M41T81 Serial access real-time clock with alarm Datasheet - production data Description 8 The M41T81 is a low-power serial RTC with a built-in 32.768 kHz oscillator (external crystal controlled). Eight bytes of the SRAM are used for 1 the clock/calendar function and are configured in SO8 binary-coded decimal (BCD) format. An additional 8-pin SOIC 12 bytes of SRAM provide status/control of alarm, watchdog and square wave functions. Addresses and data are transferred serially via a two line, 2 Features bidirectional I C interface. The built-in address register is incremented automatically after each For all new designs use M41T81S WRITE or READ data byte. Counters for tenths/hundredths of seconds, The M41T81 has a built-in power sense circuit seconds, minutes, hours, day, date, month, which detects power failures and automatically year, and century switches to the battery supply when a power 32 KHz crystal oscillator integrating load failure occurs. The energy needed to sustain the capacitance (12.5 pF) providing exceptional SRAM and clock operations can be supplied by a oscillator stability and high crystal series small lithium button supply when a power failure resistance operation occurs. 2 Serial interface supports I C bus (400 kHz Functions available to the user include a non- protocol) volatile, time-of-day clock/calendar, alarm Ultra-low battery supply current of 0.6 A interrupts, watchdog timer and programmable (typ at 3 V) square wave output. The eight clock address locations contain the century, year, month, date, 2.0 to 5.5 V clock operating voltage day, hour, minute, second and tenths/hundredths Automatic switchover and deselect circuitry (for of a second in 24-hour BCD format. Corrections 3 V application select M41T81S datasheet) for 28, 29 (leap year - valid until year 2100), 30 Power-down time stamp (HT bit) allowing and 31 day months are made automatically. determination of time elapsed in battery The M41T81 is supplied in an 8-pin SOIC. backup 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 crystal deviation due to temperature Operating temperature of 40 to 85 C ECOPACK package available February 2014 DocID007529 Rev 11 1/29 This is information on a product in full production. www.st.comContents M41T81 Contents 1 Device overview 5 2 Operation 7 2.1 2-wire bus characteristics 7 2.1.1 Bus not busy 7 2.1.2 Start data transfer 8 2.1.3 Stop data transfer 8 2.1.4 Data valid . 8 2.1.5 Acknowledge 8 2.2 READ mode 9 2.3 WRITE mode 11 2.4 Data retention mode . 11 3 Clock operation 12 3.1 Power-down time-stamp 12 3.2 Clock registers . 12 3.3 Calibrating the clock . 14 3.4 Setting alarm clock registers 16 3.5 Watchdog timer 17 3.6 Square wave output . 18 3.7 Century bit 18 3.8 Output driver pin 19 3.9 Preferred initial power-on default . 19 4 Maximum ratings . 20 5 DC and AC parameters 21 6 Package mechanical information . 25 7 Part numbering 27 8 Revision history . 28 2/29 DocID007529 Rev 11