M41T81S Serial access real-time clock (RTC) with alarms − Datasheet production data Features ■ Counters for tenths/hundredths of seconds, seconds, minutes, hours, day, date, month, 8 year, and century ■ 32 KHz crystal oscillator with integrated load 1 capacitance (12.5 pf) which provides exceptional oscillator stability and high crystal SO8 series resistance operation) 8-pin SOIC ■ Oscillator stop detection (monitors clock operation) ■ Serial interface supports I2C bus (400 kHz protocol) ■ Ultra-low battery supply current of 0.6 µA (typ) 18 ■ 2.0 to 5.5 V clock operating voltage 1 ■ Automatic switchover and deselect circuitry SOX18 (fixed reference) which provides full operation 18-pin (300 mil) SOIC in 3.0 V applications) with embedded crystal ■ V = 2.7 to 5.5 V CC ■ 2.5 V ≤ V ≤ 2.7 V PFD ■ Power-down time-stamp (HT bit) which allows determination of time elapsed in battery backup ■ Battery low flag ■ Programmable alarm and interrupt function (valid even during battery backup mode) ■ Accurate programmable watchdog timer (from 62.5 ms to 128 s) ■ Software clock calibration (to compensate for crystal deviation due to temperature) ■ Operating temperature of –40 to 85 °C ■ Package options include an 8-lead SOIC or 18-lead embedded crystal SOIC May 2012 Doc ID 10773 Rev 7 1/32 This is information on a product in full production. www.st.com 1

Contents M41T81S Contents 1 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2 Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2-wire bus characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Bus not busy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Start data transfer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Stop data transfer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Data valid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Acknowledge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 READ mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 WRITE mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Data retention mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3 Clock operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Power-down time-stamp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Clock registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Calibrating the clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Setting alarm clock registers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Watchdog timer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Square wave output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Century bit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Battery low warning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Oscillator fail detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Oscillator fail interrupt enable. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Output driver pin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Preferred initial power-on default . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 5 DC and AC parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 6 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 2/32 Doc ID 10773 Rev 7

M41T81S Contents 7 Part numbering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 8 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Doc ID 10773 Rev 7 3/32

List of tables M41T81S List of tables Table 1. Signal names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Table 2. Clock register map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Table 3. Alarm repeat modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Table 4. Square wave output frequency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Table 5. Preferred default values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Table 6. Absolute maximum ratings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Table 7. Operating and AC measurement conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Table 8. Capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Table 9. DC characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Table 10. Crystal electrical characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Table 11. Power down/up AC characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Table 12. Power down/up trip points DC characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Table 13. AC characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Table 14. SO8 – 8-lead plastic small outline (150 mils body width), package mechanical data. . . . . 28 Table 15. SOX18 – 18-lead plastic small outline, 300 mils, embedded crystal, package mechanical data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Table 16. Ordering information. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 4/32 Doc ID 10773 Rev 7

M41T81S List of figures List of figures Figure 1. Logic diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 2. 8-pin SOIC (M) connections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 3. 18-pin, 300 mil SOIC (MY) connections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 4. Block diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 5. Serial bus data transfer sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 6. Acknowledgement sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 7. Slave address location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 8. READ mode sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 9. Alternative READ mode sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 10. WRITE mode sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 11. Crystal accuracy across temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 12. Clock calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 13. Alarm interrupt reset waveform. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 14. Backup mode alarm waveform. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 15. AC measurement I/O waveform. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Figure 16. Power down/up mode AC waveforms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Figure 17. Bus timing requirements sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Figure 18. SO8 – 8-lead plastic small package outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Figure 19. SOX18 – 18-lead plastic small outline, 300 mils, embedded crystal, outline . . . . . . . . . . . 29 Doc ID 10773 Rev 7 5/32

Description M41T81S 1 Description The M41T81S is a low-power serial real-time clock (RTC) with a built-in 32.768 kHz oscillator (external crystal controlled). Eight bytes of the SRAM are used for the clock/calendar function and are configured in binary-coded decimal (BCD) format. An additional 12 bytes of SRAM provide status/control of alarm, watchdog and square wave functions. Addresses and data are transferred serially via a two line, bidirectional I2C interface. The built-in address register is incremented automatically after each WRITE or READ data byte. The M41T81S has a built-in power sense circuit which detects power failures and automatically switches to the battery supply when a power failure occurs. The energy needed to sustain the clock operations can be supplied by a small lithium button supply when a power failure occurs. Functions available to the user include a non-volatile, time-of- day clock/calendar, alarm interrupts, watchdog timer and programmable square wave output. The eight clock address locations contain the century, year, month, date, day, hour, minute, second and tenths/hundredths of a second in 24-hour BCD format. Corrections for 28, 29 (leap year - valid until year 2100), 30 and 31 day months are made automatically. The M41T81S is supplied in either an 8-pin SOIC or an 18-pin 300 mil SOIC package which includes an embedded 32 KHz crystal. The 18-pin, embedded crystal SOIC requires only a user-supplied battery to provide non- volatile operation. Figure 1. Logic diagram VCC VBAT XI(1) XO(1) M41T81S IRQ/FT/OUT/SQW SCL SDA VSS AI09160 1. For SO8 package only 6/32 Doc ID 10773 Rev 7

M41T81S Description Table 1. Signal names XI(1) Oscillator input XO(1) Oscillator output IRQ/OUT/FT/SQW Interrupt / output driver / frequency test / square wave (open drain) SDA Serial data input/output SCL Serial clock input V Battery supply voltage BAT V Supply voltage CC V Ground SS NC(2) No connect NF(2) No function 1. For SO8 package only. 2. NC and NF pins should be tied to V . SS Figure 2. 8-pin SOIC (M) connections XI 1 8 V CC XO 2 7 IRQ/FT/OUT/SQW(1) VBAT 3 M41T81S 6 SCL V 4 5 SDA SS AI09161 1. Open drain output Figure 3. 18-pin, 300 mil SOIC (MY) connections NC 1 18 NC NF(1) 2 17 NF(1) NF(1) 3 16 NF(1) NC 4 15 VCC NC 5 M41T81S 14 NC NC 6 13 IRQ/FT/OUT/SQW(2) NC 7 12 NC VBAT 8 11 SCL VSS 9 10 SDA AI09162 1. NC and NF pins should be tied to V . Pins 2 and 3 are internally shorted together. Pins 17 and 16 are SS internally shorted together. 2. Open drain output Doc ID 10773 Rev 7 7/32

Description M41T81S Figure 4. Block diagram REAL TIME CLOCK CALENDAR OSCILLATOR FAIL OFIE CIRCUIT 32KHz CRYSTAL OSCILLATOR RTC W/ALARM AFE & CALIBRATION WATCHDOG IRQ/FT/OUT/SQW(1) SDA I2C INTERFACE SQWE(2) SQUARE WAVE SCL FREQUENCY TEST FT WRITE PROTECT OUTPUT DRIVER OUT INTERNAL POWER V CC V BAT V SO COMPARE V PFD AI09163 1. Open drain output 2. Square wave function has the highest priority on IRQ/FT/OUT/SQW output. 8/32 Doc ID 10773 Rev 7

M41T81S Operation 2 Operation The M41T81S clock operates as a slave device on the serial bus. Access is obtained by implementing a start condition followed by the correct slave address (D0h). The 20 bytes contained in the device can then be accessed sequentially in the following order: 1. Tenths/hundredths of a second register 2. Seconds register 3. Minutes register 4. Century/hours register 5. Day register 6. Date register 7. Month register 8. Year register 9. Calibration register 10. Watchdog register 11 - 15. Alarm registers 16. Flags register 17 - 19. Reserved 20. Square wave register The M41T81S clock continually monitors V for an out-of-tolerance condition. Should V CC CC fall below V , the device terminates an access in progress and resets the device address PFD counter. Inputs to the device will not be recognized at this time to prevent erroneous data from being written to the device from a an out-of-tolerance system. Once V falls below the CC switchover voltage (V ), the device automatically switches over to the battery and powers SO down into an ultra-low current mode of operation to preserve battery life. If V is less than BAT V , the device power is switched from V to V when V drops below V . If V is PFD CC BAT CC BAT BAT greater than V , the device power is switched from V to V when V drops below PFD CC BAT CC V . Upon power-up, the device switches from battery to V at V . When V rises PFD CC SO CC above V , it will recognize the inputs. PFD For more information on battery storage life refer to application note AN1012, "Predicting the battery life and data retention period of NVRAMs and serial RTCs" . 2-wire bus characteristics The bus is intended for communication between different ICs. It consists of two lines: a bi- directional data signal (SDA) and a clock signal (SCL). Both the SDA and SCL lines must be connected to a positive supply voltage via a pull-up resistor. Doc ID 10773 Rev 7 9/32

Operation M41T81S The following protocol has been defined: ● Data transfer may be initiated only when the bus is not busy. ● During data transfer, the data line must remain stable whenever the clock line is high. ● Changes in the data line, while the clock line is high, will be interpreted as control signals. Accordingly, the following bus conditions have been defined: Bus not busy Both data and clock lines remain high. Start data transfer A change in the state of the data line, from high to low, while the clock is high, defines the START condition. Stop data transfer A change in the state of the data line, from low to high, while the clock is high, defines the STOP condition. Data valid The state of the data line represents valid data when after a start condition, the data line is stable for the duration of the high period of the clock signal. The data on the line may be changed during the low period of the clock signal. There is one clock pulse per bit of data. Each data transfer is initiated with a start condition and terminated with a stop condition. The number of data bytes transferred between the start and stop conditions is not limited. The information is transmitted byte-wide and each receiver acknowledges with a ninth bit. By definition a device that gives out a message is called “transmitter,” the receiving device that gets the message is called “receiver.” The device that controls the message is called “master.” The devices that are controlled by the master are called “slaves.” Acknowledge Each byte of eight bits is followed by one acknowledge bit. This acknowledge bit is a low level put on the bus by the receiver whereas the master generates an extra acknowledge related clock pulse. A slave receiver which is addressed is obliged to generate an acknowledge after the reception of each byte that has been clocked out of the slave transmitter. The device that acknowledges has to pull down the SDA line during the acknowledge clock pulse in such a way that the SDA line is a stable low during the high period of the acknowledge related clock pulse. Of course, setup and hold times must be taken into account. A master receiver must signal an end of data to the slave transmitter by not generating an acknowledge on the last byte that has been clocked out of the slave. In this 10/32 Doc ID 10773 Rev 7

M41T81S Operation case the transmitter must leave the data line high to enable the master to generate the STOP condition. Figure 5. Serial bus data transfer sequence DATA LINE STABLE DATA VALID CLOCK DATA START CHANGE OF STOP CONDITION DATA ALLOWED CONDITION AI00587 Figure 6. Acknowledgement sequence CLOCK PULSE FOR START ACKNOWLEDGEMENT SCL FROM 1 2 8 9 MASTER DATA OUTPUT MSB LSB BY TRANSMITTER DATA OUTPUT BY RECEIVER AI00601 READ mode In this mode the master reads the M41T81S slave after setting the slave address (see Figure8 on page12). Following the WRITE mode control bit (R/W=0) and the acknowledge bit, the word address 'An' is written to the on-chip address pointer. Next the START condition and slave address are repeated followed by the READ mode control bit (R/W=1). At this point the master transmitter becomes the master receiver. The data byte which was addressed will be transmitted and the master receiver will send an acknowledge bit to the slave transmitter. The address pointer is only incremented on reception of an acknowledge clock. The M41T81S slave transmitter will now place the data byte at address An+1 on the bus, the master receiver reads and acknowledges the new byte and the address pointer is incremented to “An+2.” This cycle of reading consecutive addresses will continue until the master receiver sends a STOP condition to the slave transmitter. The system-to-user transfer of clock data will be halted whenever the address being read is a clock address (00h to 07h). The update will resume due to a stop condition or when the pointer increments to any non-clock address (08h-13h). Doc ID 10773 Rev 7 11/32

Operation M41T81S Note: This is true both in READ mode and WRITE mode. An alternate READ mode may also be implemented whereby the master reads the M41T81S slave without first writing to the (volatile) address pointer. The first address that is read is the last one stored in the pointer (see Figure9 on page12). Figure 7. Slave address location R/W START SLAVE ADDRESS A B S B M S L 1 1 0 1 0 0 0 AI00602 Figure 8. READ mode sequence T T R R BMUASS TAECRTIVITY: STA R/W STA R/W WORD SDA LINE S S DATA n DATA n+1 ADDRESS (An) K K K K K BUS ACTIVITY: C C C C C A A A A A SLAVE SLAVE ADDRESS ADDRESS P O T S DATA n+X P K C A O N AI00899 Figure 9. Alternative READ mode sequence T R P BMUASS TAECRTIVITY: STA R/W STO SDA LINE S DATA n DATA n+1 DATA n+X P K K K K K BUS ACTIVITY: C C C C C A A A A A O SLAVE N ADDRESS AI00895 12/32 Doc ID 10773 Rev 7

M41T81S Operation WRITE mode In this mode the master transmitter transmits to the M41T81S slave receiver. Bus protocol is shown in Figure10 on page13. Following the START condition and slave address, a logic '0' (R/W=0) is placed on the bus and indicates to the addressed device that word address “An” will follow and is to be written to the on-chip address pointer. The data word to be written to the memory is strobed in next and the internal address pointer is incremented to the next address location on the reception of an acknowledge clock. The M41T81S slave receiver will send an acknowledge clock to the master transmitter after it has received the slave address see Figure7 on page12 and again after it has received the word address and each data byte. Data retention mode With valid V applied, the M41T81S can be accessed as described above with READ or CC WRITE cycles. Should the supply voltage decay, the power input will be switched from the V pin to the battery when V falls below the battery backup switchover voltage (V ). At CC CC SO this time the clock registers will be maintained by the attached battery supply. On power-up, when V returns to a nominal value, write protection continues for t . CC REC For a further, more detailed review of lifetime calculations, please see application note AN1012. Figure 10. WRITE mode sequence T R P BUS ACTIVITY: A W O MASTER ST R/ ST WORD SDA LINE S DATA n DATA n+1 DATA n+X P ADDRESS (An) K K K K K BUS ACTIVITY: C C C C C A A A A A SLAVE ADDRESS AI00591 Doc ID 10773 Rev 7 13/32

Clock operation M41T81S 3 Clock operation The 20-byte register map (see Table2: Clock register map on page15) is used to both set the clock and to read the date and time from the clock, in a binary coded decimal format. Tenths/hundredths of seconds, seconds, minutes, and hours are contained within the first four registers. Note: Tenths/hundredths of seconds cannot be written to any value other than “00.” Bits D6 and D7 of clock register 03h (century/hours register) contain the CENTURY ENABLE bit (CEB) and the CENTURY bit (CB). Setting CEB to a '1' will cause CB to toggle, either from '0' to '1' or from '1' to '0' at the turn of the century (depending upon its initial state). If CEB is set to a '0,' CB will not toggle. Bits D0 through D2 of register 04h contain the day (day of week). Registers 05h, 06h, and 07h contain the date (day of month), month and years. The ninth clock register is the calibration register (this is described in the clock calibration section). Bit D7 of register 01h contains the STOP bit (ST). Setting this bit to a '1' will cause the oscillator to stop. If the device is expected to spend a significant amount of time on the shelf, the oscillator may be stopped to reduce current drain. When reset to a '0' the oscillator restarts within one second. The eight clock registers may be read one byte at a time, or in a sequential block. Provision has been made to assure that a clock update does not occur while any of the eight clock addresses are being read. If a clock address is being read, an update of the clock registers will be halted. This will prevent a transition of data during the READ. Power-down time-stamp When a power failure occurs, the HALT (HT) bit will automatically be set to a '1.' This will prevent the clock from updating the registers, and will allow the user to read the exact time of the power-down event. Resetting the HT bit to a '0' will allow the clock to update the registers with the current time. For more information, please refer to AN1572, “Power-down time-stamp function in serial real-time clocks (RTCs)”. Clock registers The M41T81S offers 20 internal registers which contain clock, alarm, watchdog, flags, square wave and calibration data. These registers are memory locations which contain external (user accessible) and internal copies of the data (usually referred to as BiPORT™ cells). The external copies are independent of internal functions except that they are updated periodically by the simultaneous transfer of the incremented internal copy. The internal divider (or clock) chain will be reset upon the completion of a WRITE to any clock address. The system-to-user transfer of clock data will be halted whenever the address being read is a clock address (00h to 07h). The update will resume either due to a stop condition or when the pointer increments to any non-clock address (08h-13h). Clock and alarm registers store data in BCD. Calibration, watchdog and square wave registers store data in binary format. 14/32 Doc ID 10773 Rev 7

M41T81S Clock operation Table 2. Clock register map Function/range BCD Addr format D7 D6 D5 D4 D3 D2 D1 D0 00h 0.1 seconds 0.01 Seconds Seconds 00-99 01h ST 10 seconds Seconds Seconds 00-59 02h 0 10 minutes Minutes Minutes 00-59 03h CEB CB 10 hours Hours (24-hour format) Century/hours 0-1/00-23 04h 0 0 0 0 0 Day of week Day 01-7 05h 0 0 10 date Date: day of month Date 01-31 06h 0 0 0 10M Month Month 01-12 07h 10 years Year Year 00-99 08h OUT FT S Calibration Calibration 09h OFIE BMB4 BMB3 BMB2 BMB1 BMB0 RB1 RB0 Watchdog 0Ah AFE SQW ABE Al Alarm month Al month 01-12 0Bh RPT4 RPT5 AI 10 date Alarm date Al date 01-31 0Ch RPT3 HT AI 10 hour Alarm hour Al hour 00-23 0Dh RPT2 Alarm 10 minutes Alarm minutes Al min 00-59 0Eh RPT1 Alarm 10 seconds Alarm seconds Al sec 00-59 0Fh WDF AF 0 BL 0 OF 0 0 Flags 10h 0 0 0 0 0 0 0 0 Reserved 11h 0 0 0 0 0 0 0 0 Reserved 12h 0 0 0 0 0 0 0 0 Reserved 13h RS3 RS2 RS1 RS0 0 0 0 0 SQW 0 = Must be set to '0' ABE = Alarm in battery backup mode enable bit AF = Alarm flag (read only) AFE = Alarm flag enable flag BL = Battery low bit BMB0-BMB4 = Watchdog multiplier bits CB = Century bit CEB = Century enable bit FT = Frequency test bit HT = Halt update bit OF = Oscillator fail flag OFIE = Oscillator fail interrupt enable OUT = Output level RB0-RB1 = Watchdog resolution bits RPT1-RPT5 = Alarm repeat mode bits RS0-RS3 = SQW frequency S = Sign bit SQWE = Square wave enable ST = Stop bit WDF = Watchdog flag (read only) Doc ID 10773 Rev 7 15/32

Clock operation M41T81S Calibrating the clock The M41T81S is driven by a quartz controlled oscillator with a nominal frequency of 32,768 Hz. The devices are tested not exceed ±35 ppm (parts per million) oscillator frequency error at 25oC, which equates to about +1.9 to –1.1 minutes per month (see Figure11 on page17). When the calibration circuit is properly employed, accuracy improves to better than ±2 ppm at 25°C. The oscillation rate of crystals changes with temperature. The M41T81S design employs periodic counter correction. The calibration circuit adds or subtracts counts from the oscillator divider circuit at the divide by 256 stage, as shown in Figure12 on page17. The number of times pulses which are blanked (subtracted, negative calibration) or split (added, positive calibration) depends upon the value loaded into the five calibration bits found in the calibration register. Adding counts speeds the clock up, subtracting counts slows the clock down. The calibration bits occupy the five lower order bits (D4-D0) in the calibration register 08h. These bits can be set to represent any value between 0 and 31 in binary form. Bit D5 is a sign bit; '1' indicates positive calibration, '0' indicates negative calibration. Calibration occurs within a 64 minute cycle. The first 62 minutes in the cycle may, once per minute, have one second either shortened by 128 or lengthened by 256 oscillator cycles. If a binary '1' is loaded into the register, only the first 2 minutes in the 64 minute cycle will be modified; if a binary 6 is loaded, the first 12 will be affected, and so on. Therefore, each calibration step has the effect of adding 512 or subtracting 256 oscillator cycles for every 125,829,120 actual oscillator cycles, that is +4.068 or –2.034 ppm of adjustment per calibration step in the calibration register (see Figure12 on page17). Assuming that the oscillator is running at exactly 32,768Hz, each of the 31 increments in the Calibration byte would represent +10.7 or –5.35 seconds per month which corresponds to a total range of +5.5 or –2.75 minutes per month. Two methods are available for ascertaining how much calibration a given M41T81S may require. The first involves setting the clock, letting it run for a month and comparing it to a known accurate reference and recording deviation over a fixed period of time. Calibration values, including the number of seconds lost or gained in a given period, can be found in application note AN934, “TIMEKEEPER® calibration.” This allows the designer to give the end user the ability to calibrate the clock as the environment requires, even if the final product is packaged in a non-user serviceable enclosure. The designer could provide a simple utility that accesses the calibration byte. The second approach is better suited to a manufacturing environment, and involves the use of the IRQ/FT/OUT/SQW pin. The pin will toggle at 512 Hz, when the stop bit (ST, D7 of 01h) is '0,' the frequency test bit (FT, D6 of 08h) is '1,' the alarm flag enable bit (AFE, D7 of 0Ah) is '0,' and the square wave enable bit (SQWE, D6 of 0Ah) is '0' and the watchdog register (09h = 0) is reset. Any deviation from 512 Hz indicates the degree and direction of oscillator frequency shift at the test temperature. For example, a reading of 512.010124 Hz would indicate a +20 ppm oscillator frequency error, requiring a –10 (XX001010) to be loaded into the calibration byte for correction. Note that setting or changing the calibration byte does not affect the frequency test output frequency. 16/32 Doc ID 10773 Rev 7

M41T81S Clock operation The IRQ/FT/OUT/SQW pin is an open drain output which requires a pull-up resistor to V CC for proper operation. A 500-10 k resistor is recommended in order to control the rise time. The FT bit is cleared on power-down. Figure 11. Crystal accuracy across temperature Frequency (ppm) 20 0 –20 –40 –60 ΔF= K x (T – T )2 –80 F O K = –0.036 ppm/°C2 ± 0.006 ppm/°C2 –100 T = 25°C ± 5°C –120 O –140 –160 –40 –30 –20 –10 0 10 20 30 40 50 60 70 80 Temperature °C AI07888 Figure 12. Clock calibration NORMAL POSITIVE CALIBRATION NEGATIVE CALIBRATION AI00594B Setting alarm clock registers Address locations 0Ah-0Eh contain the alarm settings. The alarm can be configured to go off at a prescribed time on a specific month, date, hour, minute, or second or repeat every year, month, day, hour, minute, or second. It can also be programmed to go off while the M41T81S is in the battery backup mode to serve as a system wake-up call. Bits RPT5-RPT1 put the alarm in the repeat mode of operation. Table3 on page19 shows the possible configurations. Codes not listed in the table default to the once per second mode to quickly alert the user of an incorrect alarm setting. Doc ID 10773 Rev 7 17/32

Clock operation M41T81S When the clock information matches the alarm clock settings based on the match criteria defined by RPT5-RPT1, the AF (alarm flag) is set. If AFE (alarm flag enable) is also set (and SQWE is '0.'), the alarm condition activates the IRQ/FT/OUT/SQW pin. Note: If the address pointer is allowed to increment to the flags register address, an alarm condition will not cause the Interrupt/Flag to occur until the address pointer is moved to a different address. It should also be noted that if the last address written is the “Alarm Seconds,” the address pointer will increment to the flag address, causing this situation to occur. The IRQ/FT/OUT/SQW output is cleared by a READ to the flags register as shown in Figure13. A subsequent READ of the flags register is necessary to see that the value of the alarm flag has been reset to '0.' The IRQ/FT/OUT/SQW pin can also be activated in the battery backup mode. The IRQ/FT/OUT/SQW will go low if an alarm occurs and both ABE (alarm in battery backup mode enable) and AFE are set. Figure14 illustrates the backup mode alarm timing. Figure 13. Alarm interrupt reset waveform 0Eh 0Fh 10h ACTIVE FLAG IRQ/FT/OUT/SQW HIGH-Z AI04617 Figure 14. Backup mode alarm waveform VCC VPFD VSO trec ABE and AFE Bits AF Bit in Flags Register IRQ/FT/OUT/SQW HIGH-Z AI09164b 18/32 Doc ID 10773 Rev 7

M41T81S Clock operation T able 3. Alarm repeat modes RPT5 RPT4 RPT3 RPT2 RPT1 Alarm setting 1 1 1 1 1 Once per second 1 1 1 1 0 Once per minute 1 1 1 0 0 Once per hour 1 1 0 0 0 Once per day 1 0 0 0 0 Once per month 0 0 0 0 0 Once per year Watchdog timer The watchdog timer can be used to detect an out-of-control microprocessor. The user programs the watchdog timer by setting the desired amount of time-out into the watchdog register, address 09h. Bits BMB4-BMB0 store a binary multiplier and the two lower order bits RB1-RB0 select the resolution, where 00=1/16 second, 01=1/4 second, 10=1 second, and 11=4 seconds. The amount of time-out is then determined to be the multiplication of the five-bit multiplier value with the resolution. (For example: writing 00001110 in the watchdog register=3*1, or 3 seconds). If the processor does not reset the timer within the specified period, the M41T81S sets the WDF (watchdog flag) and generates a watchdog interrupt. The watchdog timer can be reset by having the microprocessor perform a WRITE of the watchdog register. The time-out period then starts over. Should the watchdog timer time-out, a value of 00h needs to be written to the watchdog register in order to clear the IRQ/FT/OUT/SQW pin. This will also disable the watchdog function until it is again programmed correctly. A READ of the flags register will reset the watchdog flag (bit D7; register 0Fh). The watchdog function is automatically disabled upon power-up and the watchdog register is cleared. If the watchdog function is set, the frequency test function is activated, and the SQWE bit is '0,' the watchdog function prevails and the frequency test function is denied. Doc ID 10773 Rev 7 19/32

Clock operation M41T81S Square wave output The M41T81S offers the user a programmable square wave function which is output on the SQW pin. RS3-RS0 bits located in 13h establish the square wave output frequency. These frequencies are listed in Table4. Once the selection of the SQW frequency has been completed, the IRQ/FT/OUT/SQW pin can be turned on and off under software control with the square wave enable bit (SQWE) located in register 0Ah. T able 4. Square wave output frequency Square wave bits Square wave RS3 RS2 RS1 RS0 Frequency Units 0 0 0 0 None - 0 0 0 1 32.768 kHz 0 0 1 0 8.192 kHz 0 0 1 1 4.096 kHz 0 1 0 0 2.048 kHz 0 1 0 1 1.024 kHz 0 1 1 0 512 Hz 0 1 1 1 256 Hz 1 0 0 0 128 Hz 1 0 0 1 64 Hz 1 0 1 0 32 Hz 1 0 1 1 16 Hz 1 1 0 0 8 Hz 1 1 0 1 4 Hz 1 1 1 0 2 Hz 1 1 1 1 1 Hz Century bit Bits D7 and D6 of clock register 03h contain the CENTURY ENABLE bit (CEB) and the CENTURY bit (CB). Setting CEB to a '1' will cause CB to toggle, either from a '0' to '1' or from '1' to '0' at the turn of the century (depending upon its initial state). If CEB is set to a '0,' CB will not toggle. 20/32 Doc ID 10773 Rev 7

M41T81S Clock operation Battery low warning The M41T81S automatically performs battery voltage monitoring upon power-up and at factory-programmed time intervals of approximately 24 hours. The battery low (BL) bit, bit D4 of flags register 0Fh, will be asserted if the battery voltage is found to be less than approximately 2.5 V. The BL bit will remain asserted until completion of battery replacement and subsequent battery low monitoring tests, either during the next power-up sequence or the next scheduled 24-hour interval. If a battery low is generated during a power-up sequence, this indicates that the battery is below approximately 2.5 volts and may not be able to maintain data integrity. Clock data should be considered suspect and verified as correct. A fresh battery should be installed. If a battery low indication is generated during the 24-hour interval check, this indicates that the battery is near end of life. However, data is not compromised due to the fact that a nominal V is supplied. In order to insure data integrity during subsequent periods of CC battery back-up mode, the battery should be replaced. The M41T81S only monitors the battery when a nominal V is applied to the device. Thus CC applications which require extensive durations in the battery backup mode should be powered-up periodically (at least once every few months) in order for this technique to be beneficial. Additionally, if a battery low is indicated, data integrity should be verified upon power-up via a checksum or other technique. Oscillator fail detection If the oscillator fail bit (OF) is internally set to '1,' this indicates that the oscillator has either stopped, or was stopped for some period of time and can be used to judge the validity of the clock and date data. In the event the OF bit is found to be set to '1' at any time other than the initial power-up, the STOP bit (ST) should be written to a '1,' then immediately reset to '0.' This will restart the oscillator. The following conditions can cause the OF bit to be set: ● The first time power is applied (defaults to a '1' on power-up). ● The voltage present on V is insufficient to support oscillation. CC ● The ST bit is set to '1.' ● External interference of the crystal. The OF bit will remain set to '1' until written to logic '0.' The oscillator must start and have run for at least 4 seconds before attempting to reset the OF bit to '0.' Oscillator fail interrupt enable If the oscillator fail interrupt bit (OFIE) is set to a '1,' the IRQ pin will also be activated. The IRQ output is cleared by resetting the OFIE or OF bit to '0' (not be reading the flags register). Doc ID 10773 Rev 7 21/32

Clock operation M41T81S Output driver pin When the FT bit, AFE bit, SQWE bit, and watchdog register are not set, the IRQ/FT/OUT/SQW pin becomes an output driver that reflects the contents of D7 of the calibration register. In other words, when D7 (OUT bit) and D6 (FT bit) of address location 08h are a '0,' then the IRQ/FT/OUT/SQW pin will be driven low. Note: The IRQ/FT/OUT/SQW pin is an open drain which requires an external pull-up resistor. Preferred initial power-on default Upon initial application of power to the device, the following register bits are set to a '0' state: watchdog register; AFE; ABE; SQWE; OFIE; and FT. The following bits are set to a '1' state: ST; OUT; OF; and HT (see Table5). Table 5. Preferred default values WATCHDOG Condition ST HT Out FT AFE SQWE ABE OF OFIE register(1) Initial power-up(2) 1 1 1 0 0 0 0 0 1 0 Subsequent power-up UC 1 UC 0 UC UC UC 0 UC UC (with battery backup)(3) 1. BMB0-BMB4, RB0, RB1 2. State of other control bits undefined 3. UC = Unchanged 22/32 Doc ID 10773 Rev 7

M41T81S Maximum ratings 4 Maximum ratings Stressing the device above the rating listed in the absolute maximum ratings table may cause permanent damage to the device. These are stress ratings only and operation of the device at these or any other conditions above those indicated in the operating sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. Table 6. Absolute maximum ratings Sym Parameter Value Unit T Storage temperature (V off, oscillator off) –55 to 125 °C STG CC V Supply voltage –0.3 to 7 V CC SO8(1) 260 °C T Lead solder temperature for 10 seconds SLD SOX18(2) 240 °C V Input or output voltages –0.3 to V + 0.3 V IO CC I Output current 20 mA O P Power dissipation 1 W D 1. For SO8 package, Lead-free (Pb-free) lead finish, reflow at peak temperature of 260 °C. The time above 255 °C must not exceed 30 seconds. 2. For SOX18 package, reflow at peak temperature of 240 °C. The time above 235 °C must not exceed 20 seconds. Caution: Negative undershoots below –0.3 volts are not allowed on any pin while in the battery backup mode. Doc ID 10773 Rev 7 23/32

DC and AC parameters M41T81S 5 DC and AC parameters This section summarizes the operating and measurement conditions, as well as the DC and AC characteristics of the device. The parameters in the following DC and AC Characteristic tables are derived from tests performed under the measurement conditions listed in the relevant tables. Designers should check that the operating conditions in their projects match the measurement conditions when using the quoted parameters. Table 7. Operating and AC measurement conditions Parameter M41T81S Supply voltage (V ) 2.7 to 5.5 V CC Ambient operating temperature (T ) –40 to 85 °C A Load capacitance (C ) 100 pF L Input rise and fall times ≤ 50 ns Input pulse voltages 0.2V to 0.8V CC CC Input and output timing ref. voltages 0.3V to 0.7V CC CC Note: Output Hi-Z is defined as the point where data is no longer driven. Figure 15. AC measurement I/O waveform 0.8VCC 0.7VCC 0.3VCC 0.2VCC AI02568 Table 8. Capacitance Symbol Parameter(1)(2) Min Max Unit C Input capacitance - 7 pF IN C (3) Output capacitance - 10 pF OUT t Low-pass filter input time constant (SDA and SCL) - 50 ns LP 1. Effective capacitance measured with power supply at 5 V; sampled only, not 100% tested. 2. At 25 °C, f = 1 MHz 3. Outputs deselected 24/32 Doc ID 10773 Rev 7

M41T81S DC and AC parameters Table 9. DC characteristics Sym Parameter Test condition(1) Min Typ Max Unit I Input leakage current 0 V ≤ V ≤ V ±1 µA LI IN CC I Output leakage current 0 V ≤ V ≤ V ±1 µA LO OUT CC I Supply current Switch freq = 400 kHz 400 µA CC1 SCL = 0 Hz All inputs I Supply current (standby) 100 µA CC2 ≥ V – 0.2 V CC ≤ V + 0.2 V SS V Input low voltage –0.3 0.3V V IL CC V + V Input high voltage 0.7V CC V IH CC 0.3 Output low voltage I = 3.0 mA 0.4 V OL VOL Output low voltage I = 10 mA 0.4 V (open drain)(2) OL Pull-up supply voltage IRQ/OUT/FT/SQW 5.5 V (open drain) V (3) Backup supply voltage 2.0 3.5(4) V BAT T = 25 °C, V = 0 V I Battery supply current A CC 0.6 1 µA BAT Oscillator ON, V = 3 V BAT 1. Valid for ambient operating temperature: T = –40 to 85 °C; V = 2.7 to 5.5 V (except where noted). A CC 2. For IRQ/FT/OUT/SQW pin (open drain) 3. STMicroelectronics recommends the RAYOVAC BR1225 or BR1632 (or equivalent) as the battery supply. 4. For rechargeable back-up, V (max) may be considered to be V . BAT CC Table 10. Crystal electrical characteristics Sym Parameter(1)and(2) Min Typ Max Units f Resonant frequency - 32.768 kHz O R Series resistance - 60(3) kΩ S C Load capacitance - 12.5 pF L 1. Externally supplied if using the SO8 package. STMicroelectronics recommends the KDS DT-38: 1TA/1TC252E127, Tuning Fork Type (thru-hole) or the DMX-26S: 1TJS125FH2A212, (SMD) quartz crystal for industrial temperature operations. KDS can be contacted at kouhou@kdsj.co.jp or http://www.kdsj.co.jp for further information on this crystal type. 2. Load capacitors are integrated within the M41T81S. Circuit board layout considerations for the 32.768 kHz crystal of minimum trace lengths and isolation from RF generating signals should be taken into account. 3. For applications requiring back-up supply operation below 2.5 V, R (max) should be considered 40 kΩ. S Figure 16. Power down/up mode AC waveforms VCC VSO tPD trec SDA SCL DON'T CARE AI00596 Doc ID 10773 Rev 7 25/32

DC and AC parameters M41T81S Table 11. Power down/up AC characteristics Symbol Parameter(1)(2) Min Typ Max Unit t SCL and SDA at V before power-down 0 - - nS PD IH t SCL and SDA at V after power-up 10 - - µS rec IH 1. V fall time should not exceed 5 mV/µs. CC 2. Valid for ambient operating temperature: T = –40 to 85 °C; V = 2.7 to 5.5 V (except where noted). A CC Table 12. Power down/up trip points DC characteristics Sym Parameter(1)(2) Min Typ Max Unit Power-fail deselect 2.5 2.6 2.7 V V PFD Hysteresis 25 mV Battery backup switchover voltage VBAT < VPFD VBAT V VSO (VCC < VBAT; VCC < VPFD) VBAT > VPFD VPFD V Hysteresis 40 mV 1. All voltages referenced to V . SS 2. Valid for ambient operating temperature: T = –40 to 85 °C; V = 2.7 to 5.5 V (except where noted). A CC Figure 17. Bus timing requirements sequence SDA tBUF tHD:STA tHD:STA tR tF SCL tHIGH tSU:DAT tSU:STA tSU:STO P S tLOW tHD:DAT SR P AI00589 26/32 Doc ID 10773 Rev 7

M41T81S DC and AC parameters Table 13. AC characteristics Sym Parameter(1) Min Typ Max Units f SCL clock frequency 0 - 400 kHz SCL t Clock low period 1.3 - µs LOW t Clock high period 600 - ns HIGH t SDA and SCL rise time - 300 ns R t SDA and SCL fall time - 300 ns F START condition hold time tHD:STA (after this period the first clock pulse is 600 - ns generated) START condition setup time t 600 - ns SU:STA (only relevant for a repeated start condition) t Data setup time 100 - ns SU:DAT t (2) Data hold time 0 - µs HD:DAT t STOP condition setup time 600 - ns SU:STO Time the bus must be free before a new t 1.3 - µs BUF transmission can start 1. Valid for ambient operating temperature: T = –40 to 85 °C; V = 2.7 to 5.5 V (except where noted). A CC 2. Transmitter must internally provide a hold time to bridge the undefined region (300 ns max) of the falling edge of SCL. Doc ID 10773 Rev 7 27/32

Package mechanical data M41T81S 6 Package mechanical data In order to meet environmental requirements, ST offers these devices in different grades of ECOPACK® packages, depending on their level of environmental compliance. ECOPACK® specifications, grade definitions and product status are available at: www.st.com. ECOPACK® is an ST trademark. Figure 18. SO8 – 8-lead plastic small package outline h x 45° A2 A c ccc b e 0.25 mm D GAUGE PLANE k 8 E1 E L 1 A1 L1 SO-A Note: Drawing is not to scale. Table 14. SO8 – 8-lead plastic small outline (150 mils body width), package mechanical data mm inches Symb Typ Min Max Typ Min Max A 1.75 0.069 A1 0.10 0.25 0.004 0.010 A2 1.25 0.049 b 0.28 0.48 0.011 0.019 c 0.17 0.23 0.007 0.009 ccc 0.10 0.004 D 4.90 4.80 5.00 0.193 0.189 0.197 E 6.00 5.80 6.20 0.236 0.228 0.244 E1 3.90 3.80 4.00 0.154 0.150 0.157 e 1.27 0.050 h 0.25 0.50 0.010 0.020 k 0° 8° 0° 8° L 0.40 0.127 0.016 0.050 L1 1.04 0.041 28/32 Doc ID 10773 Rev 7

M41T81S Package mechanical data Figure 19. SOX18 – 18-lead plastic small outline, 300 mils, embedded crystal, outline SOX18 Note: Drawing is not to scale. Table 15. SOX18 – 18-lead plastic small outline, 300 mils, embedded crystal, package mechanical data millimeters inches Symbol Typ Min Max Typ Min Max A 2.57 2.44 2.69 0.101 0.096 0.106 A1 0.23 0.15 0.31 0.009 0.006 0.012 A2 2.34 2.29 2.39 0.092 0.090 0.094 B 0.46 0.41 0.51 0.018 0.016 0.020 c 0.25 0.20 0.31 0.010 0.008 0.012 D 11.61 11.56 11.66 0.457 0.455 0.459 E 7.62 7.57 7.67 0.300 0.298 0.302 E1 10.34 10.16 10.52 0.407 0.400 0.414 e 1.27 0.050 L 0.66 0.51 0.81 0.026 0.020 0.032 Doc ID 10773 Rev 7 29/32

Part numbering M41T81S 7 Part numbering Table 16. Ordering information Example: M41T 81S M 6 F Device type M41T Supply voltage and write protect voltage 81S = V = 2.7 to 5.5 V CC Package M = SO8 MY(1) = SOX18 Temperature range 6 = –40 °C to 85 °C Shipping method E = ECOPACK® package, tubes(2) F = ECOPACK® package, tape & reel 1. The SOX18 package includes an embedded 32,768 Hz crystal. Contact local ST sales office for availability. 2. Shipment in tubes is not recommended for new design. Contact local ST sales office for availability. For other options, or for more information on any aspect of this device, please contact the ST sales office nearest you. 30/32 Doc ID 10773 Rev 7

M41T81S Revision history 8 Revision history Date Revision Changes 22-Jan-2004 0.1 First draft Update BL information, characteristics, ratings, and Lead (Pb)-free information 06-Feb-2004 0.2 (Table12, Table6, Table10, Table16) 20-Feb-2004 0.3 Update characteristics (Table11, Table12, Table7, Part numbering) Product promoted; reformatted; update characteristics, including Lead-free 14-Apr-2004 1 package information (Figure3, Figure4, Figure11, Figure14; Table13, Table16) 05-May-2004 1.1 Update DC characteristics (Table9) 16-Jun-2004 1.2 Add shipping package (Table16) 13-Sep-2004 2 Update maximum ratings (Table6) Promote document; update characteristics and marketing status (cover page, 26-Nov-2004 3 Figure5) 23-Sep-2005 4 Update features; added Lead-free information (cover page; Figure4) Remove TIMEKEEPER references and update package mechanical data 22-Jan-2007 5 (Figure18 and Figure19) 13-Sep-2010 6 Updated Section4, ECOPACK® text in Section6; reformatted document. Added reference to AN1572 in Power-down time-stamp on page14; updated 16-May-2012 7 footnote 1 of Table6; updated Table16: Ordering information. Doc ID 10773 Rev 7 31/32

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