of the part. The JTAGEN Fuse must be programmed and the JTD bit in the I/O Register MCUCR must be cleared to enable the JTAG Test Access Port. When using the JTAG interface for Boundary-scan, using a JTAG TCK clock frequency higher than the internal chip frequency is possible. The chip clock is not required to run. 26.3 Data Registers The Data Registers relevant for Boundary-scan operations are:  Bypass Register  Device Identification Register  Reset Register  Boundary-scan Chain ATmega16U4/32U4 [DATASHEET] 325 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

26.3.1 Bypass Register The Bypass Register consists of a single Shift Register stage. When the Bypass Register is selected as path between TDI and TDO, the register is reset to 0 when leaving the Capture-DR controller state. The Bypass Register can be used to shorten the scan chain on a system when the other devices are to be tested. 26.3.2 Device Identification Register Figure 26-1 shows the structure of the Device Identification Register. Figure 26-1. The Format of the Device Identification Register MSB LSB Bit 31 28 27 12 11 1 0 Device ID Version Part Number Manufacturer ID 1 4 bits 16 bits 11 bits 1-bit 26.3.2.1Version Version is a 4-bit number identifying the revision of the component. The JTAG version number follows the revision of the device. Revision A is 0x0, revision B is 0x1 and so on. 26.3.2.2Part Number The part number is a 16-bit code identifying the component. The JTAG Part Number for ATmega16U4/ATmega32U4 is listed in this table. Part Number JTAG Part Number (Hex) AVR USB 0x9782 26.3.2.3Manufacturer ID The Manufacturer ID is a 11-bit code identifying the manufacturer. The JTAG manufacturer ID for Atmel is listed below. Manufacturer JTAG Manufacturer ID (Hex) Atmel 0x01F 26.3.3 Reset Register The Reset Register is a test Data Register used to reset the part. Since the AVR tri-states Port Pins when reset, the Reset Register can also replace the function of the un-implemented optional JTAG instruction HIGHZ. A high value in the Reset Register corresponds to pulling the external Reset low. The part is reset as long as there is a high value present in the Reset Register. Depending on the fuse settings for the clock options, the part will remain reset for a reset time-out period (refer to “Clock Sources” on page28) after releasing the Reset Register. The output from this Data Register is not latched, so the reset will take place immediately, as shown in Figure 26-2. ATmega16U4/32U4 [DATASHEET] 326 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 26-2. Reset Register To TDO From Other Internal and External Reset Sources From Internal reset D Q TDI ClockDR · AVR_RESET 26.3.4 Boundary-scan Chain The Boundary-scan Chain has the capability of driving and observing the logic levels on the digital I/O pins, as well as the boundary between digital and analog logic for analog circuitry having off-chip connections. See “Boundary-scan Chain” on page329 for a complete description. 26.4 Boundary-scan Specific JTAG Instructions The Instruction Register is 4-bit wide, supporting up to 16 instructions. Listed below are the JTAG instructions useful for Boundary-scan operation. Note that the optional HIGHZ instruction is not implemented, but all outputs with tri-state capability can be set in high-impedance state by using the AVR_RESET instruction, since the initial state for all port pins is tri-state. As a definition in this datasheet, the LSB is shifted in and out first for all Shift Registers. The OPCODE for each instruction is shown behind the instruction name in hex format. The text describes which Data Register is selected as path between TDI and TDO for each instruction. 26.4.1 EXTEST; 0x0 Mandatory JTAG instruction for selecting the Boundary-scan Chain as Data Register for testing circuitry external to the AVR package. For port-pins, Pull-up Disable, Output Control, Output Data, and Input Data are all accessible in the scan chain. For Analog circuits having off-chip connections, the interface between the analog and the digital logic is in the scan chain. The contents of the latched outputs of the Boundary-scan chain is driven out as soon as the JTAG IR-Register is loaded with the EXTEST instruction. The active states are:  Capture-DR: Data on the external pins are sampled into the Boundary-scan Chain  Shift-DR: The Internal Scan Chain is shifted by the TCK input  Update-DR: Data from the scan chain is applied to output pins 26.4.2 IDCODE; 0x1 Optional JTAG instruction selecting the 32-bit ID-Register as Data Register. The ID-Register consists of a version number, a device number and the manufacturer code chosen by JEDEC. This is the default instruction after power-up. The active states are:  Capture-DR: Data in the IDCODE Register is sampled into the Boundary-scan Chain  Shift-DR: The IDCODE scan chain is shifted by the TCK input ATmega16U4/32U4 [DATASHEET] 327 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

26.4.3 SAMPLE_PRELOAD; 0x2 Mandatory JTAG instruction for pre-loading the output latches and taking a snap-shot of the input/output pins without affecting the system operation. However, the output latches are not connected to the pins. The Boundary-scan Chain is selected as Data Register. The active states are:  Capture-DR: Data on the external pins are sampled into the Boundary-scan Chain  Shift-DR: The Boundary-scan Chain is shifted by the TCK input  Update-DR: Data from the Boundary-scan chain is applied to the output latches. However, the output latches are not connected to the pins. 26.4.4 AVR_RESET; 0xC The AVR specific public JTAG instruction for forcing the AVR device into the Reset mode or releasing the JTAG reset source. The TAP controller is not reset by this instruction. The one bit Reset Register is selected as Data Register. Note that the reset will be active as long as there is a logic “one” in the Reset Chain. The output from this chain is not latched. The active states are:  Shift-DR: The Reset Register is shifted by the TCK input 26.4.5 BYPASS; 0xF Mandatory JTAG instruction selecting the Bypass Register for Data Register. The active states are:  Capture-DR: Loads a logic “0” into the Bypass Register  Shift-DR: The Bypass Register cell between TDI and TDO is shifted 26.5 Boundary-scan Related Register in I/O Memory 26.5.1 MCU Control Register – MCUCR The MCU Control Register contains control bits for general MCU functions. Bit 7 6 5 4 3 2 1 0 JTD – – PUD – – IVSEL IVCE MCUCR Read/Write R/W R R R/W R R R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bits 7 – JTD: JTAG Interface Disable When this bit is zero, the JTAG interface is enabled if the JTAGEN Fuse is programmed. If this bit is one, the JTAG interface is disabled. In order to avoid unintentional disabling or enabling of the JTAG interface, a timed sequence must be followed when changing this bit: The application software must write this bit to the desired value twice within four cycles to change its value. Note that this bit must not be altered when using the On-chip Debug system. 26.5.2 MCU Status Register – MCUSR The MCU Status Register provides information on which reset source caused an MCU reset. Bit 7 6 5 4 3 2 1 0 – – – JTRF WDRF BORF EXTRF PORF MCUSR Read/Write R R R R/W R/W R/W R/W R/W Initial Value 0 0 0 See Bit Description ATmega16U4/32U4 [DATASHEET] 328 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

• Bit 4 – JTRF: JTAG Reset Flag This bit is set if a reset is being caused by a logic one in the JTAG Reset Register selected by the JTAG instruction AVR_RESET. This bit is reset by a Power-on Reset, or by writing a logic zero to the flag. 26.6 Boundary-scan Chain The Boundary-scan chain has the capability of driving and observing the logic levels on the digital I/O pins, as well as the boundary between digital and analog logic for analog circuitry having off-chip connection. 26.6.1 Scanning the Digital Port Pins Figure 26-3 on page 330 shows the Boundary-scan Cell for a bi-directional port pin. The pull-up function is disabled during Boundary-scan when the JTAG IC contains EXTEST or SAMPLE_PRELOAD. The cell consists of a bi-directional pin cell that combines the three signals Output Control - OCxn, Output Data - ODxn, and Input Data - IDxn, into only a two-stage Shift Register. The port and pin indexes are not used in the following description. The Boundary-scan logic is not included in the figures in the datasheet. Figure 26-4 on page 331 shows a simple digital port pin as described in the section “I/O-Ports” on page67. The Boundary-scan details from Figure 26-3 on page 330 replaces the dashed box in Figure 26-4 on page 331. When no alternate port function is present, the Input Data - ID - corresponds to the PINxn Register value (but ID has no synchronizer), Output Data corresponds to the PORT Register, Output Control corresponds to the Data Direction - DD Register, and the Pull-up Enable - PUExn - corresponds to logic expression PUD · DDxn · PORTxn. Digital alternate port functions are connected outside the dotted box in Figure 26-4 on page 331 to make the scan chain read the actual pin value. For analog function, there is a direct connection from the external pin to the analog circuit. There is no scan chain on the interface between the digital and the analog circuitry, but some digital control signal to analog circuitry are turned off to avoid driving contention on the pads. When JTAG IR contains EXTEST or SAMPLE_PRELOAD the clock is not sent out on the port pins even if the CKOUT fuse is programmed. Even though the clock is output when the JTAG IR contains SAMPLE_PRELOAD, the clock is not sampled by the boundary scan. ATmega16U4/32U4 [DATASHEET] 329 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 26-3. Boundary-scan Cell for bi-directional Port Pin with Pull-up Function ShiftDR To Next Cell EXTEST Vcc Pull-up Enable (PUE) 0 1 Output Control (OC) FF1 LD1 0 0 D Q D Q 1 1 G Output Data (OD) n) X 0 FF0 LD0 0 P 1 01 D Q D Q 1 Port Pin ( G Input Data (ID) From Last Cell ClockDR UpdateDR ATmega16U4/32U4 [DATASHEET] 330 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 26-4. General Port Pin Schematic Diagram See Boundary-scan Description for Details! PUExn PUD Q D DDxn QCLR WDx RESET OCxn RDx S U B Pxn ODxn QPORTxDn TA IDxn QCLR WRx DA RESET SLEEP RRx SYNCHRONIZER RPx D Q D Q PINxn L Q Q CLKI/O PUD: PULLUP DISABLE WDx: WRITE DDRx PUExn: PULLUP ENABLE for pin Pxn RDx: READ DDRx OCxn: OUTPUT CONTROL for pin Pxn WRx: WRITE PORTx ODxn: OUTPUT DATA to pin Pxn RRx: READ PORTx REGISTER IDxn: INPUT DATA from pin Pxn RPx: READ PORTx PIN SLEEP: SLEEP CONTROL CLK I / O : I/O CLOCK 26.6.2 Scanning the RESET Pin The RESET pin accepts 5V active low logic for standard reset operation, and 12V active high logic for High Voltage Parallel programming. An observe-only cell as shown in Figure 26-5 is inserted for the 5V reset signal. Figure 26-5. Observe-only Cell To Next ShiftDR Cell From System Pin To System Logic FF1 0 D Q 1 From ClockDR Previous Cell ATmega16U4/32U4 [DATASHEET] 331 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

26.7 Boundary-scan Order The table below shows the Scan order between TDI and TDO when the Boundary-scan chain is selected as data path. Bit 0 is the LSB; the first bit scanned in, and the first bit scanned out. The scan order follows the pin- out order as far as possible. Exceptions from the rules are the Scan chains for the analog circuits, which constitute the most significant bits of the scan chain regardless of which physical pin they are connected to. In Figure 26-3 on page 330, PXn. Data corresponds to FF0, PXn. Control corresponds to FF1, PXn. Bit 4, 5, 6, and 7 of Port F is not in the scan chain, since these pins constitute the TAP pins when the JTAG is enabled. The USB pads are not included in the boundary-scan. Table 26-1. ATmega16U4/ATmega32U4 Boundary-scan Order Bit Number Signal Name Module 88 PE6.Data 87 PE6.Control 86 Reserved Port E 85 Reserved 84 Reserved 83 Reserved 82 PB0.Data 81 PB0.Control 80 PB1.Data 79 PB1.Control 78 PB2.Data 77 PB2.Control 76 PB3.Data 75 PB3.Control Port B 74 PB4.Data 73 PB4.Control 72 PB5.Data 71 PB5.Control 70 PB6.Data 69 PB6.Control 68 PB7.Data 67 PB7.Control 66 Reserved 65 Reserved PORTE 64 Reserved 63 Reserved 62 RSTT Reset Logic (Observe Only) ATmega16U4/32U4 [DATASHEET] 332 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 26-1. ATmega16U4/ATmega32U4 Boundary-scan Order Bit Number Signal Name Module 61 PD0.Data 60 PD0.Control 59 PD1.Data 58 PD1.Control 57 PD2.Data 56 PD2.Control 55 PD3.Data 54 PD3.Control Port D 53 PD4.Data 52 PD4.Control 51 PD5.Data 50 PD5.Control 49 PD6.Data 48 PD6.Control 47 PD7.Data 46 PD7.Control 45 Reserved 44 Reserved Port E 43 Reserved 42 Reserved ATmega16U4/32U4 [DATASHEET] 333 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 26-1. ATmega16U4/ATmega32U4 Boundary-scan Order Bit Number Signal Name Module 41 Reserved 40 Reserved 39 Reserved 38 Reserved 37 Reserved 36 Reserved 35 Reserved 34 Reserved Reserved 33 Reserved 32 Reserved 31 Reserved 30 Reserved 29 Reserved 28 Reserved 27 Reserved 26 Reserved 25 PE2.Data Port E 24 PE2.Control ATmega16U4/32U4 [DATASHEET] 334 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 26-1. ATmega16U4/ATmega32U4 Boundary-scan Order Bit Number Signal Name Module 23 Reserved 22 Reserved 21 Reserved 20 Reserved 19 Reserved 18 Reserved 17 Reserved 16 Reserved Reserved 15 Reserved 14 Reserved 13 Reserved 12 Reserved 11 Reserved 10 Reserved 9 Reserved 8 Reserved 7 Reserved 6 Reserved 5 Reserved 4 Reserved Port F 3 PF1.Data 2 PF1.Control 1 PF0.Data 0 PF0.Control 26.8 Boundary-scan Description Language Files Boundary-scan Description Language (BSDL) files describe Boundary-scan capable devices in a standard format used by automated test-generation software. The order and function of bits in the Boundary-scan Data Register are included in this description. BSDL files are available for the device. ATmega16U4/32U4 [DATASHEET] 335 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

27. Boot Loader Support – Read-While-Write Self-Programming The Boot Loader Support provides a real Read-While-Write Self-Programming mechanism for downloading and uploading program code by the MCU itself. This feature allows flexible application software updates controlled by the MCU using a Flash-resident Boot Loader program. The Boot Loader program can use any available data interface and associated protocol to read code and write (program) that code into the Flash memory, or read the code from the program memory. The program code within the Boot Loader section has the capability to write into the entire Flash, including the Boot Loader memory. The Boot Loader can thus even modify itself, and it can also erase itself from the code if the feature is not needed anymore. The size of the Boot Loader memory is configurable with fuses and the Boot Loader has two separate sets of Boot Lock bits which can be set independently. This gives the user a unique flexibility to select different levels of protection. General information on SPM and ELPM is provided in “AVR CPU Core” on page9. 27.1 Boot Loader Features • Read-While-Write Self-Programming • Flexible Boot Memory Size • High Security (Separate Boot Lock Bits for a Flexible Protection) • Separate Fuse to Select Reset Vector • Optimized Page(1) Size • Code Efficient Algorithm • Efficient Read-Modify-Write Support Note: 1. A page is a section in the Flash consisting of several bytes (see Table28-11 on page359) used during programming. The page organization does not affect normal operation. 27.2 Application and Boot Loader Flash Sections The Flash memory is organized in two main sections, the Application section and the Boot Loader section (see Figure 27-2 on page 338). The size of the different sections is configured by the BOOTSZ Fuses as shown in Table27-8 on page349 and Figure 27-2 on page 338. These two sections can have different level of protection since they have different sets of Lock bits. 27.2.1 Application Section The Application section is the section of the Flash that is used for storing the application code. The protection level for the Application section can be selected by the application Boot Lock bits (Boot Lock bits 0). The Application section can never store any Boot Loader code since the SPM instruction is disabled when executed from the Application section. Refer to Table27-2 on page339. 27.2.2 BLS – Boot Loader Section While the Application section is used for storing the application code, the The Boot Loader software must be located in the BLS since the SPM instruction can initiate a programming when executing from the BLS only. The SPM instruction can access the entire Flash, including the BLS itself. The protection level for the Boot Loader section can be selected by the Boot Loader Lock bits (Boot Lock bits 1). Refer to Table27-3 on page339. 27.3 Read-While-Write and No Read-While-Write Flash Sections Whether the CPU supports Read-While-Write or if the CPU is halted during a Boot Loader software update is dependent on which address that is being programmed. In addition to the two sections that are configurable by the BOOTSZ Fuses as described above, the Flash is also divided into two fixed sections, the Read-While-Write (RWW) section and the No Read-While-Write (NRWW) section. ATmega16U4/32U4 [DATASHEET] 336 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

The limit between the RWW- and NRWW sections is given in Table27-1 on page337 and Figure 27-1 on page 338. The main difference between the two sections is:  When erasing or writing a page located inside the RWW section, the NRWW section can be read during the operation  When erasing or writing a page located inside the NRWW section, the CPU is halted during the entire operation Note that the user software can never read any code that is located inside the RWW section during a Boot Loader software operation. The syntax “Read-While-Write section” refers to which section that is being programmed (erased or written), not which section that actually is being read during a Boot Loader software update. 27.3.1 RWW – Read-While-Write Section If a Boot Loader software update is programming a page inside the RWW section, it is possible to read code from the Flash, but only code that is located in the NRWW section. During an on-going programming, the software must ensure that the RWW section never is being read. If the user software is trying to read code that is located inside the RWW section (i.e., by load program memory, call, or jump instructions or an interrupt) during programming, the software might end up in an unknown state. To avoid this, the interrupts should either be disabled or moved to the Boot Loader section. The Boot Loader section is always located in the NRWW section. The RWW Section Busy bit (RWWSB) in the Store Program Memory Control and Status Register (SPMCSR) will be read as logical one as long as the RWW section is blocked for reading. After a programming is completed, the RWWSB must be cleared by software before reading code located in the RWW section. Refer to “Store Program Memory Control and Status Register – SPMCSR” on page341 for details on how to clear RWWSB. 27.3.2 NRWW – No Read-While-Write Section The code located in the NRWW section can be read when the Boot Loader software is updating a page in the RWW section. When the Boot Loader code updates the NRWW section, the CPU is halted during the entire Page Erase or Page Write operation. Table 27-1. Read-While-Write Features Which Section Can Which Section does the Z-pointer be Read During Is the CPU Read-While-Write Address During the Programming? Programming? Halted? Supported? RWW Section NRWW Section No Yes NRWW Section None Yes No ATmega16U4/32U4 [DATASHEET] 337 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 27-1. Read-While-Write vs. No Read-While-Write Read-While-Write (RWW) Section Z-pointer Addresses NRWW Section Z-pointer Addresses RWW No Read-While-Write Section (NRWW) Section CPU is Halted During the Operation Code Located in NRWW Section Can be Read During the Operation Figure 27-2. Memory Sections Program Memory Program Memory BOOTSZ = '11' BOOTSZ = '10' 0x0000 0x0000 Section Section Write Application Flash Section Write Application Flash Section While- While- Read- Read- Section ESntadr tR NWRWWW Section ESntadr tR NWRWWW Write Application Flash Section Write Application Flash Section While- End Application While- ESntadr tA Bpopolitc Laotioander Read- Boot Loader Flash Section FSltaasrth Benodot Loader Read- Boot Loader Flash Section Flashend No No Program Memory Program Memory BOOTSZ = '01' BOOTSZ = '00' 0x0000 0x0000 Section Section Write Application Flash Section Write Application Flash Section While- While- Read- Read- Write Section Application Flash Section ESEntnaddr tRA NpWRpWWlicWation Write Section ESntadr tR NWRWW,W E,n Sdt aArpt pBliocoatt iLoonader While- Boot Loader Flash Section Start Boot Loader While- Boot Loader Flash Section Read- Flashend Read- Flashend No No Note: 1. The parameters in Figure 27-2 are given in Table27-8 on page349. ATmega16U4/32U4 [DATASHEET] 338 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

27.4 Boot Loader Lock Bits If no Boot Loader capability is needed, the entire Flash is available for application code. The Boot Loader has two separate sets of Boot Lock bits which can be set independently. This gives the user a unique flexibility to select different levels of protection. The user can select:  To protect the entire Flash from a software update by the MCU  To protect only the Boot Loader Flash section from a software update by the MCU  To protect only the Application Flash section from a software update by the MCU  Allow software update in the entire Flash See the following two tables for further details. The Boot Lock bits can be set by software and in Serial or in Parallel Programming mode. They can only be cleared by a Chip Erase command only. The general Write Lock (Lock Bit mode 2) does not control the programming of the Flash memory by SPM instruction. Similarly, the general Read/Write Lock (Lock Bit mode 1) does not control reading nor writing by (E)LPM/SPM, if it is attempted. Table 27-2. Boot Lock Bit0 Protection Modes (Application Section)(1) BLB0 Mode BLB02 BLB01 Protection 1 1 1 No restrictions for SPM or (E)LPM accessing the Application section 2 1 0 SPM is not allowed to write to the Application section SPM is not allowed to write to the Application section, and (E)LPM executing from the Boot Loader section is not allowed to read from the Application section. If 3 0 0 Interrupt Vectors are placed in the Boot Loader section, interrupts are disabled while executing from the Application section. (E)LPM executing from the Boot Loader section is not allowed to read from the 4 0 1 Application section. If Interrupt Vectors are placed in the Boot Loader section, interrupts are disabled while executing from the Application section. Note: 1. “1” means unprogrammed, “0” means programmed Table 27-3. Boot Lock Bit1 Protection Modes (Boot Loader Section)(1) BLB1 Mode BLB12 BLB11 Protection 1 1 1 No restrictions for SPM or (E)LPM accessing the Boot Loader section 2 1 0 SPM is not allowed to write to the Boot Loader section SPM is not allowed to write to the Boot Loader section, and (E)LPM executing from the Application section is not allowed to read from the Boot Loader section. If 3 0 0 Interrupt Vectors are placed in the Application section, interrupts are disabled while executing from the Boot Loader section. (E)LPM executing from the Application section is not allowed to read from the Boot 4 0 1 Loader section. If Interrupt Vectors are placed in the Application section, interrupts are disabled while executing from the Boot Loader section. Note: 1. “1” means unprogrammed, “0” means programmed. ATmega16U4/32U4 [DATASHEET] 339 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

27.5 Entering the Boot Loader Program The bootloader can be executed with three different conditions: 27.5.1 Regular Application Conditions A jump or call from the application program. This may be initiated by a trigger such as a command received via USART, SPI, or USB. 27.5.2 Boot Reset Fuse The Boot Reset Fuse (BOOTRST) can be programmed so that the Reset Vector is pointing to the Boot Flash start address after a reset. In this case, the Boot Loader is started after a reset. After the application code is loaded, the program can start executing the application code. Note that the fuses cannot be changed by the MCU itself. This means that once the Boot Reset Fuse is programmed, the Reset Vector will always point to the Boot Loader Reset and the fuse can only be changed through the serial or parallel programming interface. Table 27-4. Boot Reset Fuse(1) BOOTRST Reset Address 1 Reset Vector = Application Reset (address 0x0000) 0 Reset Vector = Boot Loader Reset (see Table27-8 on page349) Note: 1. “1” means unprogrammed, “0” means programmed. 27.5.3 External Hardware conditions The Hardware Boot Enable Fuse (HWBE) can be programmed (see the table below) so that upon special hardware conditions under reset, the bootloader execution is forced after reset. Table 27-5. Hardware Boot Enable Fuse(1) HWBE Reset Address 1 ALE/HWB pin can not be used to force Boot Loader execution after reset 0 ALE/HWB pin is used during reset to force bootloader execution after reset Note: 1. “1” means unprogrammed, “0” means programmed. When the HWBE fuse is enable the ALE/HWB pin is configured as input during reset and sampled during reset rising edge. When ALE/HWB pin is ‘0’ during reset rising edge, the reset vector will be set as the Boot Loader Reset address and the Boot Loader will be executed (See Figure 27-3). ATmega16U4/32U4 [DATASHEET] 340 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 27-3. Boot Process Description RESET t t SHRH HHRH ALE/HWB HWBE? Ext. Hardware Conditions ? BOOTRST ? Reset Vector = Application Reset Reset Vector =Boot Lhoader Reset 27.5.4 Store Program Memory Control and Status Register – SPMCSR The Store Program Memory Control and Status Register contains the control bits needed to control the Boot Loader operations. Bit 7 6 5 4 3 2 1 0 SPMIE RWWSB SIGRD RWWSRE BLBSET PGWRT PGERS SPMEN SPMCSR Read/Write R/W R R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 • Bit 7 – SPMIE: SPM Interrupt Enable When the SPMIE bit is written to one, and the I-bit in the Status Register is set (one), the SPM ready interrupt will be enabled. The SPM ready Interrupt will be executed as long as the SPMEN bit in the SPMCSR Register is cleared. • Bit 6 – RWWSB: Read-While-Write Section Busy When a Self-Programming (Page Erase or Page Write) operation to the RWW section is initiated, the RWWSB will be set (one) by hardware. When the RWWSB bit is set, the RWW section cannot be accessed. The RWWSB bit will be cleared if the RWWSRE bit is written to one after a Self-Programming operation is completed. Alternatively the RWWSB bit will automatically be cleared if a page load operation is initiated. • Bit 5 – SIGRD: Signature Row Read If this bit is written to one at the same time as SPMEN, the next LPM instruction within three clock cycles will read a byte from the signature row into the destination register. see “Reading the Signature Row from Software” ATmega16U4/32U4 [DATASHEET] 341 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

on page346 for details. An SPM instruction within four cycles after SIGRD and SPMEN are set will have no effect. This operation is reserved for future use and should not be used. • Bit 4 – RWWSRE: Read-While-Write Section Read Enable When programming (Page Erase or Page Write) to the RWW section, the RWW section is blocked for reading (the RWWSB will be set by hardware). To re-enable the RWW section, the user software must wait until the programming is completed (SPMEN will be cleared). Then, if the RWWSRE bit is written to one at the same time as SPMEN, the next SPM instruction within four clock cycles re-enables the RWW section. The RWW section cannot be re-enabled while the Flash is busy with a Page Erase or a Page Write (SPMEN is set). If the RWWSRE bit is written while the Flash is being loaded, the Flash load operation will abort and the data loaded will be lost. • Bit 3 – BLBSET: Boot Lock Bit Set If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock cycles sets Boot Lock bits, according to the data in R0. The data in R1 and the address in the Z-pointer are ignored. The BLBSET bit will automatically be cleared upon completion of the Lock bit set, or if no SPM instruction is executed within four clock cycles. An (E)LPM instruction within three cycles after BLBSET and SPMEN are set in the SPMCSR Register, will read either the Lock bits or the Fuse bits (depending on Z0 in the Z-pointer) into the destination register. See “Reading the Fuse and Lock Bits from Software” on page345 for details. • Bit 2 – PGWRT: Page Write If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock cycles executes Page Write, with the data stored in the temporary buffer. The page address is taken from the high part of the Z-pointer. The data in R1 and R0 are ignored. The PGWRT bit will auto-clear upon completion of a Page Write, or if no SPM instruction is executed within four clock cycles. The CPU is halted during the entire Page Write operation if the NRWW section is addressed. • Bit 1 – PGERS: Page Erase If this bit is written to one at the same time as SPMEN, the next SPM instruction within four clock cycles executes Page Erase. The page address is taken from the high part of the Z-pointer. The data in R1 and R0 are ignored. The PGERS bit will auto-clear upon completion of a Page Erase, or if no SPM instruction is executed within four clock cycles. The CPU is halted during the entire Page Write operation if the NRWW section is addressed. • Bit 0 – SPMEN: Store Program Memory Enable This bit enables the SPM instruction for the next four clock cycles. If written to one together with either RWWSRE, BLBSET, PGWRT’ or PGERS, the following SPM instruction will have a special meaning, see description above. If only SPMEN is written, the following SPM instruction will store the value in R1:R0 in the temporary page buffer addressed by the Z-pointer. The LSB of the Z-pointer is ignored. The SPMEN bit will auto-clear upon completion of an SPM instruction, or if no SPM instruction is executed within four clock cycles. During Page Erase and Page Write, the SPMEN bit remains high until the operation is completed. Writing any other combination than “10001”, “01001”, “00101”, “00011”, or “00001” in the lower five bits will have no effect. Note: Only one SPM instruction should be active at any time. ATmega16U4/32U4 [DATASHEET] 342 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

27.6 Addressing the Flash During Self-Programming The Z-pointer is used to address the SPM commands. The Z pointer consists of the Z-registers ZL and ZH in the register file, and RAMPZ in the I/O space. The number of bits actually used is implementation dependent. Note that the RAMPZ register is only implemented when the program space is larger than 64KB. Bit 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 RAMPZ RAMPZ7 RAMPZ6 RAMPZ5 RAMPZ4 RAMPZ3 RAMPZ2 RAMPZ1 RAMPZ0 ZH (R31) Z15 Z14 Z13 Z12 Z11 Z10 Z9 Z8 ZL (R30) Z7 Z6 Z5 Z4 Z3 Z2 Z1 Z0 7 6 5 4 3 2 1 0 Since the Flash is organized in pages, the Program Counter can be treated as having two different sections. One section, consisting of the least significant bits, is addressing the words within a page, while the most significant bits are addressing the pages. This is shown in Figure 27-4 on page 343. Note that the Page Erase and Page Write operations are addressed independently. Therefore it is of major importance that the Boot Loader software addresses the same page in both the Page Erase and Page Write operation. Once a programming operation is initiated, the address is latched and the Z-pointer can be used for other operations. The (E)LPM instruction use the Z-pointer to store the address. Since this instruction addresses the Flash byte- by-byte, also bit Z0 of the Z-pointer is used. Figure 27-4. Addressing the Flash During SPM(1) BIT 23 ZPCMSB ZPAGEMSB 1 0 0 Z - POINTER PCMSB PAGEMSB PROGRAM COUNTER PCPAGE PCWORD PAGE ADDRESS WORD ADDRESS WITHIN THE FLASH WITHIN A PAGE PROGRAM MEMORY PAGE PCWORD[PAGEMSB:0]: PAGE INSTRUCTION WORD 00 01 02 PAGEEND Note: 1. The different variables used in Figure 27-4 are listed in Table27-10 on page350. ATmega16U4/32U4 [DATASHEET] 343 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

27.7 Self-Programming the Flash The program memory is updated in a page by page fashion. Before programming a page with the data stored in the temporary page buffer, the page must be erased. The temporary page buffer is filled one word at a time using SPM and the buffer can be filled either before the Page Erase command or between a Page Erase and a Page Write operation: Alternative 1, fill the buffer before a Page Erase  Fill temporary page buffer  Perform a Page Erase  Perform a Page Write Alternative 2, fill the buffer after Page Erase  Perform a Page Erase  Fill temporary page buffer  Perform a Page Write If only a part of the page needs to be changed, the rest of the page must be stored (for example in the temporary page buffer) before the erase, and then be rewritten. When using alternative 1, the Boot Loader provides an effective Read-Modify-Write feature which allows the user software to first read the page, do the necessary changes, and then write back the modified data. If alternative 2 is used, it is not possible to read the old data while loading since the page is already erased. The temporary page buffer can be accessed in a random sequence. It is essential that the page address used in both the Page Erase and Page Write operation is addressing the same page. See “Simple Assembly Code Example for a Boot Loader” on page347 for an assembly code example. 27.7.1 Performing Page Erase by SPM To execute Page Erase, set up the address in the Z-pointer, write “X0000011” to SPMCSR and execute SPM within four clock cycles after writing SPMCSR. The data in R1 and R0 is ignored. The page address must be written to PCPAGE in the Z-register. Other bits in the Z-pointer will be ignored during this operation.  Page Erase to the RWW section: The NRWW section can be read during the Page Erase  Page Erase to the NRWW section: The CPU is halted during the operation 27.7.2 Filling the Temporary Buffer (Page Loading) To write an instruction word, set up the address in the Z-pointer and data in R1:R0, write “00000001” to SPMCSR and execute SPM within four clock cycles after writing SPMCSR. The content of PCWORD in the Z- register is used to address the data in the temporary buffer. The temporary buffer will auto-erase after a Page Write operation or by writing the RWWSRE bit in SPMCSR. It is also erased after a system reset. Note that it is not possible to write more than one time to each address without erasing the temporary buffer. If the EEPROM is written in the middle of an SPM Page Load operation, all data loaded will be lost. 27.7.3 Performing a Page Write To execute Page Write, set up the address in the Z-pointer, write “X0000101” to SPMCSR and execute SPM within four clock cycles after writing SPMCSR. The data in R1 and R0 is ignored. The page address must be written to PCPAGE. Other bits in the Z-pointer must be written to zero during this operation.  Page Write to the RWW section: The NRWW section can be read during the Page Write  Page Write to the NRWW section: The CPU is halted during the operation ATmega16U4/32U4 [DATASHEET] 344 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

27.7.4 Using the SPM Interrupt If the SPM interrupt is enabled, the SPM interrupt will generate a constant interrupt when the SPMEN bit in SPMCSR is cleared. This means that the interrupt can be used instead of polling the SPMCSR Register in software. When using the SPM interrupt, the Interrupt Vectors should be moved to the BLS section to avoid that an interrupt is accessing the RWW section when it is blocked for reading. How to move the interrupts is described in “Interrupts” on page63. 27.7.5 Consideration While Updating BLS Special care must be taken if the user allows the Boot Loader section to be updated by leaving Boot Lock bit11 unprogrammed. An accidental write to the Boot Loader itself can corrupt the entire Boot Loader, and further software updates might be impossible. If it is not necessary to change the Boot Loader software itself, it is recommended to program the Boot Lock bit11 to protect the Boot Loader software from any internal software changes. 27.7.6 Prevent Reading the RWW Section During Self-Programming During Self-Programming (either Page Erase or Page Write), the RWW section is always blocked for reading. The user software itself must prevent that this section is addressed during the self programming operation. The RWWSB in the SPMCSR will be set as long as the RWW section is busy. During Self-Programming the Interrupt Vector table should be moved to the BLS as described in “Interrupts” on page63, or the interrupts must be disabled. Before addressing the RWW section after the programming is completed, the user software must clear the RWWSB by writing the RWWSRE. See “Simple Assembly Code Example for a Boot Loader” on page347 for an example. 27.7.7 Setting the Boot Loader Lock Bits by SPM To set the Boot Loader Lock bits, write the desired data to R0, write “X0001001” to SPMCSR and execute SPM within four clock cycles after writing SPMCSR. The only accessible Lock bits are the Boot Lock bits that may prevent the Application and Boot Loader section from any software update by the MCU. Bit 7 6 5 4 3 2 1 0 R0 1 1 BLB12 BLB11 BLB02 BLB01 1 1 See Table27-2 on page339 and Table27-3 on page339 for how the different settings of the Boot Loader bits affect the Flash access. If bits 5..2 in R0 are cleared (zero), the corresponding Boot Lock bit will be programmed if an SPM instruction is executed within four cycles after BLBSET and SPMEN are set in SPMCSR. The Z-pointer is don’t care during this operation, but for future compatibility it is recommended to load the Z-pointer with 0x0001 (same as used for reading the lO bits). For future compatibility it is also recommended to set bits 7, 6, 1, and 0 in R0 to “1” when ck writing the Lock bits. When programming the Lock bits the entire Flash can be read during the operation. 27.7.8 EEPROM Write Prevents Writing to SPMCSR Note that an EEPROM write operation will block all software programming to Flash. Reading the Fuses and Lock bits from software will also be prevented during the EEPROM write operation. It is recommended that the user checks the status bit (EEPE) in the EECR Register and verifies that the bit is cleared before writing to the SPMCSR Register. 27.7.9 Reading the Fuse and Lock Bits from Software It is possible to read both the Fuse and Lock bits from software. To read the Lock bits, load the Z-pointer with 0x0001 and set the BLBSET and SPMEN bits in SPMCSR. When an (E)LPM instruction is executed within ATmega16U4/32U4 [DATASHEET] 345 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

three CPU cycles after the BLBSET and SPMEN bits are set in SPMCSR, the value of the Lock bits will be loaded in the destination register. The BLBSET and SPMEN bits will auto-clear upon completion of reading the Lock bits or if no (E)LPM instruction is executed within three CPU cycles or no SPM instruction is executed within four CPU cycles. When BLBSET and SPMEN are cleared, (E)LPM will work as described in the Instruction set Manual. Bit 7 6 5 4 3 2 1 0 Rd – – BLB12 BLB11 BLB02 BLB01 LB2 LB1 The algorithm for reading the Fuse Low byte is similar to the one described above for reading the Lock bits. To read the Fuse Low byte, load the Z-pointer with 0x0000 and set the BLBSET and SPMEN bits in SPMCSR. When an (E)LPM instruction is executed within three cycles after the BLBSET and SPMEN bits are set in the SPMCSR, the value of the Fuse Low byte (FLB) will be loaded in the destination register as shown below. Refer to Table27-5 on page340 for a detailed description and mapping of the Fuse Low byte. Bit 7 6 5 4 3 2 1 0 Rd FLB7 FLB6 FLB5 FLB4 FLB3 FLB2 FLB1 FLB0 Similarly, when reading the Fuse High byte, load 0x0003 in the Z-pointer. When an (E)LPM instruction is executed within three cycles after the BLBSET and SPMEN bits are set in the SPMCSR, the value of the Fuse High byte (FHB) will be loaded in the destination register as shown below. Refer to Table27-4 on page340 for detailed description and mapping of the Fuse High byte. Bit 7 6 5 4 3 2 1 0 Rd FHB7 FHB6 FHB5 FHB4 FHB3 FHB2 FHB1 FHB0 When reading the Extended Fuse byte, load 0x0002 in the Z-pointer. When an (E)LPM instruction is executed within three cycles after the BLBSET and SPMEN bits are set in the SPMCSR, the value of the Extended Fuse byte (EFB) will be loaded in the destination register as shown below. Refer to Table27-3 on page339 for detailed description and mapping of the Extended Fuse byte. Bit 7 6 5 4 3 2 1 0 Rd – – – – – EFB2 EFB1 EFB0 Fuse and Lock bits that are programmed, will be read as zero. Fuse and Lock bits that are unprogrammed, will be read as one. 27.7.10Reading the Signature Row from Software To read the Signature Row from software, load the Z-pointer with the signature byte address given in the table below and set the SIGRD and SPMEN bits in SPMCSR. When an LPM instruction is executed within three CPU cycles after the SIGRD and SPMEN bits are set in SPMCSR, the signature byte value will be loaded in the destination register. The SIGRD and SPMEN bits will auto-clear upon completion of reading the Signature Row Lock bits or if no LPM instruction is executed within three CPU cycles. When SIGRD and SPMEN are cleared, LPM will work as described in the Instruction set Manual. ATmega16U4/32U4 [DATASHEET] 346 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 27-6. Signature Row Addressing Signature Byte Z-Pointer Address Device Signature Byte 1 0x0000 Device Signature Byte 2 0x0002 Device Signature Byte 3 0x0004 RC Oscillator Calibration Byte 0x0001 Note: All other addresses are reserved for future use. 27.7.11 Preventing Flash Corruption During periods of low V , the Flash program can be corrupted because the supply voltage is too low for the CC CPU and the Flash to operate properly. These issues are the same as for board level systems using the Flash, and the same design solutions should be applied. A Flash program corruption can be caused by two situations when the voltage is too low. First, a regular write sequence to the Flash requires a minimum voltage to operate correctly. Secondly, the CPU itself can execute instructions incorrectly, if the supply voltage for executing instructions is too low. Flash corruption can easily be avoided by following these design recommendations (one is sufficient): 1. If there is no need for a Boot Loader update in the system, program the Boot Loader Lock bits to pre- vent any Boot Loader software updates. 2. Keep the AVR RESET active (low) during periods of insufficient power supply voltage. This can be done by enabling the internal Brown-out Detector (BOD) if the operating voltage matches the detection level. If not, an external low V reset protection circuit can be used. If a reset occurs while a write operation is in CC progress, the write operation will be completed provided that the power supply voltage is sufficient. 3. Keep the AVR core in Power-down sleep mode during periods of low V . This will prevent the CPU from CC attempting to decode and execute instructions, effectively protecting the SPMCSR Register and thus the Flash from unintentional writes. 27.7.12Programming Time for Flash when Using SPM The calibrated RC Oscillator is used to time Flash accesses. This table shows the typical programming time for Flash accesses from the CPU. Table 27-7. SPM Programming Time Symbol Min. Programming Time Max. Programming Time Flash write (Page Erase, Page Write, and write Lock bits by 3.7ms 4.5ms SPM) 27.7.13Simple Assembly Code Example for a Boot Loader ;-the routine writes one page of data from RAM to Flash ; the first data location in RAM is pointed to by the Y pointer ; the first data location in Flash is pointed to by the Z-pointer ;-error handling is not included ;-the routine must be placed inside the Boot space ; (at least the Do_spm sub routine). Only code inside NRWW section can ; be read during Self-Programming (Page Erase and Page Write). ;-registers used: r0, r1, temp1 (r16), temp2 (r17), looplo (r24), ATmega16U4/32U4 [DATASHEET] 347 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

; loophi (r25), spmcrval (r20) ; storing and restoring of registers is not included in the routine ; register usage can be optimized at the expense of code size ;-It is assumed that either the interrupt table is moved to the Boot ; loader section or that the interrupts are disabled. .equ PAGESIZEB = PAGESIZE*2 ;PAGESIZEB is page size in BYTES, not words .org SMALLBOOTSTART Write_page: ; Page Erase ldi spmcrval, (1<<PGERS) | (1<<SPMEN) call Do_spm ; re-enable the RWW section ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN) call Do_spm ;transfer data from RAM to Flash page buffer ldi looplo, low(PAGESIZEB) ;init loop variable ldi loophi, high(PAGESIZEB) ;not required for PAGESIZEB<=256 Wrloop: ld r0, Y+ ld r1, Y+ ldi spmcrval, (1<<SPMEN) call Do_spm adiw ZH:ZL, 2 sbiw loophi:looplo, 2 ;use subi for PAGESIZEB<=256 brne Wrloop ; execute Page Write subi ZL, low(PAGESIZEB) ;restore pointer sbci ZH, high(PAGESIZEB) ;not required for PAGESIZEB<=256 ldi spmcrval, (1<<PGWRT) | (1<<SPMEN) call Do_spm ; re-enable the RWW section ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN) call Do_spm ; read back and check, optional ldi looplo, low(PAGESIZEB) ;init loop variable ldi loophi, high(PAGESIZEB) ;not required for PAGESIZEB<=256 subi YL, low(PAGESIZEB) ;restore pointer sbci YH, high(PAGESIZEB) Rdloop: elpm r0, Z+ ld r1, Y+ cpse r0, r1 jmp Error sbiw loophi:looplo, 1 ;use subi for PAGESIZEB<=256 brne Rdloop ; return to RWW section ; verify that RWW section is safe to read Return: in temp1, SPMCSR ATmega16U4/32U4 [DATASHEET] 348 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

sbrs temp1, RWWSB ; If RWWSB is set, the RWW section is not ready yet ret ; re-enable the RWW section ldi spmcrval, (1<<RWWSRE) | (1<<SPMEN) call Do_spm rjmp Return Do_spm: ; check for previous SPM complete Wait_spm: in temp1, SPMCSR sbrc temp1, SPMEN rjmp Wait_spm ; input: spmcrval determines SPM action ; disable interrupts if enabled, store status in temp2, SREG cli ; check that no EEPROM write access is present Wait_ee: sbic EECR, EEPE rjmp Wait_ee ; SPM timed sequence out SPMCSR, spmcrval spm ; restore SREG (to enable interrupts if originally enabled) out SREG, temp2 ret 27.7.14Boot Loader Parameters The parameters used in the description of the Self-Programming are given throughout the following tables. Table 27-8. Boot Size Configuration (Word Addresses)(1) s 1 0 e on ron es n) Device BOOTSZ BOOTSZ Boot Siz Pages ApplicationFlash Secti Boot LoadeFlash Secti EndApplicationSection BootReset Addr(Start BootLoader Sectio 1 1 256 words 4 0x0000 - 0x3EFF 0x3F00 - 0x3FFF 0x3EFF 0x3F00 4 U 2 1 0 512 words 8 0x0000 - 0x3DFF 0x3E00 - 0x3FFF 0x3DFF 0x3E00 3 a g e 0 1 1024 words 16 0x0000 - 0x3BFF 0x3C00 - 0x3FFF 0x3BFF 0x3C00 m T A 0 0 2048 words 32 0x0000 - 0x37FF 0x3800 - 0x3FFF 0x37FF 0x3800 1 1 256 words 4 0x0000 - 0x1EFF 0x1F00 - 0x1FFF 0x1EFF 0x1F00 4 U 6 1 0 512 words 8 0x0000 - 0x1DFF 0x1E00 - 0x1FFF 0x1DFF 0x1E00 1 a g e 0 1 1024 words 16 0x0000 - 0x1BFF 0x1C00 - 0x1FFF 0x1BFF 0x1C00 m T A 0 0 2048 words 32 0x0000 - 0x17FF 0x1800 - 0x1FFF 0x17FF 0x1800 Note: 1. The different BOOTSZ Fuse configurations are shown in Figure 27-2 on page 338 ATmega16U4/32U4 [DATASHEET] 349 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 27-9. Read-While-Write Limit (Word Addresses)(1) Device Section Pages Address Read-While-Write section (RWW) 224 0x0000 - 0x37FF ATmega32U4 No Read-While-Write section (NRWW) 32 0x3800 - 0x3FFF Read-While-Write section (RWW) 97 0x0000 - 0x17FF ATmega16U4 No Read-While-Write section (NRWW) 32 0x1800 - 0x1FFF Note: 1. For details about these two section, see “NRWW – No Read-While-Write Section” on page337 and “RWW – Read-While-Write Section” on page337. Table 27-10. Explanation of different variables used in Figure 27-4 and the mapping to the Z-pointer Corresponding Variable Z-value(1) Description Most significant bit in the Program Counter. (The Program Counter is 14 PCMSB 13 bits PC[13:0]) Most significant bit which is used to address the words within one page PAGEMSB 6 (64 words in a page requires six bits PC [5:0]) Bit in Z-pointer that is mapped to PCMSB. Because Z0 is not used, the ZPCMSB Z14 ZPCMSB equals PCMSB + 1 ZPAGEMS Bit in Z-pointer that is mapped to PCMSB. Because Z0 is not used, the Z7 B ZPAGEMSB equals PAGEMSB + 1 Program Counter page address: Page select, for Page Erase and Page PCPAGE PC[13:6] Z14:Z7 Write Program Counter word address: Word select, for filling temporary buffer PCWORD PC[5:0] Z6:Z1 (must be zero during Page Write operation) Note: 1. Z0: should be zero for all SPM commands, byte select for the (E)LPM instruction. Note: See “Addressing the Flash During Self-Programming” on page343 for details about the use of Z-pointer during Self-Programming. ATmega16U4/32U4 [DATASHEET] 350 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

ATmega16U4/32U4 [DATASHEET] 351 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

ATmega16U4/32U4 [DATASHEET] 352 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

28. Memory Programming 28.1 Program And Data Memory Lock Bits The device provides six Lock bits which can be left unprogrammed (“1”) or can be programmed (“0”) to obtain the additional features listed in Table 28-2. The Lock bits can only be erased to “1” with the Chip Erase command. Table 28-1. Lock Bit Byte() Lock Bit Byte Bit No Description Default Value ATmega16U4/32U4 ATmega16U4RC/32U4RC 7 – 1 6 – 1 BLB12 5 Boot Lock bit 1 BLB11 4 Boot Lock bit 0 1 BLB02 3 Boot Lock bit 1 BLB01 2 Boot Lock bit 1 LB2 1 Lock bit 0 1 LB1 0 Lock bit 0 1 Note: 1. “1”: unprogrammed, “0”: programmed Table 28-2. Lock Bit Protection Modes(1)(2) Memory Lock Bits Protection Type LB Mode LB2 LB1 1 1 1 No memory lock features enabled. Further programming of the Flash and EEPROM is disabled in 2 1 0 Parallel and Serial Programming mode. The Fuse bits are locked in both Serial and Parallel Programming mode.(1) Further programming and verification of the Flash and EEPROM is disabled in Parallel and Serial Programming mode. The Boot Lock 3 0 0 bits and Fuse bits are locked in both Serial and Parallel Programming mode.(1) BLB0 Mode BLB02 BLB01 No restrictions for SPM or (E)LPM accessing the Application 1 1 1 section. 2 1 0 SPM is not allowed to write to the Application section. SPM is not allowed to write to the Application section, and (E)LPM executing from the Boot Loader section is not allowed to read from 3 0 0 the Application section. If Interrupt Vectors are placed in the Boot Loader section, interrupts are disabled while executing from the Application section. ATmega16U4/32U4 [DATASHEET] 353 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 28-2. Lock Bit Protection Modes(1)(2) Memory Lock Bits Protection Type (E)LPM executing from the Boot Loader section is not allowed to read from the Application section. If Interrupt Vectors are placed in 4 0 1 the Boot Loader section, interrupts are disabled while executing from the Application section. BLB1 Mode BLB12 BLB11 No restrictions for SPM or (E)LPM accessing the Boot Loader 1 1 1 section. 2 1 0 SPM is not allowed to write to the Boot Loader section. SPM is not allowed to write to the Boot Loader section, and (E)LPM executing from the Application section is not allowed to read from 3 0 0 the Boot Loader section. If Interrupt Vectors are placed in the Application section, interrupts are disabled while executing from the Boot Loader section. (E)LPM executing from the Application section is not allowed to read from the Boot Loader section. If Interrupt Vectors are placed in 4 0 1 the Application section, interrupts are disabled while executing from the Boot Loader section. Notes: 1. Program the Fuse bits and Boot Lock bits before programming the LB1 and LB2. 2. “1” means unprogrammed, “0” means programmed. 28.2 Fuse Bits The device has three bytes. Table 28-3 to Table28-5 on page355 describe briefly the functionality of all the fuses and how they are mapped into the Fuse bytes. Note that the fuses are read as logical zero, “0”, if they are programmed. Table 28-3. Extended Fuse Byte(1)(2) Fuse Low Byte Bit No Description Default Value ATmega16/32U4 ATmega16/32U4RC – 7 – 1 – 6 – 1 – 5 – 1 – 4 – 1 HWBE 3 Hardware Boot Enable 0 (programmed) 1 (unprogrammed) Brown-out Detector trigger BODLEVEL2(1) 2 0 (programmed) level Brown-out Detector trigger BODLEVEL1(1) 1 1 (unprogrammed) level Brown-out Detector trigger BODLEVEL0(1) 0 1 (unprogrammed) level Notes: 1. See Table8-1 on page53 for BODLEVEL Fuse decoding. 2. “1” means unprogrammed, “0” means programmed. ATmega16U4/32U4 [DATASHEET] 354 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 28-4. Fuse High Byte Fuse High Byte Bit No Description Default Value OCDEN(4) 7 Enable OCD 1 (unprogrammed, OCD disabled) JTAGEN 6 Enable JTAG 0 (programmed, JTAG enabled) Enable Serial Program and Data SPIEN(1) 5 0 (programmed, SPI prog. enabled) Downloading WDTON(3) 4 Watchdog Timer always on 1 (unprogrammed) EEPROM memory is preserved 1 (unprogrammed, EEPROM EESAVE 3 through the Chip Erase preserved) Select Boot Size (see Table 28-7 BOOTSZ1 2 0 (programmed)(2) for details) Select Boot Size (see Table 28-7 BOOTSZ0 1 0 (programmed)(2) for details) Select Bootloader Address as 1 (unprogrammed, Reset vector BOOTRST 0 Reset Vector @0x0000) Note: 1. The SPIEN Fuse is not accessible in serial programming mode. 2. The default value of BOOTSZ1..0 results in maximum Boot Size. See Table 28-7 for details. 3. See “Watchdog Timer” on page55 for details. 4. Never ship a product with the OCDEN Fuse programmed regardless of the setting of Lock bits and JTAGEN Fuse. A programmed OCDEN Fuse enables some parts of the clock system to be running in all sleep modes. This may increase the power consumption. Table 28-5. Fuse Low Byte Fuse Low Byte Bit Nr Description Default Value ATmega16U4/32U4 ATmega16U4RC/32U4RC CKDIV8(3) 7 Divide clock by 8 0 (programmed) CKOUT(2) 6 Clock output 1 (unprogrammed) SUT1 5 Select start-up time 0 (programmed) SUT0 4 Select start-up time 1 (unprogrammed) CKSEL3 3 Select Clock source 1 (unprogrammed)(1) 0 (programmed)(1) CKSEL2 2 Select Clock source 1 (unprogrammed)(1) 0 (programmed)(1) CKSEL1 1 Select Clock source 1 (unprogrammed)(1) 1 (unprogrammed)(1) CKSEL0 0 Select Clock source 0 (programmed)(1) 0 (programmed)(1) Note: 1. The default setting of CKSEL3..0 results in Low Power Crystal Oscillator for ATmega16U4 and ATmega32U4, and Internal RC oscillator for ATmega16U4RC and ATmega32U4RC. 2. The CKOUT Fuse allow the system clock to be output on PORTC7. See “CLKPR – Clock Prescaler Register” on page39 for details. 3. See “System Clock Prescaler” on page35 for details. The status of the Fuse bits is not affected by Chip Erase. Note that the Fuse bits are locked if Lock bit1 (LB1) is programmed. Program the Fuse bits before programming the Lock bits. ATmega16U4/32U4 [DATASHEET] 355 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

28.2.1 Latching of Fuses The fuse values are latched when the device enters programming mode and changes of the fuse values will have no effect until the part leaves Programming mode. This does not apply to the EESAVE Fuse which will take effect once it is programmed. The fuses are also latched on Power-up in Normal mode. 28.3 Signature Bytes All Atmel microcontrollers have a three-byte signature code which identifies the device. This code can be read in both serial and parallel mode, also when the device is locked. The three bytes reside in a separate address space. ATmega16U4 Signature Bytes: 1. 0x000: 0x1E (indicates manufactured by Atmel). 2. 0x001: 0x94 (indicates 16KB Flash memory). 3. 0x002: 0x88 (indicates ATmega16U4 device). ATmega32U4 Signature Bytes: 1. 0x000: 0x1E (indicates manufactured by Atmel). 2. 0x001: 0x95 (indicates 32KB Flash memory). 3. 0x002: 0x87 (indicates ATmega32U4 device). 28.4 Calibration Byte The device has a byte calibration value for the internal RC Oscillator. This byte resides in the high byte of address 0x000 in the signature address space. During reset, this byte is automatically written into the OSCCAL Register to ensure correct frequency of the calibrated RC Oscillator. 28.5 Parallel Programming Parameters, Pin Mapping, and Commands This section describes how to parallel program and verify Flash Program memory, EEPROM Data memory, Memory Lock bits, and Fuse bits in the device. Pulses are assumed to be at least 250ns unless otherwise noted. 28.5.1 Signal Names In this section, some pins of the device are referenced by signal names describing their functionality during parallel programming, see Figure 28-1 on page 357 and Table28-6 on page357. Pins not described in the following table are referenced by pin names. The XA1/XA0 pins determine the action executed when the XTAL1 pin is given a positive pulse. The bit coding is shown in Table28-9 on page358. When pulsing WR or OE, the command loaded determines the action executed. The different commands are shown in Table28-10 on page358. ATmega16U4/32U4 [DATASHEET] 356 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 28-1. Parallel Programming(1) +5V RDY/BSY PD1 VCC OE PD2 +5V WR PD3 AVCC BS1 PD4 PB7 - PB0 DATA XA0 PD5 XA1 PD6 PAGEL PD7 +12 V RESET BS2 PE6 XTAL1 GND Note: 1. Unused Pins should be left floating. Table 28-6. Pin Name Mapping Signal Name in Programming Mode Pin Name I/O Function 0: Device is busy programming, 1: Device is ready for new RDY/BSY PD1 O command OE PD2 I Output Enable (Active low) WR PD3 I Write Pulse (Active low) BS1 PD4 I Byte Select 1 XA0 PD5 I XTAL Action Bit 0 XA1 PD6 I XTAL Action Bit 1 PAGEL PD7 I Program Memory and EEPROM data Page Load BS2 PE6 I Byte Select 2 DATA PB7-0 I/O Bi-directional Data bus (Output when OE is low) ATmega16U4/32U4 [DATASHEET] 357 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 28-7. BS2 and BS1 Encoding Flash Data Flash / EEPROM Loading / Fuse Reading Fuse BS2 BS1 Address Reading Programming and Lock Bits 0 0 Low Byte Low Byte Low Byte Fuse Low Byte 0 1 High Byte High Byte High Byte Lock bits Extended High Extended Fuse 1 0 Reserved Extended Byte Byte Byte 1 1 Reserved Reserved Reserved Fuse High Byte Table 28-8. Pin Values Used to Enter Programming Mode Pin Symbol Value PAGEL Prog_enable[3] 0 XA1 Prog_enable[2] 0 XA0 Prog_enable[1] 0 BS1 Prog_enable[0] 0 Table 28-9. XA1 and XA0 Enoding XA1 XA0 Action when XTAL1 is Pulsed Load Flash or EEPROM Address (High or low address byte determined by 0 0 BS2 and BS1) 0 1 Load Data (High or Low data byte for Flash determined by BS1) 1 0 Load Command 1 1 No Action, Idle Table 28-10. Command Byte Bit Encoding Command Byte Command Executed 1000 0000 Chip Erase 0100 0000 Write Fuse bits 0010 0000 Write Lock bits 0001 0000 Write Flash 0001 0001 Write EEPROM 0000 1000 Read Signature Bytes and Calibration byte 0000 0100 Read Fuse and Lock bits 0000 0010 Read Flash 0000 0011 Read EEPROM ATmega16U4/32U4 [DATASHEET] 358 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 28-11. No. of Words in a Page and No. of Pages in the Flash No. of Device Flash Size Page Size PCWORD Pages PCPAGE PCMSB ATmega16U4 8K words (16KB) 64 words PC[5:0] 128 PC[12:6] 12 ATmega32U4 16K words (32KB) 64 words PC[5:0] 256 PC[13:6] 13 Table 28-12. No. of Words in a Page and No. of Pages in the EEPROM No. of Device EEPROM Size Page Size PCWORD Pages PCPAGE EEAMSB ATmega16U4 512 bytes 4 bytes EEA[1:0] 128 EEA[8:2] 8 ATmega32U4 1KB 4 bytes EEA[1:0] 256 EEA[9:2] 9 28.6 Parallel Programming 28.6.1 Enter Programming Mode The following algorithm puts the device in parallel programming mode: 1. Apply 4.5 - 5.5V between V and GND. CC 2. Set RESET to “0” and toggle XTAL1 at least six times. 3. Set the Prog_enable pins listed in Table28-8 on page358 to “0000” and wait at least 100ns. 4. Apply 11.5 - 12.5V to RESET. Any activity on Prog_enable pins within 100ns after +12V has been applied to RESET, will cause the device to fail entering programming mode. 5. Wait at least 50µs before sending a new command. 28.6.2 Considerations for Efficient Programming The loaded command and address are retained in the device during programming. For efficient programming, the following should be considered.  The command needs only be loaded once when writing or reading multiple memory locations  Skip writing the data value 0xFF, that is the contents of the entire EEPROM (unless the EESAVE Fuse is programmed) and Flash after a Chip Erase  Address high byte needs only be loaded before programming or reading a new 256 word window in Flash or 256 byte EEPROM. This consideration also applies to Signature bytes reading. 28.6.3 Chip Erase The Chip Erase will erase the Flash and EEPROM(1) memories plus Lock bits. The Lock bits are not reset until the program memory has been completely erased. The Fuse bits are not changed. A Chip Erase must be performed before the Flash and/or EEPROM are reprogrammed. Note: 1. The EEPRPOM memory is preserved during Chip Erase if the EESAVE Fuse is programmed. Load Command “Chip Erase”: 1. Set XA1, XA0 to “10”. This enables command loading. 2. Set BS1 to “0”. 3. Set DATA to “1000 0000”. This is the command for Chip Erase. 4. Give XTAL1 a positive pulse. This loads the command. ATmega16U4/32U4 [DATASHEET] 359 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

5. Give WR a negative pulse. This starts the Chip Erase. RDY/BSY goes low. 6. Wait until RDY/BSY goes high before loading a new command. 28.6.4 Programming the Flash The Flash is organized in pages, see Table28-11 on page359. When programming the Flash, the program data is latched into a page buffer. This allows one page of program data to be programmed simultaneously. The following procedure describes how to program the entire Flash memory: A. Load Command “Write Flash” 1. Set XA1, XA0 to “10”. This enables command loading. 2. Set BS1 to “0”. 3. Set DATA to “0001 0000”. This is the command for Write Flash. 4. Give XTAL1 a positive pulse. This loads the command. B. Load Address Low byte (Address bits 7..0) 1. Set XA1, XA0 to “00”. This enables address loading. 2. Set BS2, BS1 to “00”. This selects the address low byte. 3. Set DATA = Address low byte (0x00 - 0xFF). 4. Give XTAL1 a positive pulse. This loads the address low byte. C. Load Data Low Byte 1. Set XA1, XA0 to “01”. This enables data loading. 2. Set DATA = Data low byte (0x00 - 0xFF). 3. Give XTAL1 a positive pulse. This loads the data byte. D. Load Data High Byte 1. Set BS1 to “1”. This selects high data byte. 2. Set XA1, XA0 to “01”. This enables data loading. 3. Set DATA = Data high byte (0x00 - 0xFF). 4. Give XTAL1 a positive pulse. This loads the data byte. E. Latch Data 1. Set BS1 to “1”. This selects high data byte. 2. Give PAGEL a positive pulse. This latches the data bytes. (See Figure 28-3 on page 361 for signal waveforms) F. Repeat B through E until the entire buffer is filled or until all data within the page is loaded. While the lower bits in the address are mapped to words within the page, the higher bits address the pages within the FLASH. This is illustrated in Figure 28-2 on page 361. Note that if less than eight bits are required to address words in the page (page size < 256), the most significant bit(s) in the address low byte are used to address the page when performing a Page Write. G. Load Address High byte (Address bits15..8) 1. Set XA1, XA0 to “00”. This enables address loading. 2. Set BS2, BS1 to “01”. This selects the address high byte. 3. Set DATA = Address high byte (0x00 - 0xFF). 4. Give XTAL1 a positive pulse. This loads the address high byte. H. Load Address Extended High byte (Address bits 23..16) 1. Set XA1, XA0 to “00”. This enables address loading. 2. Set BS2, BS1 to “10”. This selects the address extended high byte. ATmega16U4/32U4 [DATASHEET] 360 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

3. Set DATA = Address extended high byte (0x00 - 0xFF). 4. Give XTAL1 a positive pulse. This loads the address high byte. I. Program Page 1. Set BS2, BS1 to “00” 2. Give WR a negative pulse. This starts programming of the entire page of data. RDY/BSY goes low. 3. Wait until RDY/BSY goes high (See Figure 28-3 on page 361 for signal waveforms). J. Repeat B through I until the entire Flash is programmed or until all data has been programmed K. End Page Programming 1. 1. Set XA1, XA0 to “10”. This enables command loading. 2. Set DATA to “0000 0000”. This is the command for No Operation. 3. Give XTAL1 a positive pulse. This loads the command, and the internal write signals are reset. Figure 28-2. Addressing the Flash Which is Organized in Pages(1) PCMSB PAGEMSB PROGRAM PCPAGE PCWORD COUNTER PAGE ADDRESS WORD ADDRESS WITHIN THE FLASH WITHIN A PAGE PROGRAM MEMORY PAGE PCWORD[PAGEMSB:0]: PAGE INSTRUCTION WORD 00 01 02 PAGEEND Note: 1. PCPAGE and PCWORD are listed in Table28-11 on page359. Figure 28-3. Programming the Flash Waveforms(1) F A B C D E B C D E G H I DATA 0x10 ADDR. LOW DATA LOW DATA HIGH XX ADDR. LOW DATA LOW DATA HIGH XX ADDR. HIGH ADDR. EXT.H XX XA1 XA0 BS1 BS2 XTAL1 WR RDY/BSY RESET+12V OE PAGEL ATmega16U4/32U4 [DATASHEET] 361 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Note: 1. “XX” is don’t care. The letters refer to the programming description above. 28.6.5 Programming the EEPROM The EEPROM is organized in pages, see Table28-12 on page359. When programming the EEPROM, the program data is latched into a page buffer. This allows one page of data to be programmed simultaneously. The programming algorithm for the EEPROM data memory is as follows (refer to “Programming the Flash” on page360 for details on Command, Address and Data loading): 1. A: Load Command “0001 0001”. 2. G: Load Address High Byte (0x00 - 0xFF). 3. B: Load Address Low Byte (0x00 - 0xFF). 4. C: Load Data (0x00 - 0xFF). 5. E: Latch data (give PAGEL a positive pulse). K: Repeat 3 through 5 until the entire buffer is filled L: Program EEPROM page 1. Set BS2, BS1 to “00”. 2. Give WR a negative pulse. This starts programming of the EEPROM page. RDY/BSY goes low. 3. Wait until to RDY/BSY goes high before programming the next page (See Figure 28-4 on page 362 for signal waveforms). Figure 28-4. Programming the EEPROM Waveforms K A G B C E B C E L DATA 0x11 ADDR. HIGH ADDR. LOW DATA XX ADDR. LOW DATA XX XA1 XA0 BS1 XTAL1 WR RDY/BSY RESET +12V OE PAGEL BS2 28.6.6 Reading the Flash The algorithm for reading the Flash memory is as follows (refer to “Programming the Flash” on page360 for details on Command and Address loading): 1. A: Load Command “0000 0010”. 2. H: Load Address Extended Byte (0x00- 0xFF). 3. G: Load Address High Byte (0x00 - 0xFF). 4. B: Load Address Low Byte (0x00 - 0xFF). 5. Set OE to “0”, and BS1 to “0”. The Flash word low byte can now be read at DATA. 6. Set BS to “1”. The Flash word high byte can now be read at DATA. 7. Set OE to “1”. ATmega16U4/32U4 [DATASHEET] 362 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

28.6.7 Reading the EEPROM The algorithm for reading the EEPROM memory is as follows (refer to “Programming the Flash” on page360 for details on Command and Address loading): 1. A: Load Command “0000 0011”. 2. G: Load Address High Byte (0x00 - 0xFF). 3. B: Load Address Low Byte (0x00 - 0xFF). 4. Set OE to “0”, and BS1 to “0”. The EEPROM Data byte can now be read at DATA. 5. Set OE to “1”. 28.6.8 Programming the Fuse Low Bits The algorithm for programming the Fuse Low bits is as follows (refer to “Programming the Flash” on page360 for details on Command and Data loading): 1. A: Load Command “0100 0000”. 2. C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit. 3. Give WR a negative pulse and wait for RDY/BSY to go high. 28.6.9 Programming the Fuse High Bits The algorithm for programming the Fuse High bits is as follows (refer to “Programming the Flash” on page360 for details on Command and Data loading): 1. A: Load Command “0100 0000”. 2. C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit. 3. Set BS2, BS1 to “01”. This selects high data byte. 4. Give WR a negative pulse and wait for RDY/BSY to go high. 5. Set BS2, BS1 to “00”. This selects low data byte. 28.6.10Programming the Extended Fuse Bits The algorithm for programming the Extended Fuse bits is as follows (refer to “Programming the Flash” on page360 for details on Command and Data loading): 1. 1. A: Load Command “0100 0000”. 2. 2. C: Load Data Low Byte. Bit n = “0” programs and bit n = “1” erases the Fuse bit. 3. 3. Set BS2, BS1 to “10”. This selects extended data byte. 4. 4. Give WR a negative pulse and wait for RDY/BSY to go high. 5. 5. Set BS2, BS1 to “00”. This selects low data byte. ATmega16U4/32U4 [DATASHEET] 363 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 28-5. Programming the FUSES Waveforms Write Fuse Low byte Write Fuse high byte Write Extended Fuse byte A C A C A C DATA 0x40 DATA XX 0x40 DATA XX 0x40 DATA XX XA1 XA0 BS1 BS2 XTAL1 WR RDY/BSY RESET +12V OE PAGEL 28.6.11 Programming the Lock Bits The algorithm for programming the Lock bits is as follows (refer to “Programming the Flash” on page360 for details on Command and Data loading): 1. A: Load Command “0010 0000”. 2. C: Load Data Low Byte. Bit n = “0” programs the Lock bit. If LB mode 3 is programmed (LB1 and LB2 is programmed), it is not possible to program the Boot Lock bits by any External Programming mode. 3. Give WR a negative pulse and wait for RDY/BSY to go high. The Lock bits can only be cleared by executing Chip Erase. 28.6.12Reading the Fuse and Lock Bits The algorithm for reading the Fuse and Lock bits is as follows (refer to “Programming the Flash” on page360 for details on Command loading): 1. A: Load Command “0000 0100”. 2. Set OE to “0”, and BS2, BS1 to “00”. The status of the Fuse Low bits can now be read at DATA (“0” means programmed). 3. Set OE to “0”, and BS2, BS1 to “11”. The status of the Fuse High bits can now be read at DATA (“0” means programmed). 4. Set OE to “0”, and BS2, BS1 to “10”. The status of the Extended Fuse bits can now be read at DATA (“0” means programmed). 5. Set OE to “0”, and BS2, BS1 to “01”. The status of the Lock bits can now be read at DATA (“0” means programmed). 6. Set OE to “1”. ATmega16U4/32U4 [DATASHEET] 364 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 28-6. Mapping Between BS1, BS2 and the Fuse and Lock Bits During Read Fuse Low Byte 0 0 Extended Fuse Byte 1 DATA BS2 Lock Bits 0 1 BS1 Fuse High Byte 1 BS2 28.6.13Reading the Signature Bytes The algorithm for reading the Signature bytes is as follows (refer to “Programming the Flash” on page360 for details on Command and Address loading): 1. A: Load Command “0000 1000”. 2. B: Load Address Low Byte (0x00 - 0x02). 3. Set OE to “0”, and BS to “0”. The selected Signature byte can now be read at DATA. 4. Set OE to “1”. 28.6.14Reading the Calibration Byte The algorithm for reading the Calibration byte is as follows (refer to “Programming the Flash” on page360 for details on Command and Address loading): 1. A: Load Command “0000 1000”. 2. B: Load Address Low Byte, 0x00. 3. Set OE to “0”, and BS1 to “1”. The Calibration byte can now be read at DATA. 4. Set OE to “1”. 28.6.15Parallel Programming Characteristics Figure 28-7. Parallel Programming Timing, Including some General Timing Requirements t XLWL t XTAL1 XHXL t t DVXH XLDX Data & Contol (DATA, XA0/1, BS1, BS2) t t t BVPH PLBX BVWL t WLBX PAGEL t PHPL t WLWH WR t PLWL WLRL RDY/BSY t WLRH ATmega16U4/32U4 [DATASHEET] 365 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 28-8. Parallel Programming Timing, Loading Sequence with Timing Requirements(1) LOAD ADDRESS LOAD DATA LOAD DATA LOAD DATA LOAD ADDRESS (LOW BYTE) (LOW BYTE) (HIGH BYTE) (LOW BYTE) tXLXH tXLPH tPLXH XTAL1 BS1 PAGEL DATA ADDR0 (Low Byte) DATA (Low Byte) DATA (High Byte) ADDR1 (Low Byte) XA0 XA1 Note: 1. The timing requirements shown in Figure 28-7 (i.e., t , t , and t ) also apply to loading operation. DVXH XHXL XLDX Figure 28-9. Parallel Programming Timing, Reading Sequence (within the Same Page) with Timing Requirements(1) LOAD ADDRESS READ DATA READ DATA LOAD ADDRESS (LOW BYTE) (LOW BYTE) (HIGH BYTE) (LOW BYTE) tXLOL XTAL1 tBVDV BS1 tOLDV OE tOHDZ DATA ADDR0 (Low Byte) DATA (Low Byte) DATA (High Byte) ADDR1 (Low Byte) XA0 XA1 Note: 1. The timing requirements shown in Figure 28-7 (i.e., t , t , and t ) also apply to reading operation. DVXH XHXL XLDX Table 28-13. Parallel Programming Characteristics, VCC = 5V ± 10% Symbol Parameter Min. Typ. Max. Units V Programming Enable Voltage 11.5 12.5 V PP I Programming Enable Current 250 A PP ATmega16U4/32U4 [DATASHEET] 366 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 28-13. Parallel Programming Characteristics, VCC = 5V ± 10% Symbol Parameter Min. Typ. Max. Units t Data and Control Valid before XTAL1 High 67 DVXH t XTAL1 Low to XTAL1 High 200 XLXH t XTAL1 Pulse Width High 150 XHXL t Data and Control Hold after XTAL1 Low 67 XLDX t XTAL1 Low to WR Low 0 XLWL t XTAL1 Low to PAGEL high 0 XLPH t PAGEL low to XTAL1 high 150 PLXH ns t BS1 Valid before PAGEL High 67 BVPH t PAGEL Pulse Width High 150 PHPL t BS1 Hold after PAGEL Low 67 PLBX t BS2/1 Hold after WR Low 67 WLBX t PAGEL Low to WR Low 67 PLWL t BS2/1 Valid to WR Low 67 BVWL t WR Pulse Width Low 150 WLWH t WR Low to RDY/BSY Low 0 1 s WLRL t WR Low to RDY/BSY High(1) 3.7 4.5 WLRH ms t WR Low to RDY/BSY High for Chip Erase(2) 7.5 9 WLRH_CE t XTAL1 Low to OE Low 0 XLOL t BS1 Valid to DATA valid 0 250 BVDV ns t OE Low to DATA Valid 250 OLDV t OE High to DATA Tri-stated 250 OHDZ Notes: 1. t is valid for the Write Flash, Write EEPROM, Write Fuse bits and Write Lock bits commands. WLRH 2. t is valid for the Chip Erase command. WLRH_CE 28.7 Serial Downloading Both the Flash and EEPROM memory arrays can be programmed using a serial programming bus while RESET is pulled to GND. The serial programming interface consists of pins SCK, PDI (input) and PDO (output). After RESET is set low, the Programming Enable instruction needs to be executed first before program/erase operations can be executed. NOTE, in Table28-14 on page368, the pin mapping for serial programming is listed. Not all packages use the SPI pins dedicated for the internal Serial Peripheral Interface - SPI. ATmega16U4/32U4 [DATASHEET] 367 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

28.8 Serial Programming Pin Mapping Table 28-14. Pin Mapping Serial Programming Symbol Pins (TQFP-64) I/O Description PDI PB2 I Serial Data in PDO PB3 O Serial Data out SCK PB1 I Serial Clock Figure 28-10. Serial Programming and Verify(1) +1.8 - 5.5V VCC +1.8 - 5.5V(2) PDI AVCC PDO SCK XTAL1 RESET GND Notes: 1. If the device is clocked by the internal Oscillator, it is no need to connect a clock source to the XTAL1 pin. 2. V - 0.3V < AVCC < V + 0.3V, however, AVCC should always be within 1.8 - 5.5V CC CC When programming the EEPROM, an auto-erase cycle is built into the self-timed programming operation (in the Serial mode ONLY) and there is no need to first execute the Chip Erase instruction. The Chip Erase operation turns the content of every memory location in both the Program and EEPROM arrays into 0xFF. Depending on CKSEL Fuses, a valid clock must be present. The minimum low and high periods for the serial clock (SCK) input are defined as follows: Low:> two CPU clock cycles for f < 12MHz, three CPU clock cycles for f >= 12MHz ck ck High:> two CPU clock cycles for f < 12MHz, three CPU clock cycles for f >= 12MHz ck ck 28.8.1 Serial Programming Algorithm When writing serial data to the device, data is clocked on the rising edge of SCK. When reading data from the device, data is clocked on the falling edge of SCK. See Figure 28-11 on page 369 for timing details. To program and verify the device in the serial programming mode, the following sequence is recommended (see four byte instruction formats in Table28-16 on page370): 1. Power-up sequence: Apply power between V and GND while RESET and SCK are set to “0”. In some systems, the pro- CC grammer can not guarantee that SCK is held low during power-up. In this case, RESET must be given a positive pulse of at least two CPU clock cycles duration after SCK has been set to “0”. 2. Wait for at least 20ms and enable serial programming by sending the Programming Enable serial instruction to pin PDI. 3. The serial programming instructions will not work if the communication is out of synchronization. When in sync. the second byte (0x53), will echo back when issuing the third byte of the Programming Enable ATmega16U4/32U4 [DATASHEET] 368 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

instruction. Whether the echo is correct or not, all four bytes of the instruction must be transmitted. If the 0x53 did not echo back, give RESET a positive pulse and issue a new Programming Enable command. 4. The Flash is programmed one page at a time. The memory page is loaded one byte at a time by supplying the seven LSB of the address and data together with the Load Program Memory Page instruction. To ensure correct loading of the page, the data low byte must be loaded before data high byte is applied for a given address. The Program Memory Page is stored by loading the Write Program Memory Page instruction with the address lines 15..8. Before issuing this command, make sure the instruction Load Extended Address Byte has been used to define the MSB of the address. The extended address byte is stored until the command is re-issued, i.e., the command needs only be issued for the first page, and when crossing the 64KWord boundary. If polling (RDY/BSY) is not used, the user must wait at least t before issuing the next page. (See Table28-15 on page369.) Accessing the serial programming WD_FLASH interface before the Flash write operation completes can result in incorrect programming. 5. The EEPROM array is programmed one byte at a time by supplying the address and data together with the appropriate Write instruction. An EEPROM memory location is first automatically erased before new data is written. If polling is not used, the user must wait at least t before issuing the next byte. WD_EEPROM (See Table28-15 on page369.) In a chip erased device, no 0xFFs in the data file(s) need to be programmed. 6. Any memory location can be verified by using the Read instruction which returns the content at the selected address at serial output PDO. When reading the Flash memory, use the instruction Load Extended Address Byte to define the upper address byte, which is not included in the Read Program Memory instruction. The extended address byte is stored until the command is re-issued, i.e., the command needs only be issued for the first page, and when crossing the 64KWord boundary. 7. At the end of the programming session, RESET can be set high to commence normal operation. 8. Power-off sequence (if needed): Set RESET to “1”. Turn V power off. CC Table 28-15. Minimum Wait Delay Before Writing the Next Flash or EEPROM Symbol Minimum Wait Delay t 4.5ms WD_FLASH t 9.0ms WD_EEPROM t 9.0ms WD_ERASE Figure 28-11. Serial Programming Waveforms SERIAL DATA INPUT MSB LSB (MOSI) SERIAL DATA OUTPUT MSB LSB (MISO) SERIAL CLOCK INPUT (SCK) SAMPLE ATmega16U4/32U4 [DATASHEET] 369 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 28-16. Serial Programming Instruction Set Instruction Format Instruction Byte 1 Byte 2 Byte 3 Byte4 Operation 1010 1100 0101 0011 xxxx xxxx xxxx xxxx Enable Serial Programming after Programming Enable RESET goes low. Chip Erase 1010 1100 100x xxxx xxxx xxxx xxxx xxxx Chip Erase EEPROM and Flash. 0100 1101 0000 0000 cccc cccc xxxx xxxx Defines Extended Address Byte for Load Extended Address Byte Read Program Memory and Write Program Memory Page. 0010 H000 aaaa aaaa bbbb bbbb oooo oooo Read H (high or low) data o from Read Program Memory Program memory at word address c:a:b. 0100 H000 xxxx xxxx xxbb bbbb iiii iiii Write H (high or low) data i to Program Memory page at word address b. Data Load Program Memory Page low byte must be loaded before Data high byte is applied within the same address. 0100 1100 aaaa aaaa bbxx xxxx xxxx xxxx Write Program Memory Page at Write Program Memory Page address c:a:b. 1010 0000 0000 aaaa bbbb bbbb oooo oooo Read data o from EEPROM memory at Read EEPROM Memory address a:b. 1100 0000 0000 aaaa bbbb bbbb iiii iiii Write data i to EEPROM memory at Write EEPROM Memory address a:b. 1100 0001 0000 0000 0000 00bb iiii iiii Load data i to EEPROM memory page Load EEPROM Memory buffer. After data is loaded, program Page (page access) EEPROM page. Write EEPROM Memory 1100 0010 0000 aaaa bbbb bb00 xxxx xxxx Write EEPROM page at address a:b. Page (page access) 0101 1000 0000 0000 xxxx xxxx xxoo oooo Read Lock bits. “0” = programmed, “1” Read Lock bits = unprogrammed. See Table28-1 on page353 for details. 1010 1100 111x xxxx xxxx xxxx 11ii iiii Write Lock bits. Set bits = “0” to Write Lock bits program Lock bits. See Table28-1 on page353 for details. Read Signature Byte 0011 0000 000x xxxx xxxx xxbb oooo oooo Read Signature Byte o at address b. 1010 1100 1010 0000 xxxx xxxx iiii iiii Set bits = “0” to program, “1” to Write Fuse bits unprogram. 1010 1100 1010 1000 xxxx xxxx iiii iiii Set bits = “0” to program, “1” to Write Fuse High bits unprogram. 1010 1100 1010 0100 xxxx xxxx iiii iiii Set bits = “0” to program, “1” to Write Extended Fuse Bits unprogram. See Table28-3 on page354 for details. 0101 0000 0000 0000 xxxx xxxx oooo oooo Read Fuse bits. “0” = programmed, “1” Read Fuse bits = unprogrammed. ATmega16U4/32U4 [DATASHEET] 370 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 28-16. Serial Programming Instruction Set Instruction Format Instruction Byte 1 Byte 2 Byte 3 Byte4 Operation 0101 1000 0000 1000 xxxx xxxx oooo oooo Read Fuse High bits. “0” = pro- Read Fuse High bits grammed, “1” = unprogrammed. 0101 0000 0000 1000 xxxx xxxx oooo oooo Read Extended Fuse bits. “0” = pro- Read Extended Fuse Bits grammed, “1” = unprogrammed. See Table28-3 on page354 for details. Read Calibration Byte 0011 1000 000x xxxx 0000 0000 oooo oooo Read Calibration Byte 1111 0000 0000 0000 xxxx xxxx xxxx xxxo If o = “1”, a programming operation is Poll RDY/BSY still busy. Wait until this bit returns to “0” before applying another command. Note: a = address high bits, b = address low bits, c = address extended bits, H = 0 - Low byte, 1 - High Byte, o = data out, i = data in, x = don’t care. 28.8.2 Serial Programming Characteristics For characteristics of the Serial Programming module see “SPI Timing Characteristics” on page 388. 28.9 Programming via the JTAG Interface Programming through the JTAG interface requires control of the four JTAG specific pins: TCK, TMS, TDI, and TDO. Control of the reset and clock pins is not required. To be able to use the JTAG interface, the JTAGEN Fuse must be programmed. The device is default shipped with the fuse programmed. In addition, the JTD bit in MCUCSR must be cleared. Alternatively, if the JTD bit is set, the external reset can be forced low. Then, the JTD bit will be cleared after two chip clocks, and the JTAG pins are available for programming. This provides a means of using the JTAG pins as normal port pins in Running mode while still allowing In-System Programming via the JTAG interface. Note that this technique can not be used when using the JTAG pins for Boundary-scan or On-chip Debug. In these cases the JTAG pins must be dedicated for this purpose. During programming the clock frequency of the TCK Input must be less than the maximum frequency of the chip. The System Clock Prescaler can not be used to divide the TCK Clock Input into a sufficiently low frequency. As a definition in this datasheet, the LSB is shifted in and out first of all Shift Registers. 28.9.1 Programming Specific JTAG Instructions The Instruction Register is 4-bit wide, supporting up to 16 instructions. The JTAG instructions useful for programming are listed below. The OPCODE for each instruction is shown behind the instruction name in hex format. The text describes which Data Register is selected as path between TDI and TDO for each instruction. The Run-Test/Idle state of the TAP controller is used to generate internal clocks. It can also be used as an idle state between JTAG sequences. The state machine sequence for changing the instruction word is shown in Figure 28-12 on page 372. ATmega16U4/32U4 [DATASHEET] 371 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 28-12. State Machine Sequence for Changing the Instruction Word 1 Test-Logic-Reset 0 0 Run-Test/Idle 1 Select-DR Scan 1 Select-IR Scan 1 0 0 1 1 Capture-DR Capture-IR 0 0 Shift-DR 0 Shift-IR 0 1 1 1 1 Exit1-DR Exit1-IR 0 0 Pause-DR 0 Pause-IR 0 1 1 0 0 Exit2-DR Exit2-IR 1 1 Update-DR Update-IR 1 0 1 0 28.9.2 AVR_RESET (0xC) The AVR specific public JTAG instruction for setting the AVR device in the Reset mode or taking the device out from the Reset mode. The TAP controller is not reset by this instruction. The one bit Reset Register is selected as Data Register. Note that the reset will be active as long as there is a logic “one” in the Reset Chain. The output from this chain is not latched. The active states are:  Shift-DR: The Reset Register is shifted by the TCK input. 28.9.3 PROG_ENABLE (0x4) The AVR specific public JTAG instruction for enabling programming via the JTAG port. The 16-bit Programming Enable Register is selected as Data Register. The active states are the following:  Shift-DR: The programming enable signature is shifted into the Data Register  Update-DR: The programming enable signature is compared to the correct value, and Programming mode is entered if the signature is valid 28.9.4 PROG_COMMANDS (0x5) The AVR specific public JTAG instruction for entering programming commands via the JTAG port. The 15-bit Programming Command Register is selected as Data Register. The active states are the following: ATmega16U4/32U4 [DATASHEET] 372 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

 Capture-DR: The result of the previous command is loaded into the Data Register  Shift-DR: The Data Register is shifted by the TCK input, shifting out the result of the previous command and shifting in the new command  Update-DR: The programming command is applied to the Flash inputs  Run-Test/Idle: One clock cycle is generated, executing the applied command 28.9.5 PROG_PAGELOAD (0x6) The AVR specific public JTAG instruction to directly load the Flash data page via the JTAG port. An 8-bit Flash Data Byte Register is selected as the Data Register. This is physically the 8 LSBs of the Programming Command Register. The active states are the following:  Shift-DR: The Flash Data Byte Register is shifted by the TCK input  Update-DR: The content of the Flash Data Byte Register is copied into a temporary register. A write sequence is initiated that within 11 TCK cycles loads the content of the temporary register into the Flash page buffer. The AVR automatically alternates between writing the low and the high byte for each new Update-DR state, starting with the low byte for the first Update-DR encountered after entering the PROG_PAGELOAD command. The Program Counter is pre-incremented before writing the low byte, except for the first written byte. This ensures that the first data is written to the address set up by PROG_COMMANDS, and loading the last location in the page buffer does not make the program counter increment into the next page. 28.9.6 PROG_PAGEREAD (0x7) The AVR specific public JTAG instruction to directly capture the Flash content via the JTAG port. An 8-bit Flash Data Byte Register is selected as the Data Register. This is physically the 8 LSBs of the Programming Command Register. The active states are the following:  Capture-DR: The content of the selected Flash byte is captured into the Flash Data Byte Register. The AVR automatically alternates between reading the low and the high byte for each new Capture-DR state, starting with the low byte for the first Capture-DR encountered after entering the PROG_PAGEREAD command. The Program Counter is post-incremented after reading each high byte, including the first read byte. This ensures that the first data is captured from the first address set up by PROG_COMMANDS, and reading the last location in the page makes the program counter increment into the next page.  Shift-DR: The Flash Data Byte Register is shifted by the TCK input 28.9.7 Data Registers The Data Registers are selected by the JTAG instruction registers described in section “Programming Specific JTAG Instructions” on page371. The Data Registers relevant for programming operations are:  Reset Register  Programming Enable Register  Programming Command Register  Flash Data Byte Register 28.9.8 Reset Register The Reset Register is a Test Data Register used to reset the part during programming. It is required to reset the part before entering Programming mode. A high value in the Reset Register corresponds to pulling the external reset low. The part is reset as long as there is a high value present in the Reset Register. Depending on the Fuse settings for the clock options, the part will remain reset for a Reset Time-out period (refer to “Clock Sources” on page28) after releasing the Reset Register. The output from this Data Register is not latched, so the reset will take place immediately, as shown in Figure 8-1 on page 51. ATmega16U4/32U4 [DATASHEET] 373 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

28.9.9 Programming Enable Register The Programming Enable Register is a 16-bit register. The contents of this register is compared to the programming enable signature, binary code 0b1010_0011_0111_0000. When the contents of the register is equal to the programming enable signature, programming via the JTAG port is enabled. The register is reset to 0 on Power-on Reset, and should always be reset when leaving Programming mode. Figure 28-13. Programming Enable Register TDI D 0xA370 = A D Q Programming Enable T A ClockDR & PROG_ENABLE TDO 28.9.10Programming Command Register The Programming Command Register is a 15-bit register. This register is used to serially shift in programming commands, and to serially shift out the result of the previous command, if any. The JTAG Programming Instruction Set is shown in Figure 28-15 on page 378. The state sequence when shifting in the programming commands is illustrated in Figure 28-15 on page 378. Figure 28-14. Programming Command Register TDI S T R O B E S Flash EEPROM A Fuses D Lock Bits D R E S S / D A T A TDO ATmega16U4/32U4 [DATASHEET] 374 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 28-17. JTAG Programming Instruction Set a = address high bits, b = address low bits, c = address extended bits, H = 0 - Low byte, 1 - High Byte, o = data out, i = data in, x = don’t care Instruction TDI Sequence TDO Sequence Notes 0100011_10000000 xxxxxxx_xxxxxxxx 0110001_10000000 xxxxxxx_xxxxxxxx 1a. Chip Erase 0110011_10000000 xxxxxxx_xxxxxxxx 0110011_10000000 xxxxxxx_xxxxxxxx 1b. Poll for Chip Erase Complete 0110011_10000000 xxxxxox_xxxxxxxx (2) 2a. Enter Flash Write 0100011_00010000 xxxxxxx_xxxxxxxx 2b. Load Address Extended High Byte 0001011_cccccccc xxxxxxx_xxxxxxxx (10) 2c. Load Address High Byte 0000111_aaaaaaaa xxxxxxx_xxxxxxxx 2d. Load Address Low Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx 2e. Load Data Low Byte 0010011_iiiiiiii xxxxxxx_xxxxxxxx 2f. Load Data High Byte 0010111_iiiiiiii xxxxxxx_xxxxxxxx 0110111_00000000 xxxxxxx_xxxxxxxx 2g. Latch Data 1110111_00000000 xxxxxxx_xxxxxxxx (1) 0110111_00000000 xxxxxxx_xxxxxxxx 0110111_00000000 xxxxxxx_xxxxxxxx 0110101_00000000 xxxxxxx_xxxxxxxx 2h. Write Flash Page (1) 0110111_00000000 xxxxxxx_xxxxxxxx 0110111_00000000 xxxxxxx_xxxxxxxx 2i. Poll for Page Write Complete 0110111_00000000 xxxxxox_xxxxxxxx (2) 3a. Enter Flash Read 0100011_00000010 xxxxxxx_xxxxxxxx 3b. Load Address Extended High Byte 0001011_cccccccc xxxxxxx_xxxxxxxx (10) 3c. Load Address High Byte 0000111_aaaaaaaa xxxxxxx_xxxxxxxx 3d. Load Address Low Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx 0110010_00000000 xxxxxxx_xxxxxxxx 3e. Read Data Low and High Byte 0110110_00000000 xxxxxxx_oooooooo Low byte 0110111_00000000 xxxxxxx_oooooooo High byte 4a. Enter EEPROM Write 0100011_00010001 xxxxxxx_xxxxxxxx 4b. Load Address High Byte 0000111_aaaaaaaa xxxxxxx_xxxxxxxx (10) 4c. Load Address Low Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx 4d. Load Data Byte 0010011_iiiiiiii xxxxxxx_xxxxxxxx 0110111_00000000 xxxxxxx_xxxxxxxx 4e. Latch Data 1110111_00000000 xxxxxxx_xxxxxxxx (1) 0110111_00000000 xxxxxxx_xxxxxxxx ATmega16U4/32U4 [DATASHEET] 375 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 28-17. JTAG Programming Instruction Set a = address high bits, b = address low bits, c = address extended bits, H = 0 - Low byte, 1 - High Byte, o = data out, i = data in, x = don’t care Instruction TDI Sequence TDO Sequence Notes 0110011_00000000 xxxxxxx_xxxxxxxx 0110001_00000000 xxxxxxx_xxxxxxxx 4f. Write EEPROM Page (1) 0110011_00000000 xxxxxxx_xxxxxxxx 0110011_00000000 xxxxxxx_xxxxxxxx 4g. Poll for Page Write Complete 0110011_00000000 xxxxxox_xxxxxxxx (2) 5a. Enter EEPROM Read 0100011_00000011 xxxxxxx_xxxxxxxx 5b. Load Address High Byte 0000111_aaaaaaaa xxxxxxx_xxxxxxxx (10) 5c. Load Address Low Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx 0110011_bbbbbbbb xxxxxxx_xxxxxxxx 5d. Read Data Byte 0110010_00000000 xxxxxxx_xxxxxxxx 0110011_00000000 xxxxxxx_oooooooo 6a. Enter Fuse Write 0100011_01000000 xxxxxxx_xxxxxxxx 6b. Load Data Low Byte(6) 0010011_iiiiiiii xxxxxxx_xxxxxxxx (3) 0111011_00000000 xxxxxxx_xxxxxxxx 0111001_00000000 xxxxxxx_xxxxxxxx 6c. Write Fuse Extended Byte (1) 0111011_00000000 xxxxxxx_xxxxxxxx 0111011_00000000 xxxxxxx_xxxxxxxx 6d. Poll for Fuse Write Complete 0110111_00000000 xxxxxox_xxxxxxxx (2) 6e. Load Data Low Byte(7) 0010011_iiiiiiii xxxxxxx_xxxxxxxx (3) 0110111_00000000 xxxxxxx_xxxxxxxx 0110101_00000000 xxxxxxx_xxxxxxxx 6f. Write Fuse High Byte (1) 0110111_00000000 xxxxxxx_xxxxxxxx 0110111_00000000 xxxxxxx_xxxxxxxx 6g. Poll for Fuse Write Complete 0110111_00000000 xxxxxox_xxxxxxxx (2) 6h. Load Data Low Byte(7) 0010011_iiiiiiii xxxxxxx_xxxxxxxx (3) 0110011_00000000 xxxxxxx_xxxxxxxx 0110001_00000000 xxxxxxx_xxxxxxxx 6i. Write Fuse Low Byte (1) 0110011_00000000 xxxxxxx_xxxxxxxx 0110011_00000000 xxxxxxx_xxxxxxxx 6j. Poll for Fuse Write Complete 0110011_00000000 xxxxxox_xxxxxxxx (2) 7a. Enter Lock Bit Write 0100011_00100000 xxxxxxx_xxxxxxxx 7b. Load Data Byte(9) 0010011_11iiiiii xxxxxxx_xxxxxxxx (4) ATmega16U4/32U4 [DATASHEET] 376 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 28-17. JTAG Programming Instruction Set a = address high bits, b = address low bits, c = address extended bits, H = 0 - Low byte, 1 - High Byte, o = data out, i = data in, x = don’t care Instruction TDI Sequence TDO Sequence Notes 0110011_00000000 xxxxxxx_xxxxxxxx 0110001_00000000 xxxxxxx_xxxxxxxx 7c. Write Lock Bits (1) 0110011_00000000 xxxxxxx_xxxxxxxx 0110011_00000000 xxxxxxx_xxxxxxxx 7d. Poll for Lock Bit Write complete 0110011_00000000 xxxxxox_xxxxxxxx (2) 8a. Enter Fuse/Lock Bit Read 0100011_00000100 xxxxxxx_xxxxxxxx 0111010_00000000 xxxxxxx_xxxxxxxx 8b. Read Extended Fuse Byte(6) 0111011_00000000 xxxxxxx_oooooooo 0111110_00000000 xxxxxxx_xxxxxxxx 8c. Read Fuse High Byte(7) 0111111_00000000 xxxxxxx_oooooooo 0110010_00000000 xxxxxxx_xxxxxxxx 8d. Read Fuse Low Byte(8) 0110011_00000000 xxxxxxx_oooooooo 0110110_00000000 xxxxxxx_xxxxxxxx 8e. Read Lock Bits(9) (5) 0110111_00000000 xxxxxxx_xxoooooo 0111010_00000000 xxxxxxx_xxxxxxxx (5) 0111110_00000000 xxxxxxx_oooooooo Fuse Ext. byte 8f. Read Fuses and Lock Bits 0110010_00000000 xxxxxxx_oooooooo Fuse High byte 0110110_00000000 xxxxxxx_oooooooo Fuse Low byte 0110111_00000000 xxxxxxx_oooooooo Lock bits 9a. Enter Signature Byte Read 0100011_00001000 xxxxxxx_xxxxxxxx 9b. Load Address Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx 0110010_00000000 xxxxxxx_xxxxxxxx 9c. Read Signature Byte 0110011_00000000 xxxxxxx_oooooooo 10a. Enter Calibration Byte Read 0100011_00001000 xxxxxxx_xxxxxxxx 10b. Load Address Byte 0000011_bbbbbbbb xxxxxxx_xxxxxxxx 0110110_00000000 xxxxxxx_xxxxxxxx 10c. Read Calibration Byte 0110111_00000000 xxxxxxx_oooooooo 0100011_00000000 xxxxxxx_xxxxxxxx 11a. Load No Operation Command 0110011_00000000 xxxxxxx_xxxxxxxx Notes: 1. This command sequence is not required if the seven MSB are correctly set by the previous command sequence (which is normally the case). 2. Repeat until o = “1”. 3. Set bits to “0” to program the corresponding Fuse, “1” to unprogram the Fuse. 4. Set bits to “0” to program the corresponding Lock bit, “1” to leave the Lock bit unchanged. 5. “0” = programmed, “1” = unprogrammed. 6. The bit mapping for Fuses Extended byte is listed in Table28-3 on page354. 7. The bit mapping for Fuses High byte is listed in Table28-4 on page355. ATmega16U4/32U4 [DATASHEET] 377 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

8. The bit mapping for Fuses Low byte is listed in Table28-5 on page355. 9. The bit mapping for Lock bits byte is listed in Table28-1 on page353. 10.Address bits exceeding PCMSB and EEAMSB (Table28-11 on page359 and Table28-12 on page359) are don’t care. 11.All TDI and TDO sequences are represented by binary digits (0b...). Figure 28-15. State Machine Sequence for Changing/Reading the Data Word 1 Test-Logic-Reset 0 0 Run-Test/Idle 1 Select-DR Scan 1 Select-IR Scan 1 0 0 1 1 Capture-DR Capture-IR 0 0 Shift-DR 0 Shift-IR 0 1 1 1 1 Exit1-DR Exit1-IR 0 0 Pause-DR 0 Pause-IR 0 1 1 0 0 Exit2-DR Exit2-IR 1 1 Update-DR Update-IR 1 0 1 0 28.9.11 Flash Data Byte Register The Flash Data Byte Register provides an efficient way to load the entire Flash page buffer before executing Page Write, or to read out/verify the content of the Flash. A state machine sets up the control signals to the Flash and senses the strobe signals from the Flash, thus only the data words need to be shifted in/out. The Flash Data Byte Register actually consists of the 8-bit scan chain and a 8-bit temporary register. During page load, the Update-DR state copies the content of the scan chain over to the temporary register and initiates a write sequence that within 11 TCK cycles loads the content of the temporary register into the Flash page buffer. The AVR automatically alternates between writing the low and the high byte for each new Update-DR state, starting with the low byte for the first Update-DR encountered after entering the PROG_PAGELOAD command. The Program Counter is pre-incremented before writing the low byte, except for the first written byte. This ensures that the first data is written to the address set up by PROG_COMMANDS, and loading the last location in the page buffer does not make the Program Counter increment into the next page. During Page Read, the content of the selected Flash byte is captured into the Flash Data Byte Register during the Capture-DR state. The AVR automatically alternates between reading the low and the high byte for each ATmega16U4/32U4 [DATASHEET] 378 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

new Capture-DR state, starting with the low byte for the first Capture-DR encountered after entering the PROG_PAGEREAD command. The Program Counter is post-incremented after reading each high byte, including the first read byte. This ensures that the first data is captured from the first address set up by PROG_COMMANDS, and reading the last location in the page makes the program counter increment into the next page. Figure 28-16. Flash Data Byte Register STROBES State Machine TDI ADDRESS Flash EEPROM Fuses Lock Bits D A T A TDO The state machine controlling the Flash Data Byte Register is clocked by TCK. During normal operation in which eight bits are shifted for each Flash byte, the clock cycles needed to navigate through the TAP controller automatically feeds the state machine for the Flash Data Byte Register with sufficient number of clock pulses to complete its operation transparently for the user. However, if too few bits are shifted between each Update-DR state during page load, the TAP controller should stay in the Run-Test/Idle state for some TCK cycles to ensure that there are at least 11 TCK cycles between each Update-DR state. 28.9.12Programming Algorithm All references below of type “1a”, “1b”, and so on, refer to Table28-17 on page375. 28.9.13Entering Programming Mode 1. Enter JTAG instruction AVR_RESET and shift 1 in the Reset Register. 2. Enter instruction PROG_ENABLE and shift 0b1010_0011_0111_0000 in the Programming Enable Register. 28.9.14Leaving Programming Mode 1. Enter JTAG instruction PROG_COMMANDS. 2. Disable all programming instructions by using no operation instruction 11a. 3. Enter instruction PROG_ENABLE and shift 0b0000_0000_0000_0000 in the programming Enable Register. 4. Enter JTAG instruction AVR_RESET and shift 0 in the Reset Register. ATmega16U4/32U4 [DATASHEET] 379 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

28.9.15Performing Chip Erase 1. Enter JTAG instruction PROG_COMMANDS. 2. Start Chip Erase using programming instruction 1a. 3. Poll for Chip Erase complete using programming instruction 1b, or wait for t (refer to Table28-13 WLRH_CE on page366). 28.9.16Programming the Flash Before programming the Flash a Chip Erase must be performed, see “Performing Chip Erase” on page 380. 1. Enter JTAG instruction PROG_COMMANDS. 2. Enable Flash write using programming instruction 2a. 3. Load address Extended High byte using programming instruction 2b. 4. Load address High byte using programming instruction 2c. 5. Load address Low byte using programming instruction 2d. 6. Load data using programming instructions 2e, 2f, and 2g. 7. Repeat steps 5 and 6 for all instruction words in the page. 8. Write the page using programming instruction 2h. 9. Poll for Flash write complete using programming instruction 2i, or wait for t (refer to Table28-13 on WLRH page366). 10. Repeat steps 3 to 9 until all data have been programmed. A more efficient data transfer can be achieved using the PROG_PAGELOAD instruction: 1. Enter JTAG instruction PROG_COMMANDS. 2. Enable Flash write using programming instruction 2a. 3. Load the page address using programming instructions 2b, 2c, and 2d. PCWORD (refer to Table28-11 on page359) is used to address within one page and must be written as 0. 4. Enter JTAG instruction PROG_PAGELOAD. 5. Load the entire page by shifting in all instruction words in the page byte-by-byte, starting with the LSB of the first instruction in the page and ending with the MSB of the last instruction in the page. Use Update- DR to copy the contents of the Flash Data Byte Register into the Flash page location and to auto- increment the Program Counter before each new word. 6. Enter JTAG instruction PROG_COMMANDS. 7. Write the page using programming instruction 2h. 8. Poll for Flash write complete using programming instruction 2i, or wait for t (refer to Table28-13 on WLRH page366). 9. Repeat steps 3 to 8 until all data have been programmed. 28.9.17Reading the Flash 1. Enter JTAG instruction PROG_COMMANDS. 2. Enable Flash read using programming instruction 3a. 3. Load address using programming instructions 3b, 3c and 3d. 4. Read data using programming instruction 3e. 5. Repeat steps 3 and 4 until all data have been read. A more efficient data transfer can be achieved using the PROG_PAGEREAD instruction: 1. Enter JTAG instruction PROG_COMMANDS. 2. Enable Flash read using programming instruction 3a. ATmega16U4/32U4 [DATASHEET] 380 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

3. Load the page address using programming instructions 3b, 3c, and 3d. PCWORD (refer to Table28-11 on page359) is used to address within one page and must be written as 0. 4. Enter JTAG instruction PROG_PAGEREAD. 5. Read the entire page (or Flash) by shifting out all instruction words in the page (or Flash), starting with the LSB of the first instruction in the page (Flash) and ending with the MSB of the last instruction in the page (Flash). The Capture-DR state both captures the data from the Flash, and also auto-increments the program counter after each word is read. Note that Capture-DR comes before the shift-DR state. Hence, the first byte which is shifted out contains valid data. 6. Enter JTAG instruction PROG_COMMANDS. 7. Repeat steps 3 to 6 until all data have been read. 28.9.18Programming the EEPROM Before programming the EEPROM a Chip Erase must be performed, see “Performing Chip Erase” on page 380. 1. Enter JTAG instruction PROG_COMMANDS. 2. Enable EEPROM write using programming instruction 4a. 3. Load address High byte using programming instruction 4b. 4. Load address Low byte using programming instruction 4c. 5. Load data using programming instructions 4d and 4e. 6. Repeat steps 4 and 5 for all data bytes in the page. 7. Write the data using programming instruction 4f. 8. Poll for EEPROM write complete using programming instruction 4g, or wait for t (refer to Table28-13 WLRH on page366). 9. Repeat steps 3 to 8 until all data have been programmed. Note that the PROG_PAGELOAD instruction can not be used when programming the EEPROM. 28.9.19Reading the EEPROM 1. Enter JTAG instruction PROG_COMMANDS. 2. Enable EEPROM read using programming instruction 5a. 3. Load address using programming instructions 5b and 5c. 4. Read data using programming instruction 5d. 5. Repeat steps 3 and 4 until all data have been read. Note that the PROG_PAGEREAD instruction can not be used when reading the EEPROM. 28.9.20Programming the Fuses 1. Enter JTAG instruction PROG_COMMANDS. 2. Enable Fuse write using programming instruction 6a. 3. Load data high byte using programming instructions 6b. A bit value of “0” will program the corresponding fuse, a “1” will unprogram the fuse. 4. Write Fuse High byte using programming instruction 6c. 5. Poll for Fuse write complete using programming instruction 6d, or wait for t (refer to Table28-13 on WLRH page366). 6. Load data low byte using programming instructions 6e. A “0” will program the fuse, a “1” will unprogram the fuse. 7. Write Fuse low byte using programming instruction 6f. 8. Poll for Fuse write complete using programming instruction 6g, or wait for t (refer to Table28-13 on WLRH page366). ATmega16U4/32U4 [DATASHEET] 381 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

28.9.21Programming the Lock Bits 1. Enter JTAG instruction PROG_COMMANDS. 2. Enable Lock bit write using programming instruction 7a. 3. Load data using programming instructions 7b. A bit value of “0” will program the corresponding lock bit, a “1” will leave the lock bit unchanged. 4. Write Lock bits using programming instruction 7c. 5. Poll for Lock bit write complete using programming instruction 7d, or wait for t (refer to Table28-13 WLRH on page366). 28.9.22Reading the Fuses and Lock Bits 1. Enter JTAG instruction PROG_COMMANDS. 2. Enable Fuse/Lock bit read using programming instruction 8a. 3. To read all Fuses and Lock bits, use programming instruction 8e. To only read Fuse High byte, use programming instruction 8b. To only read Fuse Low byte, use programming instruction 8c. To only read Lock bits, use programming instruction 8d. 28.9.23Reading the Signature Bytes 1. Enter JTAG instruction PROG_COMMANDS. 2. Enable Signature byte read using programming instruction 9a. 3. Load address 0x00 using programming instruction 9b. 4. Read first signature byte using programming instruction 9c. 5. Repeat steps 3 and 4 with address 0x01 and address 0x02 to read the second and third signature bytes, respectively. 28.9.24Reading the Calibration Byte 1. Enter JTAG instruction PROG_COMMANDS. 2. Enable Calibration byte read using programming instruction 10a. 3. Load address 0x00 using programming instruction 10b. 4. Read the calibration byte using programming instruction 10c. ATmega16U4/32U4 [DATASHEET] 382 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

29. Electrical Characteristics 29.1 Absolute Maximum Ratings* Operating Temperature. . . . . . . . . . . . -40C to +85C *NOTICE: Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent dam- age to the device. This is a stress rating only Storage Temperature . . . . . . . . . . . . -65°C to +150°C and functional operation of the device at these or other conditions beyond those indicated in the operational sections of this specification is Voltage on any Pin except RESET and VBUS not implied. Exposure to absolute maximum rat- with respect to Ground(8) . . . . . . . . -0.5V to V +0.5V CC ing conditions for extended periods may affect device reliability. Voltage on RESET with respect to Ground-0.5V to +13.0V Voltage on VBUS with respect to Ground-0.5V to +6.0V Maximum Operating Voltage . . . . . . . . . . . . . . . .6.0V DC Current per I/O Pin. . . . . . . . . . . . . . . . . . 40.0mA DC Current V and GND Pins . . . . . . . . . . 200.0mA CC 29.2 DC Characteristics Table 29-1. DC Characteristic, TA = -40°C to 85°C, VCC = 2.7V to 5.5V (unless otherwise noted) Symbol Parameter Condition Min.(5) Typ. Max.(5) Units Input Low Voltage, 0.2VCC- V Except XTAL1 and V = 2.7V - 5.5V -0.5 0.1V(1) IL CC Reset pin (LVTTL) Input Low Voltage, V V = 2.7V - 5.5V -0.5 0.1V (1) IL1 XTAL1 pin CC CC Input Low Voltage, V V = 2.7V - 5.5V -0.5 0.1V (1) IL2 RESET pin CC CC Input High Voltage, 0.2VCC+0.9 V Except XTAL1 and V = 2.7V - 5.5V V(2) V + 0.5 IH CC CC V RESET pins (LVTTL) Input High Voltage, V V = 2.7V - 5.5V 0.7V (2) V + 0.5 IH1 XTAL1 pin CC CC CC Input High Voltage, V V = 2.7V - 5.5V 0.9V (2) V + 0.5 IH2 RESET pin CC CC CC I = 10mA, V = 5V 0.7 V Output Low Voltage(3), OL CC OL I = 5mA, V = 3V 0.5 OL CC I = -10mA, V = 5V 4.2 V Output High Voltage(4), OH CC OH I = -5mA, V = 3V 2.3 OH CC Input Leakage V = 5.5V, pin low I CC 1 µA IL Current I/O Pin (absolute value) ATmega16U4/32U4 [DATASHEET] 383 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 29-1. DC Characteristic, TA = -40°C to 85°C, VCC = 2.7V to 5.5V (unless otherwise noted) Symbol Parameter Condition Min.(5) Typ. Max.(5) Units Input Leakage V = 5.5V, pin high I CC 1 µA IH Current I/O Pin (absolute value) R Reset Pull-up Resistor 30 60 RST k R I/O Pin Pull-up Resistor 20 50 PU Active 4MHz, V = 3V CC 5 (ATmega16U4/ATmega32U4) Active 8MHz, V = 5V CC 10 15 (ATmega16U4/ATmega32U4) Active 16MHz, V = 5V Power Supply Current(6) CC 27 mA (ATmega16U4/ATmega32U4) Idle 4MHz, V = 3V I CC 2 CC (ATmega16U4/ATmega32U4) Idle 8MHz, V = 5V CC 6 (ATmega16U4/ATmega32U4) WDT enabled, V = 3V, CC <10 12 Regulator Disabled Power-down mode µA WDT disabled, V = CC 1 5 3V,Regulator Disabled Analog Comparator V = 5V V CC <10 40 mV ACIO Input Offset Voltage V = V /2 in CC Analog Comparator V = 5V I CC -50 50 nA ACLK Input Leakage Current V = V /2 in CC Analog Comparator V = 2.7V 750 t CC ns ACID Propagation Delay V = 4.0V 500 CC USB Series resistor Rusb 22±5%  (external) C = 1µF ±20%, UCAP Regulator Output UVcc  4.0V, I80mA(7), Vreg 3.0 3.3 3.6 V Voltage or UVcc  3.4V, I55mA(7) Note: 1. "Max" means the highest value where the pin is guaranteed to be read as low 2. "Min" means the lowest value where the pin is guaranteed to be read as high 3. Although each I/O port can sink more than the test conditions (20mA at V = 5V, 10mA at V = 3V) under steady state CC CC conditions (non-transient), the following must be observed: ATmega16U4/ATmega32U4: 1.)The sum of all IOL, for ports A0-A7, G2, C4-C7 should not exceed 100mA. 2.)The sum of all IOL, for ports C0-C3, G0-G1, D0-D7 should not exceed 100mA. 3.)The sum of all IOL, for ports G3-G5, B0-B7, E0-E7 should not exceed 100mA. 4.)The sum of all IOL, for ports F0-F7 should not exceed 100mA. If IOL exceeds the test condition, VOL may exceed the related specification. Pins are not guaranteed to sink current greater than the listed test condition. 4. Although each I/O port can source more than the test conditions (20mA at V = 5V, 10mA at V = 3V) under steady CC CC state conditions (non-transient), the following must be observed: ATmega16U4/ATmega32U4: ATmega16U4/32U4 [DATASHEET] 384 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

1)The sum of all IOH, for ports A0-A7, G2, C4-C7 should not exceed 100mA. 2)The sum of all IOH, for ports C0-C3, G0-G1, D0-D7 should not exceed 100mA. 3)The sum of all IOH, for ports G3-G5, B0-B7, E0-E7 should not exceed 100mA. 4)The sum of all IOH, for ports F0-F7 should not exceed 100mA. 5. All DC Characteristics contained in this datasheet are based on simulation and characterization of other AVR microcontrollers manufactured in the same process technology. These values are preliminary values representing design targets, and will be updated after characterization of actual silicon 6. Values with “Power Reduction Register 1 - PRR1” disabled (0x00). 7. Maximum regulator output current should be reduced by the USB buffer current required when USB is active (about 25mA). The remaining regulator output current can be used for the external application. 8. As specified on the USB Electrical chapter, the D+/D- pads can withstand voltages down to -1V applied through a 39 resistor 29.3 External Clock Drive Waveforms Figure 29-1. External Clock Drive Waveforms VIH1 VIL1 29.4 External Clock Drive Table 29-2. External Clock Drive V =2.7-5.5V V =4.5-5.5V CC CC Symbol Parameter Min. Max. Min. Max. Units 1/t Oscillator Frequency 0 8 0 16 MHz CLCL t Clock Period 125 62.5 ns CLCL t High Time 50 25 ns CHCX t Low Time 50 25 ns CLCX t Rise Time 1.6 0.5 s CLCH t Fall Time 1.6 0.5 s CHCL Change in period from one clock t 2 2 % CLCL cycle to the next Note: All DC Characteristics contained in this datasheet are based on simulation and characterization of other AVR microcontrollers manufactured in the same process technology. ATmega16U4/32U4 [DATASHEET] 385 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

29.5 System and Reset Characteristics Table 29-3. Reset and Brown-out Detection Characteristics Symbol Parameter Condition Min. Typ. Max. Units Power-on Reset Threshold Voltage (rising) 1.4 2.3 V V POT Power-on Reset Threshold Voltage (falling)() 1.3 2.3 V V V Start Voltage to ensure internal Power-on Reset signal -0.1 +0.1 V POR CC V V Rise Rate to ensure internal Power_on Reset signal 0.3 V/ms CCRR CC V RESET Pin Threshold Voltage 0.2V 0.85V V RST CC CC t Minimum pulse width on RESET Pin 5V, 25°C 400 ns RST The Power-on Reset will not work unless the supply voltage has been below V (falling). POT 29.6 Maximum speed vs. V CC Maximum frequency is depending on V As shown in Figure 29-2 on page 386, the Maximum Frequency vs. CC. V curve is linear between 2.7V < V < 5.5V. CC CC Figure 29-2. Maximum Frequency vs. V CC 16 MHz 8 MHz Safe Operating Area 2.7V 4.5V 5.5V 29.7 2-wire Serial Interface Characteristics The following table describes the requirements for devices connected to the 2-wire Serial Bus. The device 2-wire Serial Interface meets or exceeds these requirements under the noted conditions. Timing symbols refer to Figure 29-3 on page 388. ATmega16U4/32U4 [DATASHEET] 386 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 29-4. 2-wire Serial Bus Requirements Symbol Parameter Condition Min. Max. Units Input Low-voltage -0.5 0.3 V VIL CC Input High-voltage 0.7V V + 0.5 VIH CC CC V (1) Hysteresis of Schmitt Trigger Inputs 0.05V (2) – Vhys CC (1) Output Low-voltage 3mA sink current 0 0.4 VOL (1) Rise Time for both SDA and SCL 20 + 0.1C (3)(2) 300 tr b (1) Output Fall Time from V to V 10pF < C < 400pF(3) 20 + 0.1C (3)(2) 250 ns tof IHmin ILmax b b (1) Spikes Suppressed by Input Filter 0 50(2) tSP I Input Current each I/O Pin 0.1V < V < 0.9V -10 10 µA i CC i CC C(1) Capacitance for each I/O Pin – 10 pF i f SCL Clock Frequency f (4) > max(16f , 250kHz)(5) 0 400 kHz SCL CK SCL fSCL  100kHz V----C----C----–-----0---.--4---V--- 1----0---0----0---n---s-- 3mA C b Rp Value of Pull-up resistor  fSCL > 100kHz VCC–0.4V 300ns ---------------------------- ---------------- 3mA C b f  100kHz 4.0 – SCL t Hold Time (repeated) START Condition HD;STA f > 100kHz 0.6 – SCL µs f  100kHz(6) 4.7 – SCL t Low Period of the SCL Clock LOW f > 100kHz(7) 1.3 – SCL f  100kHz 4.0 – SCL t High period of the SCL clock HIGH f > 100kHz 0.6 – SCL f  100kHz 4.7 – SCL t Set-up time for a repeated START condition µs SU;STA f > 100kHz 0.6 – SCL f  100kHz 0 3.45 SCL t Data hold time HD;DAT f > 100kHz 0 0.9 SCL f  100kHz 250 – SCL t Data setup time ns SU;DAT f > 100kHz 100 – SCL f  100kHz 4.0 – SCL t Setup time for STOP condition SU;STO f > 100kHz 0.6 – SCL µs f  100kHz 4.7 – Bus free time between a STOP and START SCL t BUF condition f > 100kHz 1.3 – SCL Notes: 1. In ATmega16U4/ATmega32U4, this parameter is characterized and not 100% tested. 2. Required only for f > 100kHz. SCL 3. C = capacitance of one bus line in pF. b ATmega16U4/32U4 [DATASHEET] 387 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

4. f = CPU clock frequency. CK 5. This requirement applies to all ATmega16U4/ATmega32U4 2-wire Serial Interface operation. Other devices connected to the 2-wire Serial Bus need only obey the general f requirement. SCL 6. The actual low period generated by the ATmega16U4/ATmega32U4 2-wire Serial Interface is (1/f - 2/f ), thus f SCL CK CK must be greater than 6MHz for the low time requirement to be strictly met at f = 100kHz. SCL 7. The actual low period generated by the ATmega16U4/ATmega32U4 2-wire Serial Interface is (1/f - 2/f ), thus the low SCL CK time requirement will not be strictly met for f > 308kHz when f = 8MHz. Still, ATmega16U4/ATmega32U4 devices SCL CK connected to the bus may communicate at full speed (400kHz) with other ATmega16U4/ATmega32U4 devices, as well as any other device with a proper t acceptance margin. LOW Figure 29-3. 2-wire Serial Bus Timing tof tHIGH tr tLOW tLOW SCL tSU;STA tHD;STA tHD;DAT tSU;DAT tSU;STO SDA tBUF 29.8 SPI Timing Characteristics See Figure 29-4 and Figure 29-5 on page 389 for details. Table 29-5. SPI Timing Parameters Description Mode Min. Typ. Max. 1 SCK period Master See Table 17-2 2 SCK high/low Master 50% duty cycle 3 Rise/Fall time Master 1600 4 Setup Master 10 5 Hold Master 10 6 Out to SCK Master 0.5 • t sck 7 SCK to out Master 10 8 SCK to out high Master 10 9 SS low to out Slave 15 ns 10 SCK period Slave 4 • t ck 11 SCK high/low(1) Slave 2 • t ck 12 Rise/Fall time Slave 1600 13 Setup Slave 10 14 Hold Slave t ck 15 SCK to out Slave 15 16 SCK to SS high Slave 20 17 SS high to tri-state Slave 10 18 SS low to SCK Slave 20 Note: 1. In SPI Programming mode the minimum SCK high/low period is: - 2 t for f < 12MHz CLCL CK - 3 t for f > 12MHz CLCL CK ATmega16U4/32U4 [DATASHEET] 388 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 29-4. SPI Interface Timing Requirements (Master Mode) SS 6 1 SCK (CPOL = 0) 2 2 SCK (CPOL = 1) 4 5 3 MISO MSB ... LSB (Data Input) 7 8 MOSI MSB ... LSB (Data Output) Figure 29-5. SPI Interface Timing Requirements (Slave Mode) SS 10 16 9 SCK (CPOL = 0) 11 11 SCK (CPOL = 1) 13 14 12 MOSI MSB ... LSB (Data Input) 15 17 MISO MSB ... LSB X (Data Output) 29.9 Hardware Boot Entrance Timing Characteristics Figure 29-6. Hardware Boot Timing Requirements RESET t t SHRH HHRH ALE/HWB Table 29-6. Hardware Boot Timings Symbol Parameter Min. Max. HWB low Setup before Reset High 0 tSHRH StartUpTime(SUT) + HWB low Hold after Reset High tHHRH Time Out Delay(TOUT) ATmega16U4/32U4 [DATASHEET] 389 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 29-7. ADC Characteristics Symbol Parameter Condition Min. Typ. Max. Units Single Ended Conversion 10 Resolution Differential conversion, gain = 1x/10x/40x 8 Bits Differential conversion, gain = 200x 8 V = 4V, V = 4V, ADC clock = 200kHz 2.0 3.0 REF CC Gain = 1x/10x/40x, V = 4V, V = 5V, REF CC 2.0 3.0 TUE Absolute accuracy ADC clock = 200 kHz LSB Gain = 200x, V = 4V, V = 5V, REF CC 2.0 4.0 ADC clock = 200kHz V = 4V, V = 4V, ADC clock = 200kHz 0.5 1.5 REF CC Gain = 1x/10x/40x, V = 4V, V = 5V, REF CC 0.3 1.5 INL Integral Non-Linearity ADC clock = 200kHz LSB Gain = 200x, V = 4V, V = 5V, REF CC 0.5 1.5 ADC clock = 200kHz V = 4V, V = 4V, ADC clock = 200kHz 0.4 0.7 REF CC Gain = 1x/10x/40x, V = 4V, V = 5V, REF CC 0.3 1.0 DNL Differential Non-Linearity ADC clock = 200kHz LSB Gain = 200x, V = 4V, V = 5V, REF CC 0.6 1.0 ADC clock = 200kHz V = 4V, V = 4V, ADC clock = 200kHz -2.5 -1.0 2.5 REF CC Gain = 1x/10x/40x, V = 4V, V = 5V, REF CC 0.0 -1.5 -2.5 Gain Error ADC clock = 200kHz LSB Gain = 200x, V = 4V, V = 5V, REF CC 0.0 -1.8 -3.0 ADC clock = 200kHz V = 4V, V = 4V, ADC clock = 200kHz -2.5 1.5 2.5 REF CC Offset Error LSB V = 4V, V = 5V, ADC clock = 200kHz, REF CC -2.0 0.0 2.0 Differential mode Single Ended Conversion 2.56 AVCC V Reference Voltage V REF Differential Conversion 2.56 AV - 0.5 CC V - AVCC Analog Supply Voltage CC V + 0.3 V 0.3 CC Single ended channels GND V REF V Input Voltage V IN Differential Conversion 0 AV CC Single Ended Channels 38.5 Input Bandwidth kHz Differential Channels 4 ATmega16U4/32U4 [DATASHEET] 390 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table 29-7. ADC Characteristics Symbol Parameter Condition Min. Typ. Max. Units Internal Voltage V 2.56V 2.4 2.56 2.8 V INT Reference Reference Input R 32 k REF Resistance R Analog Input Resistance 100 M AIN ATmega16U4/32U4 [DATASHEET] 391 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

30. Typical Characteristics The following charts show typical behavior. These figures are not tested during manufacturing. All current con- sumption measurements are performed with all I/O pins configured as inputs and with internal pull-ups enabled. A sine wave generator with rail-to-rail output is used as clock source. All Active- and Idle current consumption measurements are done with all bits in the PRR registers set and thus, the corresponding I/O modules are turned off. Also the Analog Comparator is disabled during these measure- ments. See “Power Reduction Register” on page45 for details. The power consumption in Power-down mode is independent of clock selection. The current consumption is a function of several factors such as: operating voltage, operating frequency, load- ing of I/O pins, switching rate of I/O pins, code executed and ambient temperature. The dominating factors are operating voltage and frequency. The current drawn from capacitive loaded pins may be estimated (for one pin) as C *V *f where C = load L CC L capacitance, V = operating voltage and f = average switching frequency of I/O pin. CC The parts are characterized at frequencies higher than test limits. Parts are not guaranteed to function properly at frequencies higher than the ordering code indicates. The difference between current consumption in Power-down mode with Watchdog Timer enabled and Power- down mode with Watchdog Timer disabled represents the differential current drawn by the Watchdog Timer. 30.1 Active Supply Current Figure 30-1. Active Supply Current vs. Low Frequency (1MHz) and T= 25°C 1.6 1.4 5.5V 5.0V 1.2 4.5V 1 4.0V mA) 0.8 3.6V ( CC I 0.6 2.7V 0.4 0.2 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Frequency (MHz) ATmega16U4/32U4 [DATASHEET] 392 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-2. Active Supply Current vs. Low Frequency (1MHz) and T=85°C 2.3 5.5V 2.1 1.9 5.0V 4.5V 1.7 A) 4.0V m 1.5 ( 3.6V CC I 1.3 2.7V 1.1 0.9 0.7 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Frequency (MHz) Figure 30-3. Active Supply Current vs. Frequency (1 - 16MHz) and T=-40°C 18 16 5.5V 14 5.0V 12 4.5V A) 10 m 4.0V I (CC 8 3.6V 6 2.7V 4 2 0 2 4 6 8 10 12 14 16 Frequency (MHz) ATmega16U4/32U4 [DATASHEET] 393 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-4. Active Supply Current vs. Frequency (1 - 16MHz) and T = 25°C 16 5.5V 14 5.0V 12 4.5V 10 A) 4.0V m 8 (C C 3.6V I 6 4 2.7V 2 0 2 4 6 8 10 12 14 16 Frequency (MHz) Figure 30-5. Active Supply Current vs. Frequency (1 - 16MHz) and T = 85°C 16 5.5V 14 5.0V 12 4.5V 10 A) 4.0V m 8 (C IC 3.6V 6 4 2.7V 2 0 2 4 6 8 10 12 14 16 Frequency (MHz) ATmega16U4/32U4 [DATASHEET] 394 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

30.2 Idle Supply Current Figure 30-6. Idle Supply Current vs. Low Frequency (1MHz) and T = 25°C 0.5 0.45 5.5V 0.4 5.0V 0.35 4.5V 0.3 4.0V A) 3.6V m 0.25 ( CC I 0.2 2.7V 0.15 0.1 0.05 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Frequency (MHz) Figure 30-7. Idle Supply Current vs. Low Frequency (1MHz) and T = 85°C 0.5 5.5V 0.45 5.0V 0.4 4.5V 0.35 4.0V 0.3 A) 3.6V m 0.25 ( ICC 0.2 2.7V 0.15 0.1 0.05 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Frequency (MHz) ATmega16U4/32U4 [DATASHEET] 395 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-8. Idle Supply Current vs. Frequency (1 - 16MHz) T = 25°C 7 5.5V 6 5.0V 5 4.5V A) 4 4.0V m (C C 3 I 3.6V 2 3.3V 2.7V 1 0 2 4 6 8 10 12 14 16 Frequency (MHz) Figure 30-9. Idle Supply Current vs. Frequency (1 - 16MHz) T = 85°C 7 5.5V 6 5.0V 5 4.5V mA) 4 4.0V (C C 3 I 3.6V 2 2.7V 1 0 2 4 6 8 10 12 14 16 Frequency (MHz) ATmega16U4/32U4 [DATASHEET] 396 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

30.3 Power-down Supply Current Figure 30-10. Power-Down Supply Current vs. V (WDT Disabled) CC 3.5 85°C 3 2.5 2 A) µ ( C C 1.5 I 1 25°C 0.5 -40°C 0 2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5 V (V) CC Figure 30-11. Power-Down Supply Current vs. V (WDT Enabled) CC 24 85°C 22 20 18 16 25°C A) µ 14 ( C C I 12 -40°C 10 8 6 4 2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5 V (V) CC ATmega16U4/32U4 [DATASHEET] 397 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-12. Power-down Supply Current vs. V (WDT Enabled, BOD EN) CC 48 85°C 45 42 39 25°C A) 36 µ ( -40°C CC 33 I 30 27 24 21 2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5 V (V) CC 30.4 Power-save Supply Current Figure 30-13. Power-save Supply Current vs. V (WDT Disabled) CC 200 -40°C 185 25°C 170 85°C 155 140 A) µ 125 ( C C I 110 95 80 65 50 2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5 V (V) CC ATmega16U4/32U4 [DATASHEET] 398 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

30.5 Pin Pull-Up Figure 30-14. I/O Pin Pull-up Resistor Current vs. Input Voltage (V = 2.7V) CC 80 70 60 50 A) µ 40 (OP I 30 20 25°C 10 -40°C 85°C 0 0 0.5 1 1.5 2 2.5 3 V (V) OP Figure 30-15. I/O Pin Pull-up Resistor Current vs. Input Voltage (V = 5V) CC 140 120 100 80 A) µ (OP 60 I 40 25°C 20 85°C -40°C 0 0 1 2 3 4 5 V (V) OP ATmega16U4/32U4 [DATASHEET] 399 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-16. Reset Pull-up Resistor Current vs. Reset Pin Voltage (V = 5V) CC 120 100 80 A) µ ( 60 RESET I 40 20 25°C 85°C -40°C 0 0 1 2 3 4 5 V (V) RESET 30.6 Pin Driver Strength Figure 30-17. I/O Pin Output Voltage vs. Sink Current (V = 3V) CC 4 85°C 3.5 3 2.5 V) (OL 2 V 1.5 25°C 1 -40°C 0.5 0 0 2 4 6 8 10 12 14 16 18 20 I (mA) OL ATmega16U4/32U4 [DATASHEET] 400 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-18. I/O Pin Output Voltage vs. Sink Current (V = 5V) CC 1 85°C 0.9 0.8 25°C 0.7 0.6 -40°C V) (OL 0.5 V 0.4 0.3 0.2 0.1 0 0 2 4 6 8 10 12 14 16 18 20 I (mA) OL Figure 30-19. I/O Pin Output Voltage vs. Source Current (V = 3V) CC 3.5 3 2.5 2 V) -40°C (H O V 1.5 25°C 1 0.5 85°C 0 0 2 4 6 8 10 12 14 16 18 20 I (mA) OH ATmega16U4/32U4 [DATASHEET] 401 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-20. I/O Pin Output Voltage vs. Source Current (V = 5V) CC 5.1 4.9 4.7 V) (H 4.5 O V -40°C 4.3 25°C 4.1 85°C 3.9 0 2 4 6 8 10 12 14 16 18 20 I (mA) OH Figure 30-21. USB DP LO Pull-Up Resistor Current vs. USB Pin Voltage 2800 2400 2000 A) 1600 µ ( USB 1200 I 800 85°C 400 25°C -40°C 0 0 0.5 1 1.5 2 2.5 3 3.5 V (V) USB ATmega16U4/32U4 [DATASHEET] 402 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

30.7 Pin Threshold and Hysteresis Figure 30-22. I/O Pin Input Threshold Voltage vs. V (V , IO Pin read as ‘1’) CC IH 1.8 -40°C 25°C 1.6 85°C 1.4 V) d ( ol 1.2 h s e hr T 1 0.8 0.6 2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5 V (V) CC Figure 30-23. I/O Pin Input Threshold Voltage vs. V (V , IO Pin read as ‘0’) CC IL 1.8 -40°C 25°C 1.6 85°C 1.4 V) d ( ol 1.2 h s e hr T 1 0.8 0.6 2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5 V (V) CC ATmega16U4/32U4 [DATASHEET] 403 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-24. USB Pin Input Threshold Voltage vs. V (V , IO Pin read as ‘1’) CC IH 2 85°C 25°C 1.9 1.8 -40°C 1.7 V) d ( ol 1.6 h s e Thr 1.5 1.4 1.3 1.2 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 V (V) CC Figure 30-25. USB Pin Input Threshold Voltage vs. V (V , I/O Pin read as ‘0’) CC IL 1.6 -40°C 85°C 1.5 25°C 1.4 d (V) 1.3 ol h es 1.2 hr T 1.1 1 0.9 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 V (V) CC ATmega16U4/32U4 [DATASHEET] 404 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-26. Vbus Pin Input Threshold Voltage vs. V (V , IO Pin read as ‘1’) CC IH 4.6 4.58 85°C 4.56 25°C 4.54 4.52 V) d ( 4.5 ol sh 4.48 e hr T 4.46 4.44 4.42 4.4 -40°C 4.38 2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5 V (V) CC Figure 30-27. Vbus Pin Input Threshold Voltage vs. V (V , I/O Pin read as ‘0’) CC IL 1.52 85°C 25°C 1.51 1.5 V) d ( ol 1.49 h s e hr T 1.48 1.47 -40°C 1.46 2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5 V (V) CC ATmega16U4/32U4 [DATASHEET] 405 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

30.8 BOD Threshold Figure 30-28. BOD Thresholds vs. Temperature (BODLEVEL is 2.6V) 2.8 Rising Vcc 2.78 2.76 V) 2.74 d ( Falling Vcc hol 2.72 s e hr T 2.7 2.68 2.66 2.64 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 Temperature (°C) Figure 30-29. BOD Thresholds vs. Temperature (BODLEVEL is 3.5V) 3.73 Rising Vcc 3.69 Falling Vcc 3.65 V) d ( 3.61 ol h s e 3.57 hr T 3.53 3.49 3.45 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 Temperature (°C) ATmega16U4/32U4 [DATASHEET] 406 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-30. BOD Thresholds vs. Temperature (BODLEVEL is 4.3V) 4.6 4.55 Rising Vcc Falling Vcc 4.5 4.45 V) 4.4 d ( hol 4.35 s e hr 4.3 T 4.25 4.2 4.15 4.1 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 Temperature (°C) Figure 30-31. Bandgap Voltage vs. V CC 1.11 1.1 V) 1.09 85°C e ( g 25°C a olt 1.08 V p a g d n 1.07 a B 1.06 -40°C 1.05 1.5 2 2.5 3 3.5 4 4.5 5 5.5 Vcc (V) ATmega16U4/32U4 [DATASHEET] 407 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-32. Bandgap Voltage vs. Temperature 1.11 1.9V 1.1 3.0V 4.5V V) 1.09 5.0V e ( 5.5V g a olt 1.08 V p a g d n 1.07 a B 1.06 1.05 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 Temperature (°C) 30.9 Internal Oscillator Speed Figure 30-33. Watchdog Oscillator Frequency vs. Temperature 124 122 120 z) H k 118 ( C R F 1.9V 116 3.0V 114 4.0V 4.5V 5.5V 112 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 Temperature (°C) ATmega16U4/32U4 [DATASHEET] 408 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-34. Watchdog Oscillator Frequency vs. V CC 124 122 -40°C 120 25°C z) 118 H k ( RC 116 F 114 85°C 112 110 1.5 2 2.5 3 3.5 4 4.5 5 5.5 V (V) CC Figure 30-35. Calibrated 8MHz RC Oscillator Frequency vs. Osccal Value 14 85°C 25°C 12 -40°C 10 z) 8 H M ( RC 6 F 4 2 0 0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240 256 OSCCAL (X1) ATmega16U4/32U4 [DATASHEET] 409 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-36. Calibrated 8MHz RC Oscillator Frequency vs. Temperature 8.4 2.7V 4.0V 8.3 5.5V 8.2 z) 8.1 H M ( RC 8 F 7.9 7.8 7.7 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 Temperature (°C) Figure 30-37. Calibrated 8MHz RC Oscillator Frequency vs. Operating Voltage 8.4 8.3 85°C 8.2 8.1 z) H M 8 ( 25°C C R F 7.9 7.8 -40°C 7.7 7.6 2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5 V (V) CC ATmega16U4/32U4 [DATASHEET] 410 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-38. OSCCAL VALUE STEP SIZE IN% (Base frequency = 0.0MHz) 1.4 1.2 1 %) e ( 0.8 g n a h c 0.6 RC F 0.4 85°C -40°C 0.2 25°C 0 0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240 256 OSCCAL (X1) 30.10 Current Consumption of Peripheral Units Figure 30-39. USB Regulator Level vs. V CC 3.5 3.4 -40°C 85°C 3.3 25°C 3.2 A) µ 3.1 (CC I 3 2.9 2.8 2.7 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5 V (V) CC ATmega16U4/32U4 [DATASHEET] 411 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-40. USB Regulator Level with load 75 vs. V CC 3.4 -40°C 3.3 85°C 25°C 3.2 3.1 A) µ 3 nt ( urre 2.9 C 2.8 2.7 2.6 2.5 2.8 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5 V (V) CC Figure 30-41. ADC Internal V vs. V REF CC 2.54 2.53 2.52 2.51 V) 85°C Ref ( 2.5 25°C nt VI e 2.49 g a olt 2.48 V 2.47 2.46 -40°C 2.45 2.6 2.9 3.2 3.5 3.8 4.1 4.4 4.7 5 5.3 5.6 Voltage (V) ATmega16U4/32U4 [DATASHEET] 412 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Figure 30-42. Internal Reference Voltage vs. Sink Current 2.52 85°C 2.51 25°C 2.5 2.49 V) Ref ( 2.48 nt VI e 2.47 g a olt 2.46 -40°C V 2.45 2.44 2.43 -7 -6 -5 -4 -3 -2 -1 0 Sink current (mA) 30.11 Current Consumption in Reset and Reset Pulse Width Figure 30-43. Reset Supply Current vs. Frequency (1 - 20MHz) 3.5 5.5V 3 5.0V 4.5V 2.5 A) 2 4.0V m (C 3.6V C 1.5 I 1 2.7V 0.5 0 2 4 6 8 10 12 14 16 Frequency (MHz) ATmega16U4/32U4 [DATASHEET] 413 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

31. Register Summary Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Page (0xFF) Reserved - - - - - - - - (0xFE) Reserved - - - - - - - - (0xFD) Reserved - - - - - - - - (0xFC) Reserved - - - - - - - - (0xFB) Reserved - - - - - - - - (0xFA) Reserved - - - - - - - - (0xF9) Reserved - - - - (0xF8) Reserved - - - - - - - - (0xF7) Reserved - - - - - - - - (0xF6) Reserved - - - - - - - - (0xF5) Reserved - - - - - - - - (0xF4) UEINT - EPINT6:0 (0xF3) UEBCHX - - - - - BYCT10:8 (0xF2) UEBCLX BYCT7:0 (0xF1) UEDATX DAT7:0 (0xF0) UEIENX FLERRE NAKINE - NAKOUTE RXSTPE RXOUTE STALLEDE TXINE (0xEF) UESTA1X - - - - - CTRLDIR CURRBK1:0 (0xEE) UESTA0X CFGOK OVERFI UNDERFI - DTSEQ1:0 NBUSYBK1:0 (0xED) UECFG1X EPSIZE2:0 EPBK1:0 ALLOC - (0xEC) UECFG0X EPTYPE1:0 - - - - - EPDIR (0xEB) UECONX - - STALLRQ STALLRQC RSTDT - - EPEN (0xEA) UERST - EPRST6:0 (0xE9) UENUM - - - - - EPNUM2:0 (0xE8) UEINTX FIFOCON NAKINI RWAL NAKOUTI RXSTPI RXOUTI STALLEDI TXINI (0xE7) Reserved - - - - (0xE6) UDMFN - - - FNCERR - - - - (0xE5) UDFNUMH - - - - - FNUM10:8 (0xE4) UDFNUML FNUM7:0 (0xE3) UDADDR ADDEN UADD6:0 (0xE2) UDIEN - UPRSME EORSME WAKEUPE EORSTE SOFE MSOFE SUSPE (0xE1) UDINT - UPRSMI EORSMI WAKEUPI EORSTI SOFI MSOFI SUSPI (0xE0) UDCON - - - - RSTCPU LSM RMWKUP DETACH (0xDF) Reserved (0xDE) Reserved (0xDD) Reserved (0xDC) Reserved (0xDB) Reserved (0xDA) USBINT - - - - - - - VBUSTI (0xD9) USBSTA - - - - - - ID VBUS (0xD8) USBCON USBE - FRZCLK OTGPADE - - - VBUSTE (0xD7) UHWCON - - - - - - - UVREGE (0xD6) Reserved (0xD5) Reserved (0xD4) DT4 DT4H3 DT4H2 DT4H1 DT4H0 DT4L3 DT4L2 DT4L1 DT4L0 (0xD3) Reserved (0xD2) OCR4D Timer/Counter4 - Output Compare Register D (0xD1) OCR4C Timer/Counter4 - Output Compare Register C (0xD0) OCR4B Timer/Counter4 - Output Compare Register B (0xCF) OCR4A Timer/Counter4 - Output Compare Register A (0xCE) UDR1 USART1 I/O Data Register (0xCD) UBRR1H - - - - USART1 Baud Rate Register High Byte (0xCC) UBRR1L USART1 Baud Rate Register Low Byte (0xCB) UCSR1D - - - - - - CTSEN RTSEN (0xCA) UCSR1C UMSEL11 UMSEL10 UPM11 UPM10 USBS1 UCSZ11 UCSZ10 UCPOL1 (0xC9) UCSR1B RXCIE1 TXCIE1 UDRIE1 RXEN1 TXEN1 UCSZ12 RXB81 TXB81 (0xC8) UCSR1A RXC1 TXC1 UDRE1 FE1 DOR1 PE1 U2X1 MPCM1 (0xC7) CLKSTA - - - - - - RCON EXTON (0xC6) CLKSEL1 RCCKSEL3 RCCKSEL2 RCCKSEL1 RCCKSEL0 EXCKSEL3 EXCKSEL2 EXCKSEL1 EXCKSEL0 (0xC5) CLKSEL0 RCSUT1 RCSUT0 EXSUT1 EXSUT0 RCE EXTE - CLKS (0xC4) TCCR4E TLOCK4 ENHC4 OC4OE5 OC4OE4 OC4OE3 OC4OE2 OC4OE1 OC4OE0 (0xC3) TCCR4D FPIE4 FPEN4 FPNC4 FPES4 FPAC4 FPF4 WGM41 WGM40 (0xC2) TCCR4C COM4A1S COM4A0S COM4B1S COM4B0S COM4D1S COM4D0S FOC4D PWM4D (0xC1) TCCR4B PWM4X PSR4 DTPS41 DTPS40 CS43 CS42 CS41 CS40 (0xC0) TCCR4A COM4A1 COM4A0 COM4B1 COM4B0 FOC4A FOC4B PWM4A PWM4B (0xBF) TC4H - - - - - Timer/Counter4 High Byte ATmega16U4/32U4 [DATASHEET SUMMARY] 414 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Page (0xBE) TCNT4 Timer/Counter4 - Counter Register Low Byte (0xBD) TWAMR TWAM6 TWAM5 TWAM4 TWAM3 TWAM2 TWAM1 TWAM0 - (0xBC) TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN - TWIE (0xBB) TWDR 2-wire Serial Interface Data Register (0xBA) TWAR TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE (0xB9) TWSR TWS7 TWS6 TWS5 TWS4 TWS3 - TWPS1 TWPS0 (0xB8) TWBR 2-wire Serial Interface Bit Rate Register (0xB7) Reserved - - - - - - - - (0xB6) Reserved - (0xB5) Reserved - - - - - - - - (0xB4) Reserved - - - - - - - - (0xB3) Reserved - - - - - - - - (0xB2) Reserved - - - - - - - - (0xB1) Reserved - - - - - - - - (0xB0) Reserved - - - - - - - - (0xAF) Reserved - - - - - - - - (0xAE) Reserved - - - - - - - - (0xAD) Reserved - - - - - - - - (0xAC) Reserved - - - - - - - - (0xAB) Reserved - - - - - - - - (0xAA) Reserved - - - - - - - - (0xA9) Reserved - - - - - - - - (0xA8) Reserved - - - - - - - - (0xA7) Reserved - - - - - - - - (0xA6) Reserved - - - - - - - - (0xA5) Reserved - - - - - - - - (0xA4) Reserved - - - - - - - - (0xA3) Reserved - - - - - - - - (0xA2) Reserved - - - - - - - - (0xA1) Reserved - - - - - - - - (0xA0) Reserved - - - - - - - - (0x9F) Reserved - - - - - - - - (0x9E) Reserved - - - - - - - - (0x9D) OCR3CH Timer/Counter3 - Output Compare Register C High Byte (0x9C) OCR3CL Timer/Counter3 - Output Compare Register C Low Byte (0x9B) OCR3BH Timer/Counter3 - Output Compare Register B High Byte (0x9A) OCR3BL Timer/Counter3 - Output Compare Register B Low Byte (0x99) OCR3AH Timer/Counter3 - Output Compare Register A High Byte (0x98) OCR3AL Timer/Counter3 - Output Compare Register A Low Byte (0x97) ICR3H Timer/Counter3 - Input Capture Register High Byte (0x96) ICR3L Timer/Counter3 - Input Capture Register Low Byte (0x95) TCNT3H Timer/Counter3 - Counter Register High Byte (0x94) TCNT3L Timer/Counter3 - Counter Register Low Byte (0x93) Reserved - - - - - - - - (0x92) TCCR3C FOC3A - - - - - - - (0x91) TCCR3B ICNC3 ICES3 - WGM33 WGM32 CS32 CS31 CS30 (0x90) TCCR3A COM3A1 COM3A0 COM3B1 COM3B0 COM3C1 COM3C0 WGM31 WGM30 (0x8F) Reserved - - - - - - - - (0x8E) Reserved - - - - - - - - (0x8D) OCR1CH Timer/Counter1 - Output Compare Register C High Byte (0x8C) OCR1CL Timer/Counter1 - Output Compare Register C Low Byte (0x8B) OCR1BH Timer/Counter1 - Output Compare Register B High Byte (0x8A) OCR1BL Timer/Counter1 - Output Compare Register B Low Byte (0x89) OCR1AH Timer/Counter1 - Output Compare Register A High Byte (0x88) OCR1AL Timer/Counter1 - Output Compare Register A Low Byte (0x87) ICR1H Timer/Counter1 - Input Capture Register High Byte (0x86) ICR1L Timer/Counter1 - Input Capture Register Low Byte (0x85) TCNT1H Timer/Counter1 - Counter Register High Byte (0x84) TCNT1L Timer/Counter1 - Counter Register Low Byte (0x83) Reserved - - - - - - - - (0x82) TCCR1C FOC1A FOC1B FOC1C - - - - - (0x81) TCCR1B ICNC1 ICES1 - WGM13 WGM12 CS12 CS11 CS10 (0x80) TCCR1A COM1A1 COM1A0 COM1B1 COM1B0 COM1C1 COM1C0 WGM11 WGM10 (0x7F) DIDR1 - - - - - - - AIN0D (0x7E) DIDR0 ADC7D ADC6D ADC5D ADC4D - - ADC1D ADC0D (0x7D) DIDR2 - - ADC13D ADC12D ADC11D ADC10D ADC9D ADC8D (0x7C) ADMUX REFS1 REFS0 ADLAR MUX4 MUX3 MUX2 MUX1 MUX0 (0x7B) ADCSRB ADHSM ACME MUX5 - ADTS3 ADTS2 ADTS1 ADTS0 ATmega16U4/32U4 [DATASHEET SUMMARY] 415 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Page (0x7A) ADCSRA ADEN ADSC ADATE ADIF ADIE ADPS2 ADPS1 ADPS0 (0x79) ADCH ADC Data Register High byte (0x78) ADCL ADC Data Register Low byte (0x77) Reserved - - - - - - - - (0x76) Reserved - - - - - - - - (0x75) Reserved - - - - - - - - (0x74) Reserved - - - - - - - - (0x73) Reserved - - - - - - - - (0x72) TIMSK4 OCIE4D OCIE4A OCIE4B - - TOIE4 - - (0x71) TIMSK3 - - ICIE3 - OCIE3C OCIE3B OCIE3A TOIE3 (0x70) Reserved - - - - - - - - (0x6F) TIMSK1 - - ICIE1 - OCIE1C OCIE1B OCIE1A TOIE1 (0x6E) TIMSK0 - - - - - OCIE0B OCIE0A TOIE0 (0x6D) Reserved - - - - - - - - (0x6C) Reserved - - - - - - - - (0x6B) PCMSK0 PCINT7 PCINT6 PCINT5 PCINT4 PCINT3 PCINT2 PCINT1 PCINT0 (0x6A) EICRB - - ISC61 ISC60 - - - - (0x69) EICRA ISC31 ISC30 ISC21 ISC20 ISC11 ISC10 ISC01 ISC00 (0x68) PCICR - - - - - - - PCIE0 (0x67) RCCTRL - - - - - - - RCFREQ (0x66) OSCCAL RC Oscillator Calibration Register (0x65) PRR1 PRUSB - - PRTIM4 PRTIM3 - - PRUSART1 (0x64) PRR0 PRTWI - PRTIM0 - PRTIM1 PRSPI - PRADC (0x63) Reserved - - - - - - - - (0x62) Reserved - - - - - - - - (0x61) CLKPR CLKPCE - - - CLKPS3 CLKPS2 CLKPS1 CLKPS0 (0x60) WDTCSR WDIF WDIE WDP3 WDCE WDE WDP2 WDP1 WDP0 0x3F (0x5F) SREG I T H S V N Z C 0x3E (0x5E) SPH SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8 0x3D (0x5D) SPL SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0 0x3C (0x5C) Reserved - - - - - - - - 0x3B (0x5B) RAMPZ - - - - - - RAMPZ1 RAMPZ0 0x3A (0x5A) Reserved - - - - - - - - 0x39 (0x59) Reserved - - - - - - - - 0x38 (0x58) Reserved - - - - - - - - 0x37 (0x57) SPMCSR SPMIE RWWSB SIGRD RWWSRE BLBSET PGWRT PGERS SPMEN 0x36 (0x56) Reserved - - - - - - - - 0x35 (0x55) MCUCR JTD - - PUD - - IVSEL IVCE 0x34 (0x54) MCUSR - - USBRF JTRF WDRF BORF EXTRF PORF 0x33 (0x53) SMCR - - - - SM2 SM1 SM0 SE 0x32 (0x52) PLLFRQ PINMUX PLLUSB PLLTM1 PLLTM0 PDIV3 PDIV2 PDIV1 PDIV0 OCDR/ OCDR7 OCDR6 OCDR5 OCDR4 OCDR3 OCDR2 OCDR1 OCDR0 0x31 (0x51) MONDR Monitor Data Register 0x30 (0x50) ACSR ACD ACBG ACO ACI ACIE ACIC ACIS1 ACIS0 0x2F (0x4F) Reserved - - - - - - - - 0x2E (0x4E) SPDR SPI Data Register 0x2D (0x4D) SPSR SPIF WCOL - - - - - SPI2X 0x2C (0x4C) SPCR SPIE SPE DORD MSTR CPOL CPHA SPR1 SPR0 0x2B (0x4B) GPIOR2 General Purpose I/O Register 2 0x2A (0x4A) GPIOR1 General Purpose I/O Register 1 0x29 (0x49) PLLCSR - - - PINDIV - - PLLE PLOCK 0x28 (0x48) OCR0B Timer/Counter0 Output Compare Register B 0x27 (0x47) OCR0A Timer/Counter0 Output Compare Register A 0x26 (0x46) TCNT0 Timer/Counter0 (8 Bit) 0x25 (0x45) TCCR0B FOC0A FOC0B - - WGM02 CS02 CS01 CS00 0x24 (0x44) TCCR0A COM0A1 COM0A0 COM0B1 COM0B0 - - WGM01 WGM00 0x23 (0x43) GTCCR TSM - - - - - PSRASY PSRSYNC 0x22 (0x42) EEARH - - - - EEPROM Address Register High Byte 0x21 (0x41) EEARL EEPROM Address Register Low Byte 0x20 (0x40) EEDR EEPROM Data Register 0x1F (0x3F) EECR - - EEPM1 EEPM0 EERIE EEMPE EEPE EERE 0x1E (0x3E) GPIOR0 General Purpose I/O Register 0 0x1D (0x3D) EIMSK - INT6 - - INT3 INT2 INT1 INT0 0x1C (0x3C) EIFR - INTF6 - - INTF3 INTF2 INTF1 INTF0 0x1B (0x3B) PCIFR - - - - - - - PCIF0 0x1A (0x3A) Reserved - - - - - - - - 0x19 (0x39) TIFR4 OCF4D OCF4A OCF4B - - TOV4 - - 0x18 (0x38) TIFR3 - - ICF3 - OCF3C OCF3B OCF3A TOV3 ATmega16U4/32U4 [DATASHEET SUMMARY] 416 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Page 0x17 (0x37) Reserved - - - - - - - - 0x16 (0x36) TIFR1 - - ICF1 - OCF1C OCF1B OCF1A TOV1 0x15 (0x35) TIFR0 - - - - - OCF0B OCF0A TOV0 0x14 (0x34) Reserved - - - - - - - - 0x13 (0x33) Reserved - - - - - - - - 0x12 (0x32) Reserved - - - - - - - - 0x11 (0x31) PORTF PORTF7 PORTF6 PORTF5 PORTF4 - - PORTF1 PORTF0 0x10 (0x30) DDRF DDF7 DDF6 DDF5 DDF4 - - DDF1 DDF0 0x0F (0x2F) PINF PINF7 PINF6 PINF5 PINF4 - - PINF1 PINF0 0x0E (0x2E) PORTE - PORTE6 - - - PORTE2 - - 0x0D (0x2D) DDRE - DDE6 - - - DDE2 - - 0x0C (0x2C) PINE - PINE6 - - - PINE2 - - 0x0B (0x2B) PORTD PORTD7 PORTD6 PORTD5 PORTD4 PORTD3 PORTD2 PORTD1 PORTD0 0x0A (0x2A) DDRD DDD7 DDD6 DDD5 DDD4 DDD3 DDD2 DDD1 DDD0 0x09 (0x29) PIND PIND7 PIND6 PIND5 PIND4 PIND3 PIND2 PIND1 PIND0 0x08 (0x28) PORTC PORTC7 PORTC6 - - - - - - 0x07 (0x27) DDRC DDC7 DDC6 - - - - - - 0x06 (0x26) PINC PINC7 PINC6 - - - - - - 0x05 (0x25) PORTB PORTB7 PORTB6 PORTB5 PORTB4 PORTB3 PORTB2 PORTB1 PORTB0 0x04 (0x24) DDRB DDB7 DDB6 DDB5 DDB4 DDB3 DDB2 DDB1 DDB0 0x03 (0x23) PINB PINB7 PINB6 PINB5 PINB4 PINB3 PINB2 PINB1 PINB0 0x02 (0x22) Reserved - - - - - - - - 0x01 (0x21) Reserved - - - - - - - - 0x00 (0x20) Reserved - - - - - - - - Note: 1. For compatibility with future devices, reserved bits should be written to zero if accessed. Reserved I/O memory addresses should never be written. 2. I/O registers within the address range $00 - $1F are directly bit-accessible using the SBI and CBI instructions. In these registers, the value of single bits can be checked by using the SBIS and SBIC instructions. 3. Some of the status flags are cleared by writing a logical one to them. Note that the CBI and SBI instructions will operate on all bits in the I/O register, writing a one back into any flag read as set, thus clearing the flag. The CBI and SBI instruc- tions work with registers 0x00 to 0x1F only. 4. When using the I/O specific commands IN and OUT, the I/O addresses $00 - $3F must be used. When addressing I/O registers as data space using LD and ST instructions, $20 must be added to these addresses. The ATmega16U4/ATmega32U4 is a complex microcontroller with more peripheral units than can be supported within the 64 location reserved in Opcode for the IN and OUT instructions. For the Extended I/O space from $60 - $1FF in SRAM, only the ST/STS/STD and LD/LDS/LDD instructions can be used. ATmega16U4/32U4 [DATASHEET SUMMARY] 417 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

32. Instruction Set Summary Mnemonics Operands Description Operation Flags #Clocks ARITHMETIC AND LOGIC INSTRUCTIONS ADD Rd, Rr Add two Registers Rd  Rd + Rr Z,C,N,V,H 1 ADC Rd, Rr Add with Carry two Registers Rd  Rd + Rr + C Z,C,N,V,H 1 ADIW Rdl,K Add Immediate to Word Rdh:Rdl  Rdh:Rdl + K Z,C,N,V,S 2 SUB Rd, Rr Subtract two Registers Rd  Rd - Rr Z,C,N,V,H 1 SUBI Rd, K Subtract Constant from Register Rd  Rd - K Z,C,N,V,H 1 SBC Rd, Rr Subtract with Carry two Registers Rd  Rd - Rr - C Z,C,N,V,H 1 SBCI Rd, K Subtract with Carry Constant from Reg. Rd  Rd - K - C Z,C,N,V,H 1 SBIW Rdl,K Subtract Immediate from Word Rdh:Rdl  Rdh:Rdl - K Z,C,N,V,S 2 AND Rd, Rr Logical AND Registers Rd Rd  Rr Z,N,V 1 ANDI Rd, K Logical AND Register and Constant Rd  Rd K Z,N,V 1 OR Rd, Rr Logical OR Registers Rd  Rd v Rr Z,N,V 1 ORI Rd, K Logical OR Register and Constant Rd Rd v K Z,N,V 1 EOR Rd, Rr Exclusive OR Registers Rd  Rd  Rr Z,N,V 1 COM Rd One’s Complement Rd  0xFF  Rd Z,C,N,V 1 NEG Rd Two’s Complement Rd  0x00  Rd Z,C,N,V,H 1 SBR Rd,K Set Bit(s) in Register Rd  Rd v K Z,N,V 1 CBR Rd,K Clear Bit(s) in Register Rd  Rd  (0xFF - K) Z,N,V 1 INC Rd Increment Rd  Rd + 1 Z,N,V 1 DEC Rd Decrement Rd  Rd  1 Z,N,V 1 TST Rd Test for Zero or Minus Rd  Rd  Rd Z,N,V 1 CLR Rd Clear Register Rd  Rd  Rd Z,N,V 1 SER Rd Set Register Rd  0xFF None 1 MUL Rd, Rr Multiply Unsigned R1:R0  Rd x Rr Z,C 2 MULS Rd, Rr Multiply Signed R1:R0  Rd x Rr Z,C 2 MULSU Rd, Rr Multiply Signed with Unsigned R1:R0  Rd x Rr Z,C 2 FMUL Rd, Rr Fractional Multiply Unsigned R1:R0  (Rd x Rr) << 1 Z,C 2 FMULS Rd, Rr Fractional Multiply Signed R1:R0  (Rd x Rr) << 1 Z,C 2 FMULSU Rd, Rr Fractional Multiply Signed with Unsigned R1:R0  (Rd x Rr) << 1 Z,C 2 BRANCH INSTRUCTIONS RJMP k Relative Jump PC PC + k + 1 None 2 IJMP Indirect Jump to (Z) PC  Z None 2 EIJMP Extended Indirect Jump to (Z) PC (EIND:Z) None 2 JMP k Direct Jump PC k None 3 RCALL k Relative Subroutine Call PC  PC + k + 1 None 4 ICALL Indirect Call to (Z) PC  Z None 4 EICALL Extended Indirect Call to (Z) PC (EIND:Z) None 4 CALL k Direct Subroutine Call PC  k None 5 RET Subroutine Return PC  STACK None 5 RETI Interrupt Return PC  STACK I 5 CPSE Rd,Rr Compare, Skip if Equal if (Rd = Rr) PC PC + 2 or 3 None 1/2/3 CP Rd,Rr Compare Rd  Rr Z, N,V,C,H 1 CPC Rd,Rr Compare with Carry Rd  Rr  C Z, N,V,C,H 1 CPI Rd,K Compare Register with Immediate Rd  K Z, N,V,C,H 1 SBRC Rr, b Skip if Bit in Register Cleared if (Rr(b)=0) PC  PC + 2 or 3 None 1/2/3 SBRS Rr, b Skip if Bit in Register is Set if (Rr(b)=1) PC  PC + 2 or 3 None 1/2/3 SBIC P, b Skip if Bit in I/O Register Cleared if (P(b)=0) PC  PC + 2 or 3 None 1/2/3 SBIS P, b Skip if Bit in I/O Register is Set if (P(b)=1) PC  PC + 2 or 3 None 1/2/3 BRBS s, k Branch if Status Flag Set if (SREG(s) = 1) then PCPC+k + 1 None 1/2 BRBC s, k Branch if Status Flag Cleared if (SREG(s) = 0) then PCPC+k + 1 None 1/2 BREQ k Branch if Equal if (Z = 1) then PC  PC + k + 1 None 1/2 BRNE k Branch if Not Equal if (Z = 0) then PC  PC + k + 1 None 1/2 BRCS k Branch if Carry Set if (C = 1) then PC  PC + k + 1 None 1/2 BRCC k Branch if Carry Cleared if (C = 0) then PC  PC + k + 1 None 1/2 BRSH k Branch if Same or Higher if (C = 0) then PC  PC + k + 1 None 1/2 BRLO k Branch if Lower if (C = 1) then PC  PC + k + 1 None 1/2 BRMI k Branch if Minus if (N = 1) then PC  PC + k + 1 None 1/2 BRPL k Branch if Plus if (N = 0) then PC  PC + k + 1 None 1/2 BRGE k Branch if Greater or Equal, Signed if (N  V= 0) then PC  PC + k + 1 None 1/2 BRLT k Branch if Less Than Zero, Signed if (N  V= 1) then PC  PC + k + 1 None 1/2 BRHS k Branch if Half Carry Flag Set if (H = 1) then PC  PC + k + 1 None 1/2 BRHC k Branch if Half Carry Flag Cleared if (H = 0) then PC  PC + k + 1 None 1/2 BRTS k Branch if T Flag Set if (T = 1) then PC  PC + k + 1 None 1/2 BRTC k Branch if T Flag Cleared if (T = 0) then PC  PC + k + 1 None 1/2 BRVS k Branch if Overflow Flag is Set if (V = 1) then PC  PC + k + 1 None 1/2 ATmega16U4/32U4 [DATASHEET SUMMARY] 418 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Mnemonics Operands Description Operation Flags #Clocks BRVC k Branch if Overflow Flag is Cleared if (V = 0) then PC  PC + k + 1 None 1/2 BRIE k Branch if Interrupt Enabled if ( I = 1) then PC  PC + k + 1 None 1/2 BRID k Branch if Interrupt Disabled if ( I = 0) then PC  PC + k + 1 None 1/2 BIT AND BIT-TEST INSTRUCTIONS SBI P,b Set Bit in I/O Register I/O(P,b)  1 None 2 CBI P,b Clear Bit in I/O Register I/O(P,b)  0 None 2 LSL Rd Logical Shift Left Rd(n+1)  Rd(n), Rd(0)  0 Z,C,N,V 1 LSR Rd Logical Shift Right Rd(n)  Rd(n+1), Rd(7)  0 Z,C,N,V 1 ROL Rd Rotate Left Through Carry Rd(0)C,Rd(n+1) Rd(n),CRd(7) Z,C,N,V 1 ROR Rd Rotate Right Through Carry Rd(7)C,Rd(n) Rd(n+1),CRd(0) Z,C,N,V 1 ASR Rd Arithmetic Shift Right Rd(n)  Rd(n+1), n=0..6 Z,C,N,V 1 SWAP Rd Swap Nibbles Rd(3..0)Rd(7..4),Rd(7..4)Rd(3..0) None 1 BSET s Flag Set SREG(s)  1 SREG(s) 1 BCLR s Flag Clear SREG(s)  0 SREG(s) 1 BST Rr, b Bit Store from Register to T T  Rr(b) T 1 BLD Rd, b Bit load from T to Register Rd(b)  T None 1 SEC Set Carry C  1 C 1 CLC Clear Carry C  0 C 1 SEN Set Negative Flag N  1 N 1 CLN Clear Negative Flag N  0 N 1 SEZ Set Zero Flag Z  1 Z 1 CLZ Clear Zero Flag Z  0 Z 1 SEI Global Interrupt Enable I  1 I 1 CLI Global Interrupt Disable I 0 I 1 SES Set Signed Test Flag S  1 S 1 CLS Clear Signed Test Flag S  0 S 1 SEV Set Twos Complement Overflow. V  1 V 1 CLV Clear Twos Complement Overflow V  0 V 1 SET Set T in SREG T  1 T 1 CLT Clear T in SREG T  0 T 1 SEH Set Half Carry Flag in SREG H  1 H 1 CLH Clear Half Carry Flag in SREG H  0 H 1 DATA TRANSFER INSTRUCTIONS MOV Rd, Rr Move Between Registers Rd  Rr None 1 MOVW Rd, Rr Copy Register Word Rd+1:Rd  Rr+1:Rr None 1 LDI Rd, K Load Immediate Rd  K None 1 LD Rd, X Load Indirect Rd  (X) None 2 LD Rd, X+ Load Indirect and Post-Inc. Rd  (X), X  X + 1 None 2 LD Rd, - X Load Indirect and Pre-Dec. X  X - 1, Rd  (X) None 2 LD Rd, Y Load Indirect Rd  (Y) None 2 LD Rd, Y+ Load Indirect and Post-Inc. Rd  (Y), Y  Y + 1 None 2 LD Rd, - Y Load Indirect and Pre-Dec. Y  Y - 1, Rd  (Y) None 2 LDD Rd,Y+q Load Indirect with Displacement Rd  (Y + q) None 2 LD Rd, Z Load Indirect Rd  (Z) None 2 LD Rd, Z+ Load Indirect and Post-Inc. Rd  (Z), Z  Z+1 None 2 LD Rd, -Z Load Indirect and Pre-Dec. Z  Z - 1, Rd  (Z) None 2 LDD Rd, Z+q Load Indirect with Displacement Rd  (Z + q) None 2 LDS Rd, k Load Direct from SRAM Rd  (k) None 2 ST X, Rr Store Indirect (X) Rr None 2 ST X+, Rr Store Indirect and Post-Inc. (X) Rr, X  X + 1 None 2 ST - X, Rr Store Indirect and Pre-Dec. X  X - 1, (X)  Rr None 2 ST Y, Rr Store Indirect (Y)  Rr None 2 ST Y+, Rr Store Indirect and Post-Inc. (Y)  Rr, Y  Y + 1 None 2 ST - Y, Rr Store Indirect and Pre-Dec. Y  Y - 1, (Y)  Rr None 2 STD Y+q,Rr Store Indirect with Displacement (Y + q)  Rr None 2 ST Z, Rr Store Indirect (Z)  Rr None 2 ST Z+, Rr Store Indirect and Post-Inc. (Z)  Rr, Z  Z + 1 None 2 ST -Z, Rr Store Indirect and Pre-Dec. Z  Z - 1, (Z)  Rr None 2 STD Z+q,Rr Store Indirect with Displacement (Z + q)  Rr None 2 STS k, Rr Store Direct to SRAM (k)  Rr None 2 LPM Load Program Memory R0  (Z) None 3 LPM Rd, Z Load Program Memory Rd  (Z) None 3 LPM Rd, Z+ Load Program Memory and Post-Inc Rd  (Z), Z  Z+1 None 3 ELPM Extended Load Program Memory R0  (RAMPZ:Z) None 3 ELPM Rd, Z Extended Load Program Memory Rd  (Z) None 3 ELPM Rd, Z+ Extended Load Program Memory Rd  (RAMPZ:Z), RAMPZ:Z RAMPZ:Z+1 None 3 SPM Store Program Memory (Z)  R1:R0 None - IN Rd, P In Port Rd  P None 1 ATmega16U4/32U4 [DATASHEET SUMMARY] 419 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Mnemonics Operands Description Operation Flags #Clocks OUT P, Rr Out Port P  Rr None 1 PUSH Rr Push Register on Stack STACK  Rr None 2 POP Rd Pop Register from Stack Rd  STACK None 2 MCU CONTROL INSTRUCTIONS NOP No Operation None 1 SLEEP Sleep (see specific description for Sleep function) None 1 WDR Watchdog Reset (see specific description for WDR/timer) None 1 BREAK Break For On-chip Debug Only None N/A ATmega16U4/32U4 [DATASHEET SUMMARY] 420 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

33. Ordering Information 33.1 ATmega16U4 Speed [MHz] Power Supply Ordering Code Default Oscillator Package Operation Range ATmega16U4-AU External XTAL 44ML ATmega16U4RC-AU Internal Calib. RC 16 2.7 - 5.5V ATmega16U4-MU Industrial (-40° to +85°C) External XTAL (1)(2)(3) 44PW ATmega16U4RC-MU Internal Calib. RC (1)(2)(3) Notes: 1. For more information on running the USB from internal RC oscillator consult application note AVR291: 8MHz Internal Oscillator Calibration for USB Low Speed on Atmel ATmega32U4RC. 2. USB operation from internal RC oscillator is only guaranteed for 0°C to 40°C. 3. These parts are shipped with no USB bootloader pre-programmed. Package Type ML, 44 - Lead, 10 x 10mm Body Size, 1.0mm Body Thickness 44ML 0.8mm Lead Pitch, Thin Profile Plastic Quad Flat Package (TQFP) PW, 44 - Lead 7.0 x 7.0mm Body, 0.50mm Pitch 44PW Quad Flat No Lead Package (QFN) ATmega16U4/32U4 [DATASHEET] 421 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

33.2 ATmega32U4 Speed [MHz] Power Supply Ordering Code Default Oscillator Package Operation Range ATmega32U4-AU External XTAL 44ML ATmega32U4RC-AU Internal Calib. RC 16 2.7 - 5.5V ATmega32U4-MU(1)(2)(3) External XTAL Industrial (-40° to +85°C) ATmega32U4RC-MU(1) 44PW Internal Calib. RC (2) (3) Notes: 1. For more information on running the USB from internal RC oscillator consult application note AVR291: 8MHz Internal Oscillator Calibration for USB Low Speed on Atmel ATmega32U4RC. 2. USB operation from internal RC oscillator is only guaranteed for 0°C to 40°C. 3. These parts are shipped with no USB bootloader pre-programmed. Package Type ML, 44 - Lead, 10 x 10mm Body Size, 1.0mm Body Thickness 44ML 0.8mm Lead Pitch, Thin Profile Plastic Quad Flat Package (TQFP) PW, 44 - Lead 7.0 x 7.0mm Body, 0.50mm Pitch 44PW Quad Flat No Lead Package (QFN) ATmega16U4/32U4 [DATASHEET] 422 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

34. Packaging Information 34.1 TQFP44 0.17 0.60 0.37 02/06/2014 J ATmega16U4/32U4 [DATASHEET] 423 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

34.2 QFN44 ATmega16U4/32U4 [DATASHEET] 424 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

ATmega16U4/32U4 [DATASHEET] 425 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

35. Errata The revision letter in this section refers to the revision of the ATmega16U4/ATmega32U4 device. 35.1 ATmega16U4/ATmega32U4 Rev E • Spike on TWI pins when TWI is enabled • High current consumption in sleep mode • MSB of OCR4A/B/D is write only in 11-bits enhanced PWM mode 1. Spike on TWI pins when TWI is enabled 100 ns negative spike occurs on SDA and SCL pins when TWI is enabled. Problem Fix/work around Enable ATmega16U4/ATmega32U4 TWI before the other nodes of the TWI network. 2. High current consumption in sleep mode If a pending interrupt cannot wake the part up from the selected mode, the current consumption will increase during sleep when executing the SLEEP instruction directly after a SEI instruction. Problem Fix/work around Before entering sleep, interrupts not used to wake up the part from the sleep mode should be disabled. 3. MSB of OCR4A/B/D is write only in 11-bits enhanced PWM mode In the 11-bits enhanced PWM mode the MSB of OCR4A/B/D is write only. A read of OCR4A/B/D will always return zero in the MSB position. Problem Fix/work around None. 35.2 ATmega16U4/ATmega32U4 Rev D • Spike on TWI pins when TWI is enabled • High current consumption in sleep mode • Timer 4 11-bits enhanced PWM mode 1. Spike on TWI pins when TWI is enabled 100 ns negative spike occurs on SDA and SCL pins when TWI is enabled. Problem Fix/work around Enable ATmega16U4/ATmega32U4 TWI before the other nodes of the TWI network. 2. High current consumption in sleep mode If a pending interrupt cannot wake the part up from the selected mode, the current consumption will increase during sleep when executing the SLEEP instruction directly after a SEI instruction. Problem Fix/work around Before entering sleep, interrupts not used to wake up the part from the sleep mode should be disabled. 3. Timer 4 11-bits enhanced PWM mode Timer 4 11-bits enhanced mode is not functional. Problem Fix/work around None. ATmega16U4/32U4 [DATASHEET] 426 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

35.3 ATmega16U4/ATmega32U4 Rev C Not sampled 35.4 ATmega16U4/ATmega32U4 Rev B • Spike on TWI pins when TWI is enabled • High current consumption in sleep mode • Incorrect execution of VBUSTI interrupt • Timer 4 11-bits enhanced PWM mode 1. Spike on TWI pins when TWI is enabled 100 ns negative spike occurs on SDA and SCL pins when TWI is enabled. Problem Fix/work around Enable ATmega16U4/ATmega32U4 TWI before the other nodes of the TWI network. 2. High current consumption in sleep mode If a pending interrupt cannot wake the part up from the selected mode, the current consumption will increase during sleep when executing the SLEEP instruction directly after a SEI instruction. Problem Fix/work around Before entering sleep, interrupts not used to wake up the part from the sleep mode should be disabled. 3. Incorrect execution of VBUSTI interrupt The CPU may incorrectly execute the interrupt vector related to the VBUSTI interrupt flag. Problem fix/work around Do not enable this interrupt. Firmware must process this USB event by polling VBUSTI. 4. Timer 4 11-bits enhanced PWM mode Timer 4 11-bits enhanced mode is not functional. Problem Fix/work around None. 35.5 ATmega16U4/ATmega32U4 Rev A • Spike on TWI pins when TWI is enabled • High current consumption in sleep mode • Increased power consumption in power-down mode • Internal RC oscillator start up may fail • Internal RC oscillator calibration • Incorrect execution of VBUSTI interrupt • Timer 4 enhanced mode issue 1. Spike on TWI pins when TWI is enabled 100 ns negative spike occurs on SDA and SCL pins when TWI is enabled. Problem Fix/work around Enable ATmega16U4/ATmega32U4 TWI before the other nodes of the TWI network. 2. High current consumption in sleep mode ATmega16U4/32U4 [DATASHEET] 427 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

If a pending interrupt cannot wake the part up from the selected mode, the current consumption will increase during sleep when executing the SLEEP instruction directly after a SEI instruction. Problem Fix/work around Before entering sleep, interrupts not used to wake up the part from the sleep mode should be disabled. 3. Increased power consumption in power-down mode The typical power consumption is increased by about 30µA in power-down mode. Problem Fix/work around None. 4. Internal RC oscillator start up may fail When the part is configured to start on internal RC oscillator, the oscillator may not start properly after power-on. Problem Fix/work around Do not configure the part to start on internal RC oscillator. 5. Internal RC oscillator calibration 8 MHz frequency can be impossible to reach with internal RC even when using maximal OSCAL value. Problem Fix/work around None. 6. Incorrect execution of VBUSTI interrupt The CPU may incorrectly execute the interrupt vector related to the VBUSTI interrupt flag. Problem fix/work around Do not enable this interrupt. Firmware must process this USB event by polling VBUSTI. 7. Timer 4 11-bits enhanced PWM mode Timer 4 11-bits enhanced mode is not functional. Problem Fix/work around None. ATmega16U4/32U4 [DATASHEET] 428 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

36. Datasheet Revision History for ATmega16U4/ATmega32U4 Note that the referring page numbers in this section are referred to this document. The referring revision in this section are referring to the document revision. 36.1 Rev. 7766J – 04/2016 “Memory Programming” on page353: Updated number of words in a page and number of 1. pages in the Flash and EEPROM for ATmega16U4 and ATmega32U4. Refer to Table 28-11 and Table28-12 on page359. 36.2 Rev. 7766I – 07/2015 1. Applied Atmel brands throughout the contents and reorganized the contents. 2. Updated “Power Management and Sleep Modes” on page43. Part of contents was missing. 36.3 Rev. 7766H – 06/2014 The first section in “Phase and Frequency Correct PWM Mode” on page154 has been 1. corrected. 2. Several corrections are made according to the new template. 3. Trademarks are added to the last page. 4 Removed preliminary on the front page 5 Updated with new datasheet template from 05-2014 Updated description of parts pre-programed with a default USB bootloader in Features on 6. page 2. Added three footnotes for the RC part numbers in Section 33., “Ordering Information” on page 7. 421. 8. Removed footnote on Frequency range inTable6-3 on page30 and Table6-7 on page32. 9. Updated values and removed footnote in Table8-3 on page55. 10. Removed column V =1.5 - 5.5V in Table29-2 on page385. CC 11. Changed footnote for Table29-2 on page385. 12. Added max value for Rise/Fall time in Table29-4 on page387. ATmega16U4/32U4 [DATASHEET] 429 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

36.4 Rev. 7766G – 02/2014 Updated the “Description” on page177 of the “Output Compare Modulator (OCM1C0A)” . 1. Specified when the logical AND and the logical OR will be performed based on the PORTB7. Updated “USART Control and Status Register n D– UCSRnD” on page213. “Bits 7:2 - 2. Reserved” are Read only. Updated “Crystal-less Operation” on page259. The temperature range changed to “within the 3. 0C and +40C. 4. MUX bit in “ADC Control and Status Register B – ADCSRB” on page294 changed to R/W. Updated Table24-6 on page318. Trigger Source: Timer/Counter0 Compare Match updated 5. to Timer/Counter0 Compare Match A. Updated “DC Characteristics” on page383. Added Active 16MHz, V = 5V, max. 27mA, in 6. CC “Icc / Power supply current”. 7. Updated “Register Summary” on page414. Added UCSRnD at the address CBh. Replaced the “TQFP44” on page423 and “QFN44” on page424 by updated package 8. drawings. 9. Updated the last page according to Atmel new Brand Style Guide (new logo). 36.5 Rev. 7766F – 11/10 1. Replaced the “QFN44” on page424 by an updated drawing. Updated “ADC Control and Status Register B – ADCSRB” on page294. Defined the 2. ADCSRB register as in “ADC Control and Status Register B – ADCSRB” on page317. 3. Updated the last page according to Atmel new Brand Style Guide. 36.6 Rev. 7766E – 04/10 1. Updated “Features” on page1. 2. Updated “Features” on page256. 3. Updated Figure 21-9 on page 261. 4. Updated Section 21.8 on page 263. 5. Updated “Features” on page297. 6. Updated “Boundary-scan Order” on page332. 7. Updated “Program And Data Memory Lock Bits” on page353. 8. Updated Table28-5 on page355. 9. Updated “Electrical Characteristics” on page383. 10. Updated Figure 29-2 on page 386. ATmega16U4/32U4 [DATASHEET] 430 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

11. Added “Typical Characteristics” on page392. 12. Updated “Ordering Information” on page421. 13. Updated “Errata” on page426. 36.7 Rev. 7766D – 01/09 1. Updated Memory section in “Features” on page1. 2. Added section “Resources” on page8. 3. Added section “Data Retention” on page8. 4. Updated “Ordering Information” on page421. 36.8 Rev. 7766C – 11/08 1. Updated Memory section in “Features” on page1. 36.9 Rev. 7766B – 11/08 1. Added ATmega16U4 device. 2. Created errata section and added ATmega16U4. 3. Updated High Speed Timer, asynchronous description Section 15. on page 139 36.10 Rev. 7766A – 07/08 1. Initial revision ATmega16U4/32U4 [DATASHEET] 431 Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

Table of Contents 1. Pin Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1 Block Diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2 Pin Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. About . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1 Disclaimer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.2 Resources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.3 Code Examples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.4 Data Retention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4. AVR CPU Core . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.2 Architectural Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.3 ALU – Arithmetic Logic Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 4.4 Status Register. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 4.5 General Purpose Register File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.6 Stack Pointer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.7 Instruction Execution Timing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.8 Reset and Interrupt Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 5. AVR Memories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.1 In-System Reprogrammable Flash Program Memory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.2 SRAM Data Memory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 5.3 EEPROM Data Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 5.4 I/O Memory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 6. System Clock and Clock Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.1 Clock Systems and their Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.2 Clock Sources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 6.3 Low Power Crystal Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 6.4 Low Frequency Crystal Oscillator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 6.5 Calibrated Internal RC Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 6.6 External Clock. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 6.7 Clock Switch. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 6.8 Clock Output Buffer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 6.9 PLL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 6.10 Clock switch Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 6.11 Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 7. Power Management and Sleep Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 7.1 Idle Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 7.2 ADC Noise Reduction Mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 7.3 Power-down Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 7.4 Power-save Mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 7.5 Standby Mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 7.6 Extended Standby Mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 7.7 Power Reduction Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 7.8 Minimizing Power Consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 ATmega16U4/32U4 [DATASHEET] i Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

7.9 Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 8. System Control and Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 8.1 Resetting the AVR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 8.2 Reset Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 8.3 Power-on Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 8.4 External Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 8.5 Brown-out Detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 8.6 Watchdog Reset. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 8.7 USB Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 8.8 Internal Voltage Reference. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 8.9 Watchdog Timer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 8.10 Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 8.11 MCU Status Register – MCUSR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 9. Interrupts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 9.1 Interrupt Vectors in ATmega16U4/ATmega32U4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 9.2 Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 10. I/O-Ports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 10.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 10.2 Ports as General Digital I/O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 10.3 Alternate Port Functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 10.4 Register Description for I/O-Ports. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 11. External Interrupts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 11.1 Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 12. Timer/Counter0, Timer/Counter1, and Timer/Counter3 Prescalers . . . . . . . . . . . . . . . 92 12.1 Internal Clock Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 12.2 Prescaler Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 12.3 External Clock Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 12.4 Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 13. 8-bit Timer/Counter0 with PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 13.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 13.2 Timer/Counter Clock Sources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 13.3 Counter Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 13.4 Output Compare Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 13.5 Compare Match Output Unit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 13.6 Modes of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 13.7 Timer/Counter Timing Diagrams. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 13.8 8-bit Timer/Counter Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 14. 16-bit Timers/Counters (Timer/Counter1 and Timer/Counter3) . . . . . . . . . . . . . . . . . . 111 14.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 14.2 Accessing 16-bit Registers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 14.3 Timer/Counter Clock Sources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 14.4 Counter Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 14.5 Input Capture Unit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 14.6 Output Compare Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 14.7 Compare Match Output Unit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 ATmega16U4/32U4 [DATASHEET] ii Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

14.8 Modes of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 14.9 Timer/Counter Timing Diagrams. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 14.10 16-bit Timer/Counter Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 15. 10-bit High Speed Timer/Counter4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 15.1 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 15.2 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 15.3 Counter Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 15.4 Output Compare Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 15.5 Dead Time Generator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 15.6 Compare Match Output Unit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 15.7 Synchronous update. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 15.8 Modes of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 15.9 Timer/Counter Timing Diagrams. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 15.10 Fault Protection Unit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 15.11 Accessing 10-bit Registers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 15.12 Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 16. Output Compare Modulator (OCM1C0A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 16.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 16.2 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 17. Serial Peripheral Interface – SPI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 17.1 SS Pin Functionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182 17.2 Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 18. USART . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 18.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 18.2 Clock Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 18.3 Frame Formats. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 18.4 USART Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 18.5 Data Transmission – The USART Transmitter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 18.6 Data Reception – The USART Receiver. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 18.7 Asynchronous Data Reception. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 18.8 Multi-processor Communication Mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 18.9 Hardware Flow Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205 18.10 Examples of Baud Rate Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 18.11 USART Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 19. USART in SPI Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 19.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 19.2 Clock Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 19.3 SPI Data Modes and Timing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 19.4 Frame Formats. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 19.5 Data Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218 19.6 AVR USART MSPIM vs. AVR SPI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 19.7 USART MSPIM Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 20. 2-wire Serial Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 20.1 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 20.2 2-wire Serial Interface Bus Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 20.3 Data Transfer and Frame Format. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 ATmega16U4/32U4 [DATASHEET] iii Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

20.4 Multi-master Bus Systems, Arbitration and Synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 20.5 Overview of the TWI Module. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 20.6 Using the TWI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 20.7 Transmission Modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 20.8 Multi-master Systems and Arbitration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250 20.9 TWI Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252 21. USB Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 21.1 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 21.2 Block Diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 21.3 Typical Application Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 21.4 Crystal-less Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 21.5 Design Guidelines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 21.6 General Operating Modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260 21.7 Power Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 21.8 Speed Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 21.9 Memory Management. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 21.10 PAD Suspend. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 21.11 Plug-in Detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 21.12 USB Software Operating Modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 21.13 Registers Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 22. USB Device Operating Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 22.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 22.2 Power-on and Reset. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 22.3 Endpoint Reset. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 22.4 USB Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 22.5 Endpoint Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 22.6 Endpoint Activation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 22.7 Address Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 22.8 Suspend, Wake-up and Resume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 22.9 Detach. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 22.10 Remote Wake-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 22.11 STALL Request. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 22.12 CONTROL Endpoint Management. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274 22.13 OUT Endpoint Management. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 22.14 IN endpoint management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276 22.15 Isochronous Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 22.16 Overflow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 22.17 Interrupts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 22.18 Registers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281 23. Analog Comparator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293 23.1 Register Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294 23.2 Analog Comparator Multiplexed Input. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295 24. Analog to Digital Converter - ADC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297 24.1 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297 24.2 Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 24.3 Starting a Conversion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 24.4 Prescaling and Conversion Timing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300 24.5 Changing Channel or Reference Selection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 ATmega16U4/32U4 [DATASHEET] iv Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

24.6 Temperature Sensor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304 24.7 ADC Noise Canceler. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305 24.8 ADC Conversion Result . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 24.9 ADC Register Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313 25. JTAG Interface and On-chip Debug System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 25.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 25.2 Test Access Port – TAP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 25.3 TAP Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321 25.4 Using the Boundary-scan Chain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322 25.5 Using the On-chip Debug System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322 25.6 On-chip Debug Specific JTAG Instructions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323 25.7 On-chip Debug Related Register in I/O Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323 25.8 Using the JTAG Programming Capabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324 25.9 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324 26. IEEE 1149.1 (JTAG) Boundary-scan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325 26.1 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325 26.2 System Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325 26.3 Data Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325 26.4 Boundary-scan Specific JTAG Instructions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 26.5 Boundary-scan Related Register in I/O Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328 26.6 Boundary-scan Chain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329 26.7 Boundary-scan Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332 26.8 Boundary-scan Description Language Files. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335 27. Boot Loader Support – Read-While-Write Self-Programming . . . . . . . . . . . . . . . . . . . 336 27.1 Boot Loader Features. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336 27.2 Application and Boot Loader Flash Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336 27.3 Read-While-Write and No Read-While-Write Flash Sections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336 27.4 Boot Loader Lock Bits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339 27.5 Entering the Boot Loader Program. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340 27.6 Addressing the Flash During Self-Programming. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343 27.7 Self-Programming the Flash. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344 28. Memory Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353 28.1 Program And Data Memory Lock Bits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353 28.2 Fuse Bits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354 28.3 Signature Bytes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356 28.4 Calibration Byte. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356 28.5 Parallel Programming Parameters, Pin Mapping, and Commands. . . . . . . . . . . . . . . . . . . . . . . . . 356 28.6 Parallel Programming. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359 28.7 Serial Downloading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367 28.8 Serial Programming Pin Mapping. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368 28.9 Programming via the JTAG Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371 29. Electrical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383 29.1 Absolute Maximum Ratings*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383 29.2 DC Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383 29.3 External Clock Drive Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385 29.4 External Clock Drive. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385 29.5 System and Reset Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386 ATmega16U4/32U4 [DATASHEET] v Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

29.6 Maximum speed vs. V 386 CC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29.7 2-wire Serial Interface Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386 29.8 SPI Timing Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388 29.9 Hardware Boot Entrance Timing Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389 30. Typical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392 30.1 Active Supply Current. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392 30.2 Idle Supply Current. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395 30.3 Power-down Supply Current. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397 30.4 Power-save Supply Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398 30.5 Pin Pull-Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399 30.6 Pin Driver Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 400 30.7 Pin Threshold and Hysteresis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403 30.8 BOD Threshold. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406 30.9 Internal Oscillator Speed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408 30.10 Current Consumption of Peripheral Units. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411 30.11 Current Consumption in Reset and Reset Pulse Width . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413 31. Register Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414 32. Instruction Set Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418 33. Ordering Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421 33.1 ATmega16U4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421 33.2 ATmega32U4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422 34. Packaging Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423 34.1 TQFP44 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423 34.2 QFN44 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424 35. Errata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426 35.1 ATmega16U4/ATmega32U4 RevE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426 35.2 ATmega16U4/ATmega32U4 RevD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426 35.3 ATmega16U4/ATmega32U4 RevC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427 35.4 ATmega16U4/ATmega32U4 RevB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427 35.5 ATmega16U4/ATmega32U4 RevA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427 36. Datasheet Revision History for ATmega16U4/ATmega32U4 . . . . . . . . . . . . . . . . . . . . 429 36.1 Rev. 7766J – 04/2016. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429 36.2 Rev. 7766I – 07/2015 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429 36.3 Rev. 7766H – 06/2014 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429 36.4 Rev. 7766G – 02/2014 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430 36.5 Rev. 7766F – 11/10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430 36.6 Rev. 7766E – 04/10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430 36.7 Rev. 7766D – 01/09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431 36.8 Rev. 7766C – 11/08 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431 36.9 Rev. 7766B – 11/08 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431 36.10 Rev. 7766A – 07/08 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431 ATmega16U4/32U4 [DATASHEET] vi Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016

X X X X X X Atmel Corporation 1600 Technology Drive, San Jose, CA 95110 USA T: (+1)(408) 441.0311 F: (+1)(408) 436.4200 | www.atmel.com © 2015 Atmel Corporation. / Rev.: Atmel-7766I-USB-ATmega16U4-32U4-Datasheet_07/2015. Atmel®, Atmel logo and combinations thereof, AVR®, AVR Studio®, Enabling Unlimited Possibilities®, and others are registered trademarks or trademarks of Atmel Corporation or its subsidiaries. Windows® is a registered trademark of Microsoft Corporation in U.S. and or other countries. Other terms and product names may be trademarks of others. DISCLAIMER: The information in this document is provided in connection with Atmel products. No license, express or implied, by estoppel or otherwise, to any intellectual property right is granted by this document or in connection with the sale of Atmel products. EXCEPT AS SET FORTH IN THE ATMEL TERMS AND CONDITIONS OF SALES LOCATED ON THE ATMEL WEBSITE, ATMEL ASSUMES NO LIABILITY WHATSOEVER AND DISCLAIMS ANY EXPRESS, IMPLIED OR STATUTORY WARRANTY RELATING TO ITS PRODUCTS INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTY OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, OR NON-INFRINGEMENT. IN NO EVENT SHALL ATMEL BE LIABLE FOR ANY DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE, SPECIAL OR INCIDENTAL DAMAGES (INCLUDING, WITHOUT LIMITATION, DAMAGES FOR LOSS AND PROFITS, BUSINESS INTERRUPTION, OR LOSS OF INFORMATION) ARISING OUT OF THE USE OR INABILITY TO USE THIS DOCUMENT, EVEN IF ATMEL HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. Atmel makes no representations or warranties with respect to the accuracy or completeness of the contents of this document and reserves the right to make changes to specifications and products descriptions at any time without notice. Atmel does not make any commitment to update the information contained herein. Unless specifically provided otherwise, Atmel products are not suitable for, and shall not be used in, automotive applications. Atmel products are not intended, authorized, or warranted for use as components in applications intended to support or sustain life. SAFETY-CRITICAL, MILITARY, AND AUTOMOTIVE APPLICATIONS DISCLAIMER: Atmel products are not designed for and will not be used in connection with any applications where the failure of such products would reasonably be expected to result in significant personal injury or death (“Safety-Critical Applications”) without an Atmel officer's specific written consent. Safety-Critical Applications include, without limitation, life support devices and systems, equipment or systems for the operation of nuclear facilities and weapons systems. Atmel products are not designed nor intended for use in military or aerospace applications or environments unless specifically designated by Atmel as military-grade. Atmel products are not designed nor intended for use in automotive applications unless specifically designated by Atmel as automotive-grade.

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