xr ST16C1550/51 2.97V TO 5.5V UART WITH 16-BYTE FIFO AUGUST 2005 REV. 4.2.1 GENERAL DESCRIPTION FEATURES The ST16C1550 and ST16C1551 UARTs (here on • Pin and functionally compatible to SSI 73M1550/ denoted as the ST16C155X) are improved versions 2550 of the SSI 73M1550 and SSI 73M2550 UART with • 16 byte Transmit FIFO higher operating speed and lower access time. The • 16 byte Receive FIFO with error flags ST16C155X provides enhanced UART functions with 16 byte FIFOs, a modem control interface, • 4 selectable Receive FIFO interrupt trigger levels independent programmable baud rate generators • Modem Control Signals (CTS#, RTS#, DSR#, with clock rates up to 1.5 Mbps. Onboard status DTR#, RI#, CD#) registers provide the user with error indications and • Programmable character lengths (5, 6, 7, 8) with operational status. System interrupt and modem even, odd or no parity control features may be tailored by external software to meet specific user requirements. An internal • Crystal or external clock input (except 28 pin loopback capability allows onboard diagnostics. The ST16C1551, external clock only) baud rate generator can be configured for either • 1.5 Mbps Transmit/Receive operation (24 MHz) crystal or external clock input with the exception of with programmable clock control the 28 pin ST16C1551 package (where an external • Power Down Mode (50 uA at 3.3 V, 200 uA at 5 V) clock must be provided). Each package type, with the exception of the 28 pin ST16C155X, provides a • Software controllable reset output buffered reset output that can be controlled through • 2.97 to 5.5 Volt operation user software. DMA monitor signals TXRDY/RXRDY APPLICATIONS are not available at the ST16C155X I/O pins but these signals are accessible through ISR register bits • Battery Operated Electronics 4-5. Except as listed above, all package versions • Internet Appliances have the same features. The ST16C155X is not compatible with the industry standard 16550 and will • Handheld Terminal not work with the standard serial port driver in MS • Personal Digital Assistants Windows (see pages 16-17 for details). For an MS • Cellular Phones DataPort Windows compatible UART, see the ST16C550. FIGURE 1. BLOCK DIAGRAM 16 Byte TX FIFO A2:A0 Transmitter TX D7:D0 IOR# DTR#, RTS# IOW# UART Modem Control Signals Configuration DSR#, CTS#, CS# Data Bus Regs CD#, RI# Interface INT Receiver RX 16 Byte RX FIFO Baud Rate Generator RESET RST XTAL1/CLK Crystal Osc/Buffer XTAL2 Exar Corporation 48720 Kato Road, Fremont CA, 94538 • (510) 668-7000 • FAX (510) 668-7017 • www.exar.com

xr ST16C1550/51 2.97V TO 5.5V UART WITH 16-BYTE FIFO REV. 4.2.1 FIGURE 2. ST16C1550 PINOUTS 48-TQFP PACKAGE NOTE: PINOUTS NOT TO SCALE. N.C. D3 D2 D1 N.C. D0 N.C. VCC CD# DSR# N.C. N.C. AISC STMUAALL LSEIRZE T OHFA NT QPFLPC CP APCAKCAKGAEGE. 8 7 6 5 4 3 2 1 0 9 8 7 4 4 4 4 4 4 4 4 4 3 3 3 N.C. 1 36 N.C. N.C. 2 35 N.C. D4 3 34 CTS# D5 4 33 RESET D6 5 32 DTR# D7 6 31 RTS# ST16C1550CQ48 RX 7 30 A0 TX 8 29 N.C. CS# 9 28 A1 N.C. 10 27 A2 N.C. 11 26 N.C. N.C. 12 25 N.C. 3 4 5 6 7 8 9 0 1 2 3 4 1 1 1 1 1 1 1 2 2 2 2 2 N.C. N.C. XTAL1 XTAL2 IOW# N.C. GND IOR# RI# RST INT N.C. 28-PLCC PACKAGES D3 D2 D1 D0 VCC CD# DSR# 4 3 2 1 28 27 26 D4 5 25 CTS# D5 6 24 RESET D6 7 23 DTR# D7 8 ST16C1550CJ28 22 RTS# RX 9 21 A0 TX 10 20 A1 CS# 11 19 A2 2 3 4 5 6 7 8 1 1 1 1 1 1 1 TAL1 TAL2 OW# GND OR# RI# INT X X I I 2

xr ST16C1550/51 REV. 4.2.1 2.97V TO 5.5V UART WITH 16-BYTE FIFO FIGURE 3. ST16C1551 PINOUTS 48-TQFP PACKAGE NOTE: PINOUTS NOT TO SCALE. N.C. D3 D2 D1 N.C. D0 N.C. VCC CD# DSR# N.C. N.C. AISC STMUAALL LSEIRZE T OHFA NT QPFLPC CP APCAKCAKGAEGE. 8 7 6 5 4 3 2 1 0 9 8 7 4 4 4 4 4 4 4 4 4 3 3 3 N.C. 1 36 N.C. N.C. 2 35 N.C. D4 3 34 CTS# D5 4 33 RESET D6 5 32 DTR# D7 6 31 RTS# ST16C1551CQ48 RX 7 30 A0 TX 8 29 N.C. CS# 9 28 A1 N.C. 10 27 A2 N.C. 11 26 N.C. N.C. 12 25 N.C. 3 4 5 6 7 8 9 0 1 2 3 4 1 1 1 1 1 1 1 2 2 2 2 2 N.C. N.C. XTAL1 XTAL2 IOW# N.C. GND IOR# RI# RST INT N.C. 28-PLCC PACKAGES D3 D2 D1 D0 VCC CD# DSR# 4 3 2 1 28 27 26 D4 5 25 CTS# D5 6 24 RESET D6 7 23 DTR# D7 8 ST16C1551CJ28 22 RTS# RX 9 21 A0 TX 10 20 A1 CS# 11 19 A2 2 3 4 5 6 7 8 1 1 1 1 1 1 1 CLK OW# GND OR# RI# RST INT I I 3

xr ST16C1550/51 2.97V TO 5.5V UART WITH 16-BYTE FIFO REV. 4.2.1 ORDERING INFORMATION OPERATING PART NUMBER PACKAGE TEMPERATURE DEVICE STATUS RANGE ST16C1550CJ28 28-Lead PLCC 0°C to +70°C Active ST16C1550CQ48 48-Lead TQFP 0°C to +70°C Active ST16C1551CJ28 28-Lead PLCC 0°C to +70°C Active ST16C1551CQ48 48-Lead TQFP 0°C to +70°C Active ST16C1550IJ28 28-Lead PLCC -40°C to +85°C Active ST16C1550IQ48 48-Lead TQFP -40°C to +85°C Active ST16C1551IJ28 28-Lead PLCC -40°C to +85°C Active ST16C1551IQ48 48-Lead TQFP -40°C to +85°C Active 4

xr ST16C1550/51 REV. 4.2.1 2.97V TO 5.5V UART WITH 16-BYTE FIFO PIN DESCRIPTIONS 28-PIN 28-PIN 48-PIN NAME PLCC PLCC TYPE DESCRIPTION TQFP (1550) (1551) DATA BUS INTERFACE A0 21 21 30 I Address data lines [2:0]. A2:A0 selects internal UART’s configuration regis- A1 20 20 28 ters. A2 19 19 27 D0 1 1 43 I/O Data bus lines [7:0] (bidirectional). D1 2 2 45 D2 3 3 46 D3 4 4 47 D4 5 5 3 D5 6 6 4 D6 7 7 5 D7 8 8 6 IOR# 16 15 20 I Input/Output Read (active low). The falling edge instigates an internal read cycle and retrieves the data byte from an internal register pointed by the address lines [A2:A0], places it on the data bus to allow the host processor to read it on the leading edge. IOW# 14 13 17 I Input/Output Write (active low). The falling edge instigates the internal write cycle and the rising edge transfers the data byte on the data bus to an internal register pointed by the address lines [A2:A0]. CS# 11 11 9 I Chip Select input (active low). A logic 0 on this pin selects the ST16C155X device. INT 18 18 23 O Interrupt Output (three-state, active high). INT output defaults to three-state mode and becomes active high when MCR bit-3 is set to a logic 1. INT output becomes a logic high level when interrupts are enabled in the interrupt enable register (IER), and whenever the transmitter, receiver, line and/or modem sta- tus register has an active condition. MODEM OR SERIAL I/O INTERFACE TX 10 10 8 O Transmit Data. This output is associated with individual serial transmit chan- nel data from the 155X. The TX signal will be a logic 1 during reset, idle (no data), or when the transmitter is disabled. During the local loopback mode, the TX output pin is disabled and TX data is internally connected to the UART RX input. RX 9 9 7 I Receive Data. This input is associated with individual serial channel data to the 155X. Normal received data input idles at logic 1 condition. This input must be connected to its idle logic state, logic 1, else the receiver may report “receive break” and/or “error” condition(s). RTS# 22 22 31 O Request to Send or general purpose output (active low). If this pin is not needed for modem communication, then it can be used as a general I/O. If it is not used, leave it unconnected. CTS# 25 25 34 I Clear to Send or general purpose input (active low). If this pin is not needed for modem communication, then it can be used as a general I/O. If it is not used, connect it to VCC. 5

xr ST16C1550/51 2.97V TO 5.5V UART WITH 16-BYTE FIFO REV. 4.2.1 28-PIN 28-PIN 48-PIN NAME PLCC PLCC TYPE DESCRIPTION TQFP (1550) (1551) DTR# 23 23 32 O Data Terminal Ready or general purpose output (active low). If this pin is not needed for modem communication, then it can be used as a general I/O. If it is not used, leave it unconnected. DSR# 26 26 39 I Data Set Ready input or general purpose input (active low). If this pin is not needed for modem communication, then it can be used as a general I/O. If it is not used, connect it to VCC. CD# 27 27 40 I Carrier Detect input or general purpose input (active low). If this pin is not needed for modem communication, then it can be used as a general I/O. If it is not used, connect it to VCC. RI# 17 16 21 I Ring Indicator input or general purpose input (active low). If this pin is not needed for modem communication, then it can be used as a general I/O. If it is not used, connect it to VCC. ANCILLARY SIGNALS CLK - 12 - I External Clock Input. This function is associated with 28 pin PDIP and 28 pin PLCC packages only. An external clock must be connected to this pin to clock the baud rate generator and internal circuitry. XTAL1 12 - 15 I Crystal or external clock input. See Figure 4 for typical oscillator connec- tions. XTAL2 13 - 16 O Crystal or buffered clock output. See Figure 4 for typical oscillator connec- tions. RESET 24 24 33 I Reset Input (active high). When it is asserted, the UART configuration regis- ters are reset to default values, see Table 8. RST - 17 22 O Reset Output (active high). This output is only available on the ST16C1551. When IER bit-5 is a logic 0, RST will follow the logical state of the RESET pin. When IER bit-5 is a logic 1, the user may send software (soft) resets via MCR bit-2. Soft resets from MCR bit-2 are “ORed” with the state of the RESET pin. VCC 28 28 41 Pwr Power supply input. GND 15 14 19 Pwr Power supply common ground. N.C. - - 1, 2, - Not connected. 10-14, 18, 24-26, 29, 35-38, 42, 44, 48 Pin type: I=Input, O=Output, I/O= Input/output, OD=Output Open Drain. 6

xr ST16C1550/51 REV. 4.2.1 2.97V TO 5.5V UART WITH 16-BYTE FIFO 1.0 PRODUCT DESCRIPTION The ST16C155X provides serial asynchronous receive data synchronization, parallel-to-serial and serial-to- parallel data conversions for both the transmitter and receiver sections. These functions are necessary for converting the serial data stream into parallel data that is required in digital data systems. Synchronization for the serial data stream is accomplished by adding start and stops bits to the transmit data to form a data character (character orientated protocol). Data integrity is ensured by attaching a parity bit to the data character. The parity bit is checked by the receiver for any transmission bit errors. ENHANCED FEATURES The ST16C155X is an upward solution that provides 16 bytes of transmit and receive FIFO memory, instead of none in the 16C145X. The 155X is designed to work with high speed modems and shared network environments, that require fast data processing time. Increased performance is realized in the 155X by the larger transmit and receive FIFOs. This allows the external processor to handle more networking tasks within a given time. For example, the ST16C550 with a 16 byte FIFO, unloads 16 bytes of receive data in 93 microseconds (This example uses a character length of 11 bits, including start/stop bits at 115.2Kbps). This means the external CPU will have to service the receive FIFO less than every 100 microseconds. However with the 16 byte FIFO in the 155X, the data buffer will not require unloading/loading for 1.53 ms. This increases the service interval giving the external CPU additional time for other applications and reducing the overall UART interrupt servicing time. In addition, the 4 selectable levels of FIFO trigger interrupt are provided for maximum data throughput performance especially when operating in a multi-channel environment. The FIFO memory greatly reduces the bandwidth requirement of the external controlling CPU, increases performance, and reduces power consumption. DATA RATE The 155X is capable of operation up to 1.5 Mbps with a 24 MHz crystal or external clock input with a 16X sampling clock (at VCC = 5.0V). With a crystal of 14.7456 MHz and through a software option, the user can select data rates up to 921.6 Kbps. The rich feature set of the 155X is available through internal registers. Selectable receive FIFO trigger levels, selectable baud rates, and modem interface controls are all standard features. Following a power on reset or an external reset, the 155X is software compatible with the ST16C145X. 7

xr ST16C1550/51 2.97V TO 5.5V UART WITH 16-BYTE FIFO REV. 4.2.1 2.0 FUNCTIONAL DESCRIPTIONS 2.1 Internal Registers The 155X has a set of enhanced registers for controlling, monitoring and data loading and unloading. These registers function as data holding registers (THR/RHR), interrupt status and control registers (ISR/IER), a FIFO control register (FCR), receive line status and control registers (LSR/LCR), modem status and control registers (MSR/MCR), programmable data rate (clock) divisor registers (DLL/DLM), and a user accessible scractchpad register (SPR). All the register functions are discussed in full detail later in “Section 3.0, UART INTERNAL REGISTERS” on page 14. 2.2 DMA Mode The DMA Mode (a legacy term) in this document does not mean “Direct Memory Access” but refers to data block transfer operation. The DMA mode affects the state of the RXRDY and TXRDY bits (ISR bits 5 and 4 respectively). The transmit and receive FIFO trigger levels provide additional flexibility to the user for block mode operation. The LSR bits 5-6 provide an indication when the transmitter is empty or has an empty location(s) for more data. The user can optionally operate the transmit and receive FIFO in the DMA mode (FCR bit-3=1). When the transmit and receive FIFO are enabled and the DMA mode is disabled (FCR bit-3 = 0), the 155X activates the TXRDY & RXRDY output pin for each data transmit or receive operation. When DMA mode is enabled (FCR bit-3 = 1), the user takes advantage of block mode operation by loading or unloading the FIFO in a block sequence determined by the programmed trigger level. In this mode, the 155X sets the TXRDY bit when the transmit FIFO becomes full, and sets the RXRDY pin when the receive FIFO becomes empty. The following table shows their behavior. TABLE 1: TXRDY AND RXRDY BITS IN FIFO AND DMA MODE FCR BIT-0=0 PINS FCR BIT-0=1 (FIFO ENABLED) (FIFO DISABLED) FCR Bit-3 = 0 FCR Bit-3 = 1 (DMA Mode Disabled) (DMA Mode Enabled) RXRDY 1 = 1 byte 1 = at least 1 byte in FIFO 1 = FIFO reaches the trigger level, or timeout 0 = no data 0 = FIFO empty occurs 0 = FIFO empty TXRDY 1 = THR empty 1 = FIFO empty 1 = FIFO has at least 1 empty location 0 = byte in THR 0 = at least 1 byte in FIFO 0 = FIFO is full 2.3 Crystal Oscillator or External Clock The 155X includes an on-chip oscillator (XTAL1 and XTAL2). The crystal oscillator provides the system clock to the Baud Rate Generators (BRG) in the UART. XTAL1 is the input to the oscillator or external clock buffer input with XTAL2 pin being the output. For programming details, see “Section 2.4, Programmable Baud Rate Generator” on page 9. The on-chip oscillator is designed to use an industry standard microprocessor crystal (parallel resonant, fundamental frequency with 10-22 pF capacitance load, ESR of 20-120 ohms and 100ppm frequency tolerance) connected externally between the XTAL1 and XTAL2 pins (see Figure 4). Alternatively, an external clock can be connected to the XTAL1 pin to clock the internal baud rate generator for standard or custom rates. Typical oscillator connections are shown in Figure 4. For further reading on oscillator circuit please see application note DAN108 on EXAR’s web site. 8

xr ST16C1550/51 REV. 4.2.1 2.97V TO 5.5V UART WITH 16-BYTE FIFO FIGURE 4. TYPICAL OSCILLATOR CONNECTIONS XTAL1 XTAL2 R1 0-120 (Optional) R2 500K - 1M 1.8432 MHz Y1 to 24 MHz C1 C2 22-47pF 22-47pF 2.4 Programmable Baud Rate Generator The UART has its own Baud Rate Generator (BRG) with a prescaler. The prescaler is controlled by a software bit in the MCR register. The MCR register bit-7 sets the prescaler to divide the input crystal or external clock by 1 or 4. The clock output of the prescaler goes to the BRG. The BRG further divides this clock by a programmable divisor between 1 and (216 -1) to obtain a 16X sampling clock of the serial data rate. The sampling clock is used by the transmitter for data bit shifting and receiver for data sampling. The BRG divisor (DLL and DLM registers) defaults to a random value upon power up or a reset. Therefore, the BRG must be programmed during initialization to the operating data rate. Programming the Baud Rate Generator Registers DLM and DLL provides the capability of selecting the operating data rate. Table 2 shows the standard data rates available with a 14.7456 MHz crystal or external clock at 16X clock rate. When using a non-standard data rate crystal or external clock, the divisor value can be calculated for DLL/DLM with the following equation. divisor (decimal) = (XTAL1 clock frequency / prescaler) / (serial data rate x 16) 9

xr ST16C1550/51 2.97V TO 5.5V UART WITH 16-BYTE FIFO REV. 4.2.1 TABLE 2: TYPICAL DATA RATES WITH A 14.7456 MHZ CRYSTAL OR EXTERNAL CLOCK DLM DLL OUTPUT Data Rate OUTPUT Data Rate DIVISOR FOR 16x DIVISOR FOR 16x DATA RATE PROGRAM PROGRAM MCR Bit-7=1 MCR Bit-7=0 Clock (Decimal) Clock (HEX) ERROR (%) VALUE (HEX) VALUE (HEX) 100 400 2304 900 09 00 0 600 2400 384 180 01 80 0 1200 4800 192 C0 00 C0 0 2400 9600 96 60 00 60 0 4800 19.2k 48 30 00 30 0 9600 38.4k 24 18 00 18 0 19.2k 76.8k 12 0C 00 0C 0 38.4k 153.6k 6 06 00 06 0 57.6k 230.4k 4 04 00 04 0 115.2k 460.8k 2 02 00 02 0 230.4k 921.6k 1 01 00 01 0 2.5 Transmitter The transmitter section comprises of an 8-bit Transmit Shift Register (TSR) and 16 bytes of FIFO which includes a byte-wide Transmit Holding Register (THR). TSR shifts out every data bit with the 16X internal clock. A bit time is 16 clock periods. The transmitter sends the start-bit followed by the number of data bits, inserts the proper parity-bit if enabled, and adds the stop-bit(s). The status of the FIFO and TSR are reported in the Line Status Register (LSR bit-5 and bit-6). 2.5.1 Transmit Holding Register (THR) - Write Only The transmit holding register is an 8-bit register providing a data interface to the host processor. The host writes transmit data byte to the THR to be converted into a serial data stream including start-bit, data bits, parity-bit and stop-bit(s). The least-significant-bit (Bit-0) becomes first data bit to go out. The THR is the input register to the transmit FIFO of 16 bytes when FIFO operation is enabled by FCR bit-0. Every time a write operation is made to the THR, the FIFO data pointer is automatically bumped to the next sequential data location. 10

xr ST16C1550/51 REV. 4.2.1 2.97V TO 5.5V UART WITH 16-BYTE FIFO 2.5.2 Transmitter Operation in non-FIFO Mode The host loads transmit data to THR one character at a time. The THR empty flag (LSR bit-5) is set when the data byte is transferred to TSR. THR flag can generate a transmit empty interrupt (ISR bit-1) when it is enabled by IER bit-1. The TSR flag (LSR bit-6) is set when TSR becomes completely empty. FIGURE 5. TRANSMITTER OPERATION IN NON-FIFO MODE Data Transmit Byte Holding Register (THR) THR Interrupt (ISR bit-1) Enabled by IER bit-1 16X Clock M L Transmit Shift Register (TSR) S S B B TXNOFIFO1 2.5.3 Transmitter Operation in FIFO Mode The host may fill the transmit FIFO with up to 16 bytes of transmit data. The THR empty flag (LSR bit-5) is set whenever the FIFO is empty. The THR empty flag can generate a transmit empty interrupt (ISR bit-1) when the amount of data in the FIFO falls below its programmed trigger level. The transmit empty interrupt is enabled by IER bit-1. The TSR flag (LSR bit-6) is set when TSR/FIFO becomes empty. 2.6 Receiver The receiver section contains an 8-bit Receive Shift Register (RSR) and 16 bytes of FIFO which includes a byte-wide Receive Holding Register (RHR). The RSR uses the 16X clock for timing. It verifies and validates every bit on the incoming character in the middle of each data bit. On the falling edge of a start or false start bit, an internal receiver counter starts counting at the 16X clock rate. After 8 clocks the start bit period should be at the center of the start bit. At this time the start bit is sampled and if it is still a logic 0 it is validated. Evaluating the start bit in this manner prevents the receiver from assembling a false character. The rest of the data bits and stop bits are sampled and validated in this same manner to prevent false framing. If there were any error(s), they are reported in the LSR register bits 2-4. Upon unloading the receive data byte from RHR, the receive FIFO pointer is bumped and the error tags are immediately updated to reflect the status of the data byte in RHR register. RHR can generate a receive data ready interrupt upon receiving a character or delay until it reaches the FIFO trigger level. Furthermore, data delivery to the host is guaranteed by a receive data ready time-out interrupt when data is not received for 4 word lengths as defined by LCR[1:0] plus 12 bits time. This is equivalent to 3.7-4.6 character times. The RHR interrupt is enabled by IER bit-0. 2.6.1 Receive Holding Register (RHR) - Read-Only The Receive Holding Register is an 8-bit register that holds a receive data byte from the Receive Shift Register. It provides the receive data interface to the host processor. The RHR register is part of the receive FIFO of 16 bytes by 11-bits wide, the 3 extra bits are for the 3 error tags to be reported in LSR register. When the FIFO is enabled by FCR bit-0, the RHR contains the first data character received by the FIFO. After the RHR is read, the next character byte is loaded into the RHR and the errors associated with the current data byte are immediately updated in the LSR bits 2-4. 11

xr ST16C1550/51 2.97V TO 5.5V UART WITH 16-BYTE FIFO REV. 4.2.1 FIGURE 6. RECEIVER OPERATION IN NON-FIFO MODE 16X Clock Receive Data Shift Data Bit Register (RSR) Validation Receive Data Characters Error Receive Receive Data Tags in Data Byte RHR Interrupt (ISR bit-2) Holding Register LSR bits and Errors (RHR) 4:2 RXFIFO1 2.7 Special (Enhanced Feature) Mode The 155X supports the standard features of the ST16C550. In addition the 155X supports some enhanced features not available for the ST16C550. These features are enabled by IER bit-5 and include a software controllable (SOFT) reset, power down feature and FIFO monitoring bits. 2.7.1 Soft Reset Soft resets are useful when the user desires the capability of resetting an externally connected device only. MCR bit-2 can be used to initiate a SOFT reset at the RST output pin. This does not reset the 155X (only the RESET input pin can reset the 155X). Soft resets from MCR bit-2 are “ORed” with the RESET input pin. Therefore both reset types will be seen at the RST output pin. 2.7.2 Power Down Mode The power down feature (controlled by MCR bit-7) provides the user with the capability to conserve power when the package is not in actual use without destroying internal register configuration data. This allows quick turnarounds from power down to normal operation. 2.7.3 TXRDY and RXRDY bits When IER bit-5 is set to a logic 1, ISR bits 4 and 5 represent the compliment (inversion) of the TXRDY status and RXRDY status, respectively. See Table 1. 12

xr ST16C1550/51 REV. 4.2.1 2.97V TO 5.5V UART WITH 16-BYTE FIFO 2.8 Internal Loopback The 155X UART provides an internal loopback capability for system diagnostic purposes. The internal loopback mode is enabled by setting MCR register bit-4 to logic 1. All regular UART functions operate normally. Figure 7 shows how the modem port signals are re-configured. Transmit data from the transmit shift register output is internally routed to the receive shift register input allowing the system to receive the same data that it was sending. The TX pin is held at logic 1 or mark condition while RTS# and DTR# are de-asserted, and CTS#, DSR# CD# and RI# inputs are ignored. Caution: the RX input must be held to a logic 1 during loopback test else upon exiting the loopback test the UART may detect and report a false “break” signal. FIGURE 7. INTERNAL LOOPBACK VCC TX Transmit Shift Register (THR/FIFO) MCR bit-4=1 als Receive Shift Register n (RHR/FIFO) g RX Si VCC ol r t n RTS# o RTS# C d n a s Lines rol Logic CTS# VCC CTS# u nt B o DTR# a C DTR# at e D s al po ern Pur DSR# DSR# nt al I er n e G OP1# m / e RI# d RI# o M OP2# CD# CD# 13

xr ST16C1550/51 2.97V TO 5.5V UART WITH 16-BYTE FIFO REV. 4.2.1 3.0 UART INTERNAL REGISTERS The 155X has a set of configuration registers selected by address lines A0, A1 and A2. The 16C550 compatible registers can be accessed when LCR[7] = 0 and the baud rate generator divisor registers can be accessed when LCR[7] = 1. The complete register set is shown on Table 3 and Table 4. TABLE 3: ST16C155X UART INTERNAL REGISTERS A2,A1,A0 ADDRESSES REGISTER READ/WRITE COMMENTS 0 0 0 RHR - Receive Holding Register Read-only LCR[7] = 0 THR - Transmit Holding Register Write-only 0 0 0 DLL - Div Latch Low Byte Read/Write LCR[7] = 1 0 0 1 DLM - Div Latch High Byte Read/Write 0 0 1 IER - Interrupt Enable Register Read/Write 0 1 0 ISR - Interrupt Status Register Read-only LCR[7] = 0 FCR - FIFO Control Register Write-only 0 1 1 LCR - Line Control Register Read/Write 1 0 0 MCR - Modem Control Register Read/Write 1 0 1 LSR - Line Status Register Read-only Reserved Write-only LCR[7] = 0 1 1 0 MSR - Modem Status Register Read-only Reserved Write-only 1 1 1 SPR - Scratch Pad Register Read/Write 14

xr ST16C1550/51 REV. 4.2.1 2.97V TO 5.5V UART WITH 16-BYTE FIFO . TABLE 4: INTERNAL REGISTERS DESCRIPTION. SHADED BITS ARE ENABLED WHEN EFR BIT-4=1 ADDRESS REG READ/ BIT-7 BIT-6 BIT-5 BIT-4 BIT-3 BIT-2 BIT-1 BIT-0 COMMENT A2-A0 NAME WRITE 16C550 Compatible Registers 0 0 0 RHR RD Bit-7 Bit-6 Bit-5 Bit-4 Bit-3 Bit-2 Bit-1 Bit-0 0 0 0 THR WR Bit-7 Bit-6 Bit-5 Bit-4 Bit-3 Bit-2 Bit-1 Bit-0 0 0 1 IER RD/WR 0 0 Special 0 Modem RX Line TX RX Mode Status Status Empty Data Enable Int. Int. Int. Int. (Enable Enable Enable Enable Enable ISR bits 5-4, FCR bits 5-4, MCR bits LCR[7] = 0 7, 2) 0 1 0 ISR RD FIFOs FIFOs 0/ 0/ INT INT INT INT Enabled Enabled Source Source Source Source RXRDY TXRDY Bit-3 Bit-2 Bit-1 Bit-0 0 1 0 FCR WR RX FIFO RX FIFO 0/ 0/ DMA TX RX FIFOs Trigger Trigger Mode FIFO FIFO Enable (MSB) (LSB) TX FIFO TX FIFO Enable Reset Reset Trigger Trigger (MSB) (LSB) 0 1 1 LCR RD/WR Divisor Set TX Set Even Parity Stop Word Word Enable Break Parity Parity Enable Bits Length Length Bit-1 Bit-0 1 0 0 MCR RD/WR 0/ 0 0 Internal (OP2#)/ (OP1#)/ RTS# DTR# Loop- INT Output Output Power back Output SOFT Control Control Down Enable Enable Reset Mode 1 0 1 LSR RD RX FIFO THR & THR RX RX RX RX RX Global TSR Empty Break Framing Parity Over- Data LCR[7] = 0 Error Empty Error Error run Ready Error 1 1 0 MSR RD CD# RI# DSR# CTS# Delta Delta Delta Delta Input Input Input Input CD# RI# DSR# CTS# 1 1 1 SPR RD/WR Bit-7 Bit-6 Bit-5 Bit-4 Bit-3 Bit-2 Bit-1 Bit-0 Baud Rate Generator Divisor 0 0 0 DLL RD/WR Bit-7 Bit-6 Bit-5 Bit-4 Bit-3 Bit-2 Bit-1 Bit-0 LCR[7] = 1 0 0 1 DLM RD/WR Bit-7 Bit-6 Bit-5 Bit-4 Bit-3 Bit-2 Bit-1 Bit-0 15

xr ST16C1550/51 2.97V TO 5.5V UART WITH 16-BYTE FIFO REV. 4.2.1 4.0 INTERNAL REGISTER DESCRIPTIONS 4.1 Receive Holding Register (RHR) - Read- Only SEE”RECEIVER” ON PAGE 11. 4.2 Transmit Holding Register (THR) - Write-Only SEE”TRANSMITTER” ON PAGE 10. 4.3 Interrupt Enable Register (IER) - Read/Write The Interrupt Enable Register (IER) masks the interrupts from receive data ready, transmit empty, line status and modem status registers. These interrupts are reported in the Interrupt Status Register (ISR). 4.3.1 IER versus Receive FIFO Interrupt Mode Operation When the receive FIFO (FCR BIT-0 = 1) and receive interrupts (IER BIT-0 = 1) are enabled, the RHR interrupts (see ISR bits 2 and 3) status will reflect the following: A. The receive data available interrupts are issued to the host when the FIFO has reached the programmed trigger level. It will be cleared when the FIFO drops below the programmed trigger level. B. FIFO level will be reflected in the ISR register when the FIFO trigger level is reached. Both the ISR register status bit and the interrupt will be cleared when the FIFO drops below the trigger level. C. The receive data ready bit (LSR BIT-0) is set as soon as a character is transferred from the shift register to the receive FIFO. It is reset when the FIFO is empty. 4.3.2 IER versus Receive/Transmit FIFO Polled Mode Operation When FCR BIT-0 equals a logic 1 for FIFO enable; resetting IER bits 0-3 enables the ST16C155X in the FIFO polled mode of operation. Since the receiver and transmitter have separate bits in the LSR either or both can be used in the polled mode by selecting respective transmit or receive control bit(s). A. LSR BIT-0 indicates there is data in RHR or RX FIFO. B. LSR BIT-1 indicates an overrun error has occurred and that data in the FIFO may not be valid. C. LSR BIT 2-4 provides the type of receive data errors encountered for the data byte in RHR, if any. D. LSR BIT-5 indicates THR is empty. E. LSR BIT-6 indicates when both the transmit FIFO and TSR are empty. F. LSR BIT-7 indicates a data error in at least one character in the RX FIFO. IER[0]: RHR Interrupt Enable The receive data ready interrupt will be issued when RHR has a data character in the non-FIFO mode or when the receive FIFO has reached the programmed trigger level in the FIFO mode. • Logic 0 = Disable the receive data ready interrupt (default). • Logic 1 = Enable the receiver data ready interrupt. IER[1]: THR Interrupt Enable This bit enables the Transmit Ready interrupt which is issued whenever the THR is empty in the non-FIFO mode or when data in the FIFO falls below the programmed trigger level in the FIFO mode. If the THR is empty when this bit is enabled, an interrupt will be generated. Note that this interrupt does not behave in the same manner as the industry standard 16C550. SEE”INTERRUPT CLEARING:” ON PAGE 17. • Logic 0 = Disable Transmit Ready interrupt (default). • Logic 1 = Enable Transmit Ready interrupt. 16

xr ST16C1550/51 REV. 4.2.1 2.97V TO 5.5V UART WITH 16-BYTE FIFO IER[2]: Receive Line Status Interrupt Enable If any of the LSR register bits 1, 2, 3 or 4 is a logic 1, it will generate an interrupt to inform the host controller about the error status of the current data byte in FIFO. LSR bit-1 generates an interrupt immediately when the character has been received. LSR bits 2-4 generate an interrupt when the character with errors is read out of the FIFO. • Logic 0 = Disable the receiver line status interrupt (default). • Logic 1 = Enable the receiver line status interrupt. IER[3]: Modem Status Interrupt Enable • Logic 0 = Disable the modem status register interrupt (default). • Logic 1 = Enable the modem status register interrupt. IER[4]: Reserved IER[5]: Special Mode Enable • Logic 0 = Disable special mode functions (default). • Logic 1 = Enable special mode functions in addition to basic ST16C1450 functions. Enables ISR bits 4-5 (TXRDY/RXRDY), MCR bit-2 (soft reset) and MCR bit-7 (power down) functions. IER[7:6]: Reserved 4.4 Interrupt Status Register (ISR) - Read-Only The UART provides multiple levels of prioritized interrupts to minimize external software interaction. The Interrupt Status Register (ISR) provides the user with six interrupt status bits. Performing a read cycle on the ISR will give the user the current highest pending interrupt level to be serviced, others are queued up to be serviced next. No other interrupts are acknowledged until the pending interrupt is serviced. The Interrupt Source Table, Table 5, shows the data values (bits 0-3) for the interrupt priority levels and the interrupt sources associated with each of these interrupt levels. 4.4.1 Interrupt Generation: • LSR is by any of the LSR bits 1, 2, 3 and 4. • RXRDY is by RX trigger level. • RXRDY Time-out is by a 4-char plus 12 bits delay timer. • TXRDY is by TX trigger level or TX FIFO empty. • MSR is by any of the MSR bits 0, 1, 2 and 3. 4.4.2 Interrupt Clearing: • LSR interrupt is cleared by a read to the LSR register (but flags and tags not cleared until character(s) that generated the interrupt(s) has been emptied or cleared from FIFO). • RXRDY interrupt is cleared by reading data until FIFO falls below the trigger level. • RXRDY Time-out interrupt is cleared by reading RHR until empty. • TXRDY interrupt is cleared by a read to the ISR register AND disabling the TXRDY interrupt (set IER bit-1 = 0), or by loading data into the TX FIFO. • MSR interrupt is cleared by a read to the MSR register. 17

xr ST16C1550/51 2.97V TO 5.5V UART WITH 16-BYTE FIFO REV. 4.2.1 ] TABLE 5: INTERRUPT SOURCE AND PRIORITY LEVEL PRIORITY LEVEL ISR REGISTER STATUS BITS SOURCE OF INTERRUPT BIT-3 BIT-2 BIT-1 BIT-0 1 0 1 1 0 LSR (Receiver Line Status Register) 2 1 1 0 0 RXRDY (Receive Data Time-out) 3 0 1 0 0 RXRDY (Received Data Ready) 4 0 0 1 0 TXRDY (Transmit Ready) 5 0 0 0 0 MSR (Modem Status Register) - 0 0 0 1 None (default) ISR[0]: Interrupt Status • Logic 0 = An interrupt is pending and the ISR contents may be used as a pointer to the appropriate interrupt service routine. • Logic 1 = No interrupt pending (default condition). ISR[3:1]: Interrupt Status These bits indicate the source for a pending interrupt at interrupt priority levels (See Interrupt Source Table 5). ISR[4]: TXRDY This bit represents the compliment (inversion) of the TXRDY status when IER bit-5 is set to a logic 1. See Table 1. ISR[5]: RXRDY This bit represents the compliment (inversion) of the RXRDY status when IER bit-5 is set to a logic 1. See Table 1. ISR[7:6]: FIFO Enable Status These bits are set to a logic 0 when the FIFOs are disabled. They are set to a logic 1 when the FIFOs are enabled. 4.5 FIFO Control Register (FCR) - Write-Only This register is used to enable the FIFOs, clear the FIFOs, set the transmit/receive FIFO trigger levels, and select the DMA mode. The DMA, and FIFO modes are defined as follows: FCR[0]: TX and RX FIFO Enable • Logic 0 = Disable the transmit and receive FIFO (default). • Logic 1 = Enable the transmit and receive FIFOs. This bit must be set to logic 1 when other FCR bits are written or they will not be programmed. FCR[1]: RX FIFO Reset This bit is only active when FCR bit-0 is a ‘1’. • Logic 0 = No receive FIFO reset (default). • Logic 1 = Reset the receive FIFO pointers and FIFO level counter logic (the receive shift register is not cleared or altered). This bit will return to a logic 0 after resetting the FIFO. 18

xr ST16C1550/51 REV. 4.2.1 2.97V TO 5.5V UART WITH 16-BYTE FIFO FCR[2]: TX FIFO Reset This bit is only active when FCR bit-0 is a ‘1’. • Logic 0 = No transmit FIFO reset (default). • Logic 1 = Reset the transmit FIFO pointers and FIFO level counter logic (the transmit shift register is not cleared or altered). This bit will return to a logic 0 after resetting the FIFO. FCR[3]: DMA Mode Select Controls the behavior of the TXRDY# and RXRDY# pins. See DMA operation section for details. • Logic 0 = Normal Operation (default). • Logic 1 = DMA Mode. FCR[5:4]: Transmit FIFO Trigger Select These 2 bits are only active when IER bit-5 is a ‘1’. (logic 0 = default, TX trigger level = 1) These 2 bits set the trigger level for the transmit FIFO. The UART will issue a transmit interrupt when the number of characters in the FIFO falls below the selected trigger level, or when it gets empty in case that the FIFO did not get filled over the trigger level on last re-load. Table 6 shows the selections. FCR[7:6]: Receive FIFO Trigger Select (logic 0 = default, RX trigger level =1) These 2 bits are used to set the trigger level for the receive FIFO. The UART will issue a receive interrupt when the number of the characters in the FIFO crosses the trigger level. Table 6 shows the complete selections. TABLE 6: TRANSMIT AND RECEIVE FIFO TRIGGER LEVEL SELECTION RECEIVE TRANSMIT FCR FCR FCR FCR TRIGGER TRIGGER BIT-7 BIT-6 BIT-5 BIT-4 LEVEL LEVEL 0 0 1 0 1 4 1 0 8 1 1 14 0 0 1 0 1 4 1 0 8 1 1 14 4.6 Line Control Register (LCR) - Read/Write The Line Control Register is used to specify the asynchronous data communication format. The word or character length, the number of stop bits, and the parity are selected by writing the appropriate bits in this register. LCR[1:0]: TX and RX Word Length Select These two bits specify the word length to be transmitted or received. BIT-1 BIT-0 WORD LENGTH 0 0 5 (default) 0 1 6 1 0 7 1 1 8 19

xr ST16C1550/51 2.97V TO 5.5V UART WITH 16-BYTE FIFO REV. 4.2.1 LCR[2]: TX and RX Stop-bit Length Select The length of stop bit is specified by this bit in conjunction with the programmed word length. WORD STOP BIT LENGTH BIT-2 LENGTH (BIT TIME(S)) 0 5,6,7,8 1 (default) 1 5 1-1/2 1 6,7,8 2 LCR[3]: TX and RX Parity Select Parity or no parity can be selected via this bit. The parity bit is a simple way used in communications for data integrity check. See Table 7 for parity selection summary below. • Logic 0 = No parity. • Logic 1 = A parity bit is generated during the transmission while the receiver checks for parity error of the data character received. LCR[4]: TX and RX Parity Select If the parity bit is enabled with LCR bit-3 set to a logic 1, LCR BIT-4 selects the even or odd parity format. • Logic 0 = ODD Parity is generated by forcing an odd number of logic 1’s in the transmitted character. The receiver must be programmed to check the same format (default). • Logic 1 = EVEN Parity is generated by forcing an even number of logic 1’s in the transmitted character. The receiver must be programmed to check the same format. LCR[5]: TX and RX Parity Select If the parity bit is enabled, LCR BIT-5 selects the forced parity format. • LCR[5] = logic 0, parity is not forced (default). • LCR[5] = logic 1 and LCR[4] = logic 0, parity bit is forced to a logical 1 for the transmit and receive data. • LCR[5] = logic 1 and LCR[4] = logic 1, parity bit is forced to a logical 0 for the transmit and receive data. TABLE 7: PARITY SELECTION LCR BIT-5 LCR BIT-4 LCR BIT-3 PARITY SELECTION X X 0 No parity 0 0 1 Odd parity 0 1 1 Even parity 1 0 1 Force parity to mark, “1” 1 1 1 Forced parity to space, “0” LCR[6]: Transmit Break Enable When enabled, the Break control bit causes a break condition to be transmitted (the TX output is forced to a “space’, logic 0, state). This condition remains, until disabled by setting LCR bit-6 to a logic 0. • Logic 0 = No TX break condition (default). • Logic 1 = Forces the transmitter output (TX) to a “space”, logic 0, for alerting the remote receiver of a line break condition. 20

xr ST16C1550/51 REV. 4.2.1 2.97V TO 5.5V UART WITH 16-BYTE FIFO LCR[7]: Baud Rate Divisors Enable Baud rate generator divisor (DLL/DLM) enable. • Logic 0 = Data registers are selected (default). • Logic 1 = Divisor latch registers are selected. 4.7 Modem Control Register (MCR) or General Purpose Outputs Control - Read/Write The MCR register is used for controlling the serial/modem interface signals or general purpose inputs/outputs. MCR[0]: DTR# Output The DTR# pin is a modem control output. If the modem interface is not used, this output may be used as a general purpose output. • Logic 0 = Force DTR# output to a logic 1 (default). • Logic 1 = Force DTR# output to a logic 0. MCR[1]: RTS# Output The RTS# pin is a modem control output. If the modem interface is not used, this output may be used as a general purpose output. • Logic 0 = Force RTS# output to a logic 1 (default). • Logic 1 = Force RTS# output to a logic 0. MCR[2]: OP1# Output/Soft Reset OP1# is not available as an output pin on the 155X. But it is available for use during Internal Loopback Mode. In the Loopback Mode, this bit is used to write the state of the modem RI# interface signal. • Logic 0 = OP1# output (RI# input) is at logic 1 (default). • Logic 1 = OP1# output (RI# input) is at logic 0. In normal operation, this bit is associated with the RST (buffered reset) output pin. The logical state of the RST pin will follow exactly the logical state of the RESET pin. When IER bit-5 = 1, soft resets from MCR bit-2 are ORed with the state of the RESET input pin. Therefore both reset types will be seen at the RST pin. Note that asserting MCR bit-2 does not reset the 155X. • Logic 0 = The RST output pin is a logic 0 (default). • Logic 1 = The RST output pin is a logic 1. MCR[3]: OP2# or INT Output Enable When not in Internal Loopback Mode: • Logic 0 = INT output is three-state (default). • Logic 1 = INT output is active high. OP2# is not available as an output pin on the 155X. But it is available for use during Internal Loopback Mode. In the Loopback Mode, this bit is used to write the state of the modem CD# interface signal. • Logic 0 = OP2# output (CD# input) is a logic 1 (default). • Logic 1 = OP2# output (CD# input) is a logic 0. MCR[4]: Internal Loopback Enable • Logic 0 = Disable loopback mode (default). • Logic 1 = Enable local loopback mode, see loopback section and Figure 7. MCR[6:5]: Reserved 21

xr ST16C1550/51 2.97V TO 5.5V UART WITH 16-BYTE FIFO REV. 4.2.1 MCR[7]: Power Down Enable This bit can only be accessed when IER bit-5 = 1. • Logic 0 = Normal mode (default). • Logic 1 = Power down mode. SEE”POWER DOWN MODE” ON PAGE 12. 4.8 Line Status Register (LSR) - Read Only This register provides the status of data transfers between the UART and the host. If IER bit-2 is set to a logic 1, an LSR interrupt will be generated when the character that is ready to be read from the RX FIFO has an error (parity, framing, overrun, break). LSR[0]: Receive Data Ready Indicator • Logic 0 = No data in receive holding register or FIFO (default). • Logic 1 = Data has been received and is saved in the receive holding register or FIFO. LSR[1]: Receiver Overrun Error Flag • Logic 0 = No overrun error (default). • Logic 1 = Overrun error. A data overrun error condition occurred in the receive shift register. This happens when additional data arrives while the FIFO is full. In this case the previous data in the receive shift register is overwritten. Note that under this condition the data byte in the receive shift register is not transferred into the FIFO, therefore the data in the FIFO is not corrupted by the error. LSR[2]: Receive Data Parity Error Tag • Logic 0 = No parity error (default). • Logic 1 = Parity error. The received character in RHR does not have correct parity information and is suspect. This error is associated with the character available for reading in RHR. LSR[3]: Receive Data Framing Error Tag • Logic 0 = No framing error (default). • Logic 1 = Framing error. The received character did not have a valid stop bit(s). This error is associated with the character available for reading in RHR. LSR[4]: Receive Break Error Tag • Logic 0 = No break condition (default). • Logic 1 = The receiver received a break signal (RX was a logic 0 for at least one character frame time). In the FIFO mode, only one break character is loaded into the FIFO. The break indication remains until the RX input returns to the idle condition, “mark” or logic 1. LSR[5]: Transmit Holding Register Empty Flag This bit is the Transmit Holding Register Empty indicator. The THR bit is set to a logic 1 when the last data byte is transferred from the transmit holding register to the transmit shift register. The bit is reset to logic 0 concurrently with the data loading to the transmit holding register by the host. In the FIFO mode this bit is set when the transmit FIFO is empty, it is cleared when the transmit FIFO contains at least 1 byte. LSR[6]: THR and TSR Empty Flag This bit is set to a logic 1 whenever the transmitter goes idle. It is set to logic 0 whenever either the THR or TSR contains a data character. In the FIFO mode this bit is set to a logic 1 whenever the transmit FIFO and transmit shift register are both empty. LSR[7]: Receive FIFO Data Error Flag • Logic 0 = No FIFO error (default). • Logic 1 = A global indicator for the sum of all error bits in the RX FIFO. At least one parity error, framing error or break indication is in the FIFO data. This bit clears when there is no more error(s) in any of the bytes in the RX FIFO. 22

xr ST16C1550/51 REV. 4.2.1 2.97V TO 5.5V UART WITH 16-BYTE FIFO 4.9 Modem Status Register (MSR) - Read Only This register provides the current state of the modem interface input signals. Lower four bits of this register are used to indicate the changed information. These bits are set to a logic 1 whenever a signal from the modem changes state. These bits may be used for general purpose inputs when they are not used with modem signals. MSR[0]: Delta CTS# Input Flag • Logic 0 = No change on CTS# input (default). • Logic 1 = The CTS# input has changed state since the last time it was monitored. A modem status interrupt will be generated if MSR interrupt is enabled (IER bit-3). MSR[1]: Delta DSR# Input Flag • Logic 0 = No change on DSR# input (default). • Logic 1 = The DSR# input has changed state since the last time it was monitored. A modem status interrupt will be generated if MSR interrupt is enabled (IER bit-3). MSR[2]: Delta RI# Input Flag • Logic 0 = No change on RI# input (default). • Logic 1 = The RI# input has changed from a logic 0 to a logic 1, ending of the ringing signal. A modem status interrupt will be generated if MSR interrupt is enabled (IER bit-3). MSR[3]: Delta CD# Input Flag • Logic 0 = No change on CD# input (default). • Logic 1 = Indicates that the CD# input has changed state since the last time it was monitored. A modem status interrupt will be generated if MSR interrupt is enabled (IER bit-3). MSR[4]: CTS Input Status CTS# (active high, logical 1). Normally this bit is the compliment of the CTS# input. In the loopback mode, this bit is equivalent to bit-1 in the MCR register. The CTS# input may be used as a general purpose input when the modem interface is not used. MSR[5]: DSR Input Status DSR# (active high, logical 1). Normally this bit is the compliment of the DSR# input. In the loopback mode, this bit is equivalent to bit-0 in the MCR register. The DSR# input may be used as a general purpose input when the modem interface is not used. MSR[6]: RI Input Status RI# (active high, logical 1). Normally this bit is the compliment of the RI# input. In the loopback mode this bit is equivalent to bit-2 in the MCR register. The RI# input may be used as a general purpose input when the modem interface is not used. MSR[7]: CD Input Status CD# (active high, logical 1). Normally this bit is the compliment of the CD# input. In the loopback mode this bit is equivalent to bit-3 in the MCR register. The CD# input may be used as a general purpose input when the modem interface is not used. 4.10 Scratch Pad Register (SPR) - Read/Write This is a 8-bit general purpose register for the user to store temporary data. The content of this register is preserved during sleep mode but becomes 0xFF (default) after a reset or a power off-on cycle. 23

xr ST16C1550/51 2.97V TO 5.5V UART WITH 16-BYTE FIFO REV. 4.2.1 TABLE 8: UART RESET CONDITIONS REGISTERS RESET STATE DLL Bits 7-0 = 0xXX DLM Bits 7-0 = 0xXX RHR Bits 7-0 = 0xXX THR Bits 7-0 = 0xXX IER Bits 7-0 = 0x00 FCR Bits 7-0 = 0x00 ISR Bits 7-0 = 0x01 LCR Bits 7-0 = 0x00 MCR Bits 7-0 = 0x00 LSR Bits 7-0 = 0x60 MSR Bits 3-0 = Logic 0 Bits 7-4 = Logic levels of the inputs inverted SPR Bits 7-0 = 0xFF I/O SIGNALS RESET STATE TX Logic 1 RTS# Logic 1 DTR# Logic 1 RST Logic 1 INT Three-State Condition 24

xr ST16C1550/51 REV. 4.2.1 2.97V TO 5.5V UART WITH 16-BYTE FIFO ABSOLUTE MAXIMUM RATINGS Power Supply Range 7 Volts Voltage at Any Pin GND-0.3 V to 7 V Operating Temperature -40o to +85oC Storage Temperature -65o to +150oC Package Dissipation 500 mW TYPICAL PACKAGE THERMAL RESISTANCE DATA (MARGIN OF ERROR: ± 15%) Thermal Resistance (48-TQFP) theta-ja = 59oC/W, theta-jc = 16oC/W Thermal Resistance (28-PLCC) theta-ja = 55oC/W, theta-jc = 28oC/W ELECTRICAL CHARACTERISTICS DC ELECTRICAL CHARACTERISTICS UNLESS OTHERWISE NOTED: TA=0O TO 70OC (-40O TO +85OC FOR INDUSTRIAL GRADE PACKAGE), VCC IS 2.97V TO 5.5V LIMITS LIMITS SYMBOL PARAMETER 3.3V 5.0V UNITS CONDITIONS MIN MAX MIN MAX V Clock Input Low Level -0.3 0.6 -0.5 0.6 V ILCK V Clock Input High Level 2.4 VCC 3.0 VCC V IHCK V Input Low Voltage -0.3 0.8 -0.5 0.8 V IL V Input High Voltage 2.0 VCC 2.2 VCC V IH V Output Low Voltage 0.4 V I = 6 mA OL OL V Output Low Voltage 0.4 V I = 4 mA OL OL V Output High Voltage 2.4 V I = -6 mA OH OH V Output High Voltage 2.0 V I = -1 mA OH OH I Input Low Leakage Current ±10 ±10 uA IL I Input High Leakage Current ±10 ±10 uA IH C Input Pin Capacitance 5 5 pF IN I Power Supply Current 1.3 3 mA CC I Power Down Current 50 200 uA See Test 1 PWRDN Test 1: The following inputs should remain steady at VCC or GND state to minimize Power Down current: A0-A2, D0-D7, IOR#, IOW#, CS# and modem inputs. Also, RX input must idle at logic 1 state while in Power Down mode. 25

xr ST16C1550/51 2.97V TO 5.5V UART WITH 16-BYTE FIFO REV. 4.2.1 AC ELECTRICAL CHARACTERISTICS TA=0O TO 70OC (-40O TO +85OC FOR INDUSTRIAL GRADE PACKAGE), VCC IS 2.97V TO 5.5V LIMITS LIMITS SYMBOL PARAMETER 3.3V 5.0V UNIT CONDITIONS MIN MAX MIN MAX CLK Clock Pulse Duration 63 21 ns OSC Oscillator/External Clock Frequency 8 24 MHz T Address Setup Time 5 0 ns AS T Address Hold Time 10 5 ns AH T Chip Select Width 50 40 ns CS T IOR# Strobe Width 35 25 ns RD T Read/Write Cycle Delay 40 30 ns DY T Data Access Time 35 25 ns RDV T Data Disable Time 0 25 0 15 ns DD T IOW# Strobe Width 40 25 ns WR T Data Setup Time 20 15 ns DS T Data Hold Time 5 5 ns DH T Delay From IOW# To Output 50 40 ns 100 pF load WDO T Delay To Set Interrupt From MODEM Input 40 35 ns 100 pF load MOD T Delay To Reset Interrupt From IOR# 40 35 ns 100 pF load RSI T Delay From Stop To Set Interrupt 1 1 Bclk SSI T Delay From IOR# To Reset Interrupt 45 40 ns 100 pF load RRI T Delay From Stop To Interrupt 45 40 ns SI T Delay From Initial INT Reset To Transmit Start 8 24 8 24 Bclk INT T Delay From Stop To Reset RXRDY 1 1 Bclk SSR T Delay From IOR# To Set RXRDY 45 40 ns RR T Delay From IOW# To Reset TXRDY 45 40 ns WT T Delay From Center of Start To Set TXRDY 8 8 Bclk SRT T Reset Pulse Width 40 40 ns RST N Baud Rate Divisor 1 216-1 1 216-1 - Bclk Baud Clock 16X of data rate Hz 26

xr ST16C1550/51 REV. 4.2.1 2.97V TO 5.5V UART WITH 16-BYTE FIFO FIGURE 8. CLOCK TIMING CLK CLK EXTERNAL CLOCK OSC FIGURE 9. MODEM INPUT/OUTPUT TIMING IOW# Active IOW T WDO RTS# Change of state Change of state DTR# CD# CTS# Change of state Change of state DSR# T T MOD MOD INT Active Active Active T RSI IOR# Active Active Active IOR T MOD RI# Change of state 27

xr ST16C1550/51 2.97V TO 5.5V UART WITH 16-BYTE FIFO REV. 4.2.1 FIGURE 10. DATA BUS READ TIMING A0- Valid Valid A2 Address Address T T AS T AS T AH AH CS2# T T CS CS T DY IOR# T T RD RD T T T DD T DD RDV RDV Valid Valid D0-D7 Data Data FIGURE 11. DATA BUS WRITE TIMING A0- Valid Valid A2 Address Address T T AS T AS T AH AH CS2# T T CS CS T DY IOW# T T WR WR T T T DH T DH DS DS Valid Valid D0-D7 Data Data 28

xr ST16C1550/51 REV. 4.2.1 2.97V TO 5.5V UART WITH 16-BYTE FIFO FIGURE 12. RECEIVE READY & INTERRUPT TIMING [NON-FIFO MODE] RX Start Stop D0:D7 D0:D7 D0:D7 Bit Bit T T T SSR SSR SSR 1 Byte 1 Byte 1 Byte in RHR in RHR in RHR INT T T T SSR SSR SSR Active Active Active RXRDY Data Data Data (ISR bit-5) Ready Ready Ready T T T RR RR RR IOR# (Reading data out of RHR) RXNFM FIGURE 13. TRANSMIT READY & INTERRUPT TIMING [NON-FIFO MODE] TX Start Stop (Unloading) D0:D7 D0:D7 D0:D7 Bit Bit IER[1] INT cleared* IER[1] INT cleared* IER[1] INT cleared* enabled enabled enabled INT* T T T SRT SRT SRT TXRDY (ISR bit-4) T T T WT WT WT IOW# (Loading data into THR) *INT is cleared when the ISR is read and IER[1] is disabled. TXNonFIFO 29

xr ST16C1550/51 2.97V TO 5.5V UART WITH 16-BYTE FIFO REV. 4.2.1 FIGURE 14. RECEIVE READY & INTERRUPT TIMING [FIFO MODE, DMA DISABLED] Start Bit RX S D0:D7 S D0:D7 T D0:D7 S D0:D7 T S D0:D7 T S D0:D7 T S D0:D7 T Stop RX FIFO drops T Bit SSI below RX Trigger Level INT T RX FIFO fills up to RX FIFO First Byte is SSR Trigger Level or RX Data Empties Received in Timeout RX FIFO RXRDY (ISR bit-5) T T RRI RR IOR# (Reading data out of RX FIFO) RXINTDMA# FIGURE 15. RECEIVE READY & INTERRUPT TIMING [FIFO MODE, DMA ENABLED] Start Stop Bit Bit RX S D0:D7 S D0:D7 T D0:D7 S D0:D7 T S D0:D7 T S D0:D7 T S D0:D7 T RX FIFO drops T SSI below RX Trigger Level INT RX FIFO fills up to RX T FIFO Trigger Level or RX Data SSR Empties Timeout RXRDY (ISR bit-5) T T RRI RR IOR# (Reading data out of RX FIFO) RXFIFODMA 30

xr ST16C1550/51 REV. 4.2.1 2.97V TO 5.5V UART WITH 16-BYTE FIFO FIGURE 16. TRANSMIT READY & INTERRUPT TIMING [FIFO MODE, DMA MODE DISABLED] Start Stop Last Data Byte TX FIFO Bit Bit Transmitted Empty TX S D0:D7 T S D0:D7 TS D0:D7 T T S D0:D7 T S D0:D7 T S D0:D7 T T INT Cleared* SI T SRT INT* TX FIFO Empty TX FIFO drops TX FIFO above trigger below trigger level level and IER[1] enabled. TXRDY (ISR bit-4) Data in TX FIFO T WT IOW# (Loading data into FIFO) *INT is cleared when the ISR is read and IER[1] is disabled. TXDMA# FIGURE 17. TRANSMIT READY & INTERRUPT TIMING [FIFO MODE, DMA MODE ENABLED] Start Stop Last Data Byte Bit Bit Transmitted TX S D0:D7 T S D0:D7 T D0:D7 S D0:D7 T S D0:D7 T S D0:D7 T S D0:D7 T INT cleared* TSRT TSI INT* TX FIFO drops TX FIFO above trigger below trigger level level and IER[1] enabled. TXRDY (ISR bit-4) At least 1 TX FIFO empty location Full in FIFO T WT IOW# (Loading data into FIFO) *INT cleared when the ISR is read and IER[1] is disabled. TXDMA 31

xr ST16C1550/51 2.97V TO 5.5V UART WITH 16-BYTE FIFO REV. 4.2.1 PACKAGE DIMENSIONS (48 PIN TQFP - 7 X 7 X 1 mm) D D 1 36 25 37 24 D1 D 48 13 1 1 2 B A e 2 C A Seating α Plane A 1 L Note: The control dimension is the millimeter column INCHES MILLIMETERS SYMBOL MIN MAX MIN MAX A 0.039 0.047 1.00 1.20 A1 0.002 0.006 0.05 0.15 A2 0.037 0.041 0.95 1.05 B 0.007 0.011 0.17 0.27 C 0.004 0.008 0.09 0.20 D 0.346 0.362 8.80 9.20 D1 0.272 0.280 6.90 7.10 e 0.020 BSC 0.50 BSC L 0.018 0.030 0.45 0.75 α 0° 7° 0° 7° 32

xr ST16C1550/51 REV. 4.2.1 2.97V TO 5.5V UART WITH 16-BYTE FIFO PACKAGE DIMENSIONS (28 PIN PLCC) Note: The control dimension is the inch column INCHES MILLIMETERS SYMBOL MIN MAX MIN MAX A 0.165 0.180 4.19 4.57 A1 0.090 0.120 2.29 3.05 A2 0.020 - 0.51 - B 0.013 0.021 0.33 0.53 B1 0.026 0.032 0.66 0.81 C 0.008 0.013 0.19 0.32 D 0.485 0.495 12.32 12.57 D1 0.450 0.456 11.43 11.58 D2 0.390 0.430 9.91 10.92 D3 0.300 typ. 7.62 typ. e 0.050 BSC 1.27 BSC H1 0.042 0.056 1.07 1.42 H2 0.042 0.048 1.07 1.22 R 0.025 0.045 0.64 1.14 33

xr ST16C1550/51 2.97V TO 5.5V UART WITH 16-BYTE FIFO REV. 4.2.1 REVISION HISTORY DATE REVISION DESCRIPTION January 2003 Rev 4.0.0 Changed to standard style format. Clarified that the TX interrupt is not MS Windows compatible. Clarified timing diagrams. Renamed Rclk (Receive Clock) to Bclk (Baud Clock) and timing symbols. Added T , T and OSC. AH CS April 2003 Rev 4.0.1 Updated Ordering Information. September 2003 Rev 4.1.0 Added Status Column to Ordering Information. October 2003 Rev 4.2.0 Clarified compatibility to industry standard 16550 and MS Windows standard serial port driver in General Description. Removed Auto RTS flow control from MCR bit-1 description since that feature is not available in this device. August 2005 Rev 4.2.1 Removed discontinued 28-pin PDIP from datasheet. NOTICE EXAR Corporation reserves the right to make changes to the products contained in this publication in order to improve design, performance or reliability. EXAR Corporation assumes no responsibility for the use of any circuits described herein, conveys no license under any patent or other right, and makes no representation that the circuits are free of patent infringement. Charts and schedules contained here in are only for illustration purposes and may vary depending upon a user’s specific application. While the information in this publication has been carefully checked; no responsibility, however, is assumed for inaccuracies. EXAR Corporation does not recommend the use of any of its products in life support applications where the failure or malfunction of the product can reasonably be expected to cause failure of the life support system or to significantly affect its safety or effectiveness. Products are not authorized for use in such applications unless EXAR Corporation receives, in writing, assurances to its satisfaction that: (a) the risk of injury or damage has been minimized; (b) the user assumes all such risks; (c) potential liability of EXAR Corporation is adequately protected under the circumstances. Copyright 2005 EXAR Corporation Datasheet August 2005. Send your UART technical inquiry with technical details to hotline: uarttechsupport@exar.com. Reproduction, in part or whole, without the prior written consent of EXAR Corporation is prohibited. 34

xr ST16C1550/51 REV. 4.2.1 2.97V TO 5.5V UART WITH 16-BYTE FIFO TABLE OF CONTENTS GENERAL DESCRIPTION.................................................................................................1 FEATURES.....................................................................................................................................................1 APPLICATIONS...............................................................................................................................................1 FIGURE 1. BLOCK DIAGRAM.............................................................................................................................................................1 FIGURE 2. ST16C1550 PINOUTS.....................................................................................................................................................2 FIGURE 3. ST16C1551 PINOUTS.....................................................................................................................................................3 ORDERING INFORMATION................................................................................................................................4 PIN DESCRIPTIONS .........................................................................................................5 DATA BUS INTERFACE.............................................................................................................................................5 MODEM OR SERIAL I/O INTERFACE.......................................................................................................................5 ANCILLARY SIGNALS................................................................................................................................................6 1.0 PRODUCT DESCRIPTION ....................................................................................................................7 Enhanced Features.....................................................................................................................................................7 ....................................................................................................................................................................................7 Data Rate....................................................................................................................................................................7 2.0 FUNCTIONAL DESCRIPTIONS ............................................................................................................8 2.1 INTERNAL REGISTERS ................................................................................................................................... 8 2.2 DMA MODE ...................................................................................................................................................... 8 TABLE 1: TXRDY AND RXRDY BITS IN FIFO AND DMA MODE........................................................................................................8 2.3 CRYSTAL OSCILLATOR OR EXTERNAL CLOCK ......................................................................................... 8 FIGURE 4. TYPICAL OSCILLATOR CONNECTIONS.................................................................................................................................9 2.4 PROGRAMMABLE BAUD RATE GENERATOR ............................................................................................. 9 TABLE 2: TYPICAL DATA RATES WITH A 14.7456 MHZ CRYSTAL OR EXTERNAL CLOCK......................................................................10 2.5 TRANSMITTER ............................................................................................................................................... 10 2.5.1 TRANSMIT HOLDING REGISTER (THR) - WRITE ONLY......................................................................................... 10 2.5.2 TRANSMITTER OPERATION IN NON-FIFO MODE.................................................................................................. 11 FIGURE 5. TRANSMITTER OPERATION IN NON-FIFO MODE..............................................................................................................11 2.5.3 TRANSMITTER OPERATION IN FIFO MODE........................................................................................................... 11 2.6 RECEIVER ...................................................................................................................................................... 11 2.6.1 RECEIVE HOLDING REGISTER (RHR) - READ-ONLY............................................................................................ 11 FIGURE 6. RECEIVER OPERATION IN NON-FIFO MODE....................................................................................................................12 2.7 SPECIAL (ENHANCED FEATURE) MODE ................................................................................................... 12 2.7.1 SOFT RESET.............................................................................................................................................................. 12 2.7.2 POWER DOWN MODE............................................................................................................................................... 12 2.7.3 TXRDY AND RXRDY BITS......................................................................................................................................... 12 2.8 INTERNAL LOOPBACK ................................................................................................................................ 13 FIGURE 7. INTERNAL LOOPBACK.....................................................................................................................................................13 3.0 UART INTERNAL REGISTERS ...........................................................................................................14 TABLE 3: ST16C155X UART INTERNAL REGISTERS...............................................................................................................14 TABLE 4: INTERNAL REGISTERS DESCRIPTION. SHADED BITS ARE ENABLED WHEN EFR BIT-4=1.........................................15 4.0 INTERNAL REGISTER DESCRIPTIONS ............................................................................................16 4.1 RECEIVE HOLDING REGISTER (RHR) - READ- ONLY ............................................................................... 16 4.2 TRANSMIT HOLDING REGISTER (THR) - WRITE-ONLY ............................................................................ 16 4.3 INTERRUPT ENABLE REGISTER (IER) - READ/WRITE ............................................................................. 16 4.3.1 IER VERSUS RECEIVE FIFO INTERRUPT MODE OPERATION............................................................................. 16 4.3.2 IER VERSUS RECEIVE/TRANSMIT FIFO POLLED MODE OPERATION................................................................ 16 4.4 INTERRUPT STATUS REGISTER (ISR) - READ-ONLY ............................................................................... 17 4.4.1 INTERRUPT GENERATION:...................................................................................................................................... 17 4.4.2 INTERRUPT CLEARING:........................................................................................................................................... 17 TABLE 5: INTERRUPT SOURCE AND PRIORITY LEVEL.......................................................................................................................18 4.5 FIFO CONTROL REGISTER (FCR) - WRITE-ONLY ..................................................................................... 18 TABLE 6: TRANSMIT AND RECEIVE FIFO TRIGGER LEVEL SELECTION..............................................................................................19 4.6 LINE CONTROL REGISTER (LCR) - READ/WRITE ..................................................................................... 19 TABLE 7: PARITY SELECTION..........................................................................................................................................................20 4.7 MODEM CONTROL REGISTER (MCR) OR GENERAL PURPOSE OUTPUTS CONTROL - READ/WRITE 21 4.8 LINE STATUS REGISTER (LSR) - READ ONLY ........................................................................................... 22 4.9 MODEM STATUS REGISTER (MSR) - READ ONLY .................................................................................... 23 4.10 SCRATCH PAD REGISTER (SPR) - READ/WRITE .................................................................................... 23 TABLE 8: UART RESET CONDITIONS........................................................................................................................................24 I

xr ST16C1550/51 2.97V TO 5.5V UART WITH 16-BYTE FIFO REV. 4.2.1 ABSOLUTE MAXIMUM RATINGS...................................................................................25 TYPICAL PACKAGE THERMAL RESISTANCE DATA (MARGIN OF ERROR: ± 15%)..................................................25 ELECTRICAL CHARACTERISTICS................................................................................25 DC ELECTRICAL CHARACTERISTICS..............................................................................................................25 AC ELECTRICAL CHARACTERISTICS..............................................................................................................26 TA=0O TO 70OC (-40O TO +85OC FOR INDUSTRIAL GRADE PACKAGE), VCC IS 2.97V TO 5.5V......................26 FIGURE 8. CLOCK TIMING...............................................................................................................................................................27 FIGURE 9. MODEM INPUT/OUTPUT TIMING......................................................................................................................................27 FIGURE 10. DATA BUS READ TIMING..............................................................................................................................................28 FIGURE 11. DATA BUS WRITE TIMING.............................................................................................................................................28 FIGURE 12. RECEIVE READY & INTERRUPT TIMING [NON-FIFO MODE]............................................................................................29 FIGURE 13. TRANSMIT READY & INTERRUPT TIMING [NON-FIFO MODE]..........................................................................................29 FIGURE 14. RECEIVE READY & INTERRUPT TIMING [FIFO MODE, DMA DISABLED]..........................................................................30 FIGURE 15. RECEIVE READY & INTERRUPT TIMING [FIFO MODE, DMA ENABLED]...........................................................................30 FIGURE 16. TRANSMIT READY & INTERRUPT TIMING [FIFO MODE, DMA MODE DISABLED]..............................................................31 FIGURE 17. TRANSMIT READY & INTERRUPT TIMING [FIFO MODE, DMA MODE ENABLED]...............................................................31 PACKAGE DIMENSIONS (48 PIN TQFP - 7 X 7 X 1 MM)....................................................................................32 PACKAGE DIMENSIONS (28 PIN PLCC).........................................................................................................33 REVISION HISTORY.......................................................................................................................................34 TABLE OF CONTENTS ............................................................................................................I II