SUMMARY DESCRIPTION
The M41T81S Serial Access TIMEKEEPER® SRAM is a low power Serial RTC with a built-in 32.768kHz oscillator (external crystal controlled). Eight bytes of the SRAM (see Table 2., page 12) 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, bi-directional 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 (MY), 300mil SOIC package which in cludes an embedded 32kHz crystal.
The 18-pin, embedded crystal SOIC requires only a user-supplied battery to provide non-volatile operation.

FEATURES SUMMARY
■ COUNTERS FOR TENTHS/HUNDREDTHS
   OF SECONDS, SECONDS, MINUTES,
   HOURS, DAY, DATE, MONTH, YEAR, AND CENTURY
■ 32KHz CRYSTAL OSCILLATOR WITH
   INTEGRATED LOAD CAPACITANCE
   (12.5pF) WHICH PROVIDES EXCEPTIONAL
   OSCILLATOR STABILITY AND HIGH CRYSTAL SERIES RESISTANCE OPERATION)
■ OSCILLATOR STOP DETECTION (MONITORS CLOCK OPERATION)
■ SERIAL INTERFACE SUPPORTS I2C BUS (400kHz PROTOCOL)
■ ULTRA-LOW BATTERY SUPPLY CURRENT OF 0.6µA (typ)
■ 2.0 TO 5.5V CLOCK OPERATING VOLTAGE
■ AUTOMATIC SWITCHOVER AND
   DESELECT CIRCUITRY (FIXED REFERENCE) WHICH PROVIDES FULL OPERATION IN 3.0V APPLICATIONS)
   – VCC = 2.7 to 5.5V 2.5V ≤ VPFD ≤ 2.7V
■ POWER-DOWN TIME STAMP (HT BIT)
   WHICH ALLOWS DETERMINATION OF TIME ELAPSED IN BATTERY BACK-UP
■ BATTERY LOW FLAG
■ PROGRAMMABLE ALARM AND
   INTERRUPT FUNCTION (valid even during Battery Back-up Mode)
■ ACCURATE PROGRAMMABLE WATCHDOG TIMER (from 62.5ms to 128s)
■ 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