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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