SUMMARY DESCRIPTION
The M41ST87Y/W Serial TIMEKEEPER®/Controller SRAM is a low power 1280-bit, static CMOS SRAM organized as 160 bytes by 8 bits. A built-in 32.768 kHz oscillator (internal crystal-controlled) and 8 bytes of the SRAM (see Table 7., page 25) are used for the clock/calendar function and are configured in binary coded decimal (BCD) format.
An additional 11 bytes of RAM provide calibration, status/control of Alarm, Watchdog, Tamper, and Square Wave functions. 8 bytes of ROM and finally 128 bytes of User RAM are also provided. Ad dresses 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 M41ST87Y/W 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 SRAM and clock operations can be supplied by a small lithium button cell supply when a power failure occurs.

FEATURES SUMMARY
■ 5.0, 3.3, OR 3.0V OPERATING VOLTAGE
■ SERIAL INTERFACE SUPPORTS I2C BUS(400kHz)
■ NVRAM SUPERVISOR FOR EXTERNAL LPSRAM
■ 2.5 TO 5.5V OSCILLATOR OPERATING VOLTAGE
■ AUTOMATIC SWITCH-OVER AND DESELECT CIRCUITRY
■ CHOICE OF POWER-FAIL DESELECT VOLTAGES
   – M41ST87Y:
      VCC = 4.75 to 5.5V;
      THS Bit = 1: 4.50V ≤ VPFD ≤ 4.75V
      VCC = 4.5 to 5.5V;
      THS Bit = 0: 4.20V ≤ VPFD ≤ 4.50V
   – M41ST87W:
      VCC = 3.0 to 3.6V;
      THS Bit = 1: 2.8V ≤ VPFD ≤ 3.0V
      VCC = 2.7 to 3.6V;
      THS Bit = 0: 2.55V ≤ VPFD ≤ 2.70V
■ TWO INDEPENDENT POWER-FAIL COMPARATORS (1.25V REFERENCE)
■ COUNTERS FOR TENTHS/HUNDREDTHS OF SECONDS, SECONDS, MINUTES, HOURS, DAY, DATE, MONTH, YEAR, AND CENTURY
■ 128 BYTES OF GENERAL PURPOSE RAM
■ PROGRAMMABLE ALARM AND INTERRUPT FUNCTION (VALID EVEN DURING BATTERY BACK-UP MODE)
■ PROGRAMMABLE WATCHDOG TIMER
■ UNIQUE ELECTRONIC SERIAL NUMBER (8-BYTE)
■ 32kHz FREQUENCY OUTPUT AVAILABLE UPON POWER-ON
■ MICROPROCESSOR POWER-ON RESET
■ BATTERY LOW FLAG
■ ULTRA-LOW BATTERY SUPPLY CURRENT OF 500nA (TYP)

SECURITY FEATURES
■ TAMPER INDICATION CIRCUITS WITH TIMESTAMP AND RAM CLEAR
■ LPSRAM CLEAR FUNCTION (TPCLR)
■ PACKAGING INCLUDES A 28-LEAD, EMBEDDED CRYSTAL SOIC
■ OSCILLATOR STOP DETECTION