Summary description
The M41T81S is a low power Serial RTC with a built-in 32.768kHz oscillator (external crystal controlled). Eight bytes of the SRAM (see Table 2: Clock register map on page 15) 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-ofday 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 includes an embedded 32kHz crystal.
The 18-pin, embedded crystal SOIC requires only a user-supplied battery to provide nonvolatile operation.

Features
■ 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 backup
■ 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