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M41ST87W：5.0 V, 3.3/3.0 V secure real-time clock , NVRAM supervisor with tamper detection and 128 bytes of clearable NVRAM

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 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.

Functions available to the user include a non-volatile, time-of-day clock/calendar, alarm interrupts, tamper detection, watchdog timer, and programmable square wave output. Other features include a power-on reset as well as two additional debounced inputs (RSTIN1and RSTIN2) which can also generate an output reset (RST). 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), 30 and 31 day months are made automatically.

Key Features

• 5.0, 3.3, or 3.0 V operation
• 400 kHz I2 C 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: (not recommended for new design, contact ST sales office for availability) 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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