Description The M41ST87Y/W secure serial RTC and NVRAM supervisor 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 ) 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. Addresses and data are transferred serially via a two line, bidirectional 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 ■ 5.0, 3.3, or 3.0 V operation ■ 400 kHz I2C bus ■ NVRAM supervisor to non-volatize external LPSRAM ■ 2.5 to 5.5 V oscillator operating voltage ■ Automatic switchover and deselect circuitry ■ Choice of power-fail deselect voltages – M41ST87Y: THS = 1: VPFD≈ 4.63 V; VCC = 4.75 to 5.5 V THS = 0: VPFD≈ 4.37 V; VCC = 4.5 to 5.5 V – M41ST87W: THS = 1: VPFD ≈ 2.9 V; VCC = 3.0 to 3.6 V THS = 0: VPFD ≈ 2.63 V; VCC = 2.7 to 3.6 V ■ Two independent power-fail comparators (1.25 V reference) ■ Counters for tenths/hundredths of seconds, seconds, minutes, hours, day, date, month, year, and century ■ 128 bytes of clearable, general purpose NVRAM ■ Programmable alarm and interrupt function (valid even during battery backup mode) ■ Programmable watchdog timer ■ Unique electronic serial number (8-byte) ■ 32 kHz frequency output available upon power on ■ Microprocessor power-on reset output ■ Battery low flag ■ Ultra-low battery supply current of 500 nA (typ) Security features ■ Tamper indication circuits with timestamp and RAM clear ■ LPSRAM clear function (TPCLR) ■ Packaging includes a 28-lead, embedded crystal SOIC and a 20-lead SSOP ■ Oscillator stop detection
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