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

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Errata Sheet
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Key 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:(not recommended for new design, contact ST sales office for availability)THS = 1: VPFD≈ 4.63 V; VCC= 4.75 to 5.5 VTHS = 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 VTHS = 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

Circuit Diagram

Design Resources


Technical Documentation

Product Specifications

Description Version Size
DS3301: 5.0 V and 3.3/3.0 V secure serial RTC and NVRAM supervisor with tamper detection and 128 bytes of clearable NVRAM
10.0 782 KB

Application Notes

Description Version Size
AN1011: Battery technology used in NVRAM and real-time clock (RTC) products from ST
4.0 248 KB
AN1012: Predicting the battery life and data retention period of NVRAMs and serial RTCs
4.1 439 KB
AN1216: Implementing a periodic alarm with TIMEKEEPER® and serial real-time clocks (RTCs)
2.0 193 KB
AN1336: Power-fail comparator for NVRAM supervisory devices
2.0 128 KB
AN1572: Power-down time-stamp function in serial real-time clocks (RTCs)
2.0 171 KB
AN1879: How to use the M41ST87W tamper detect and RAM clear
5.0 107 KB
AN3060: Applications guide for serial real-time clocks (RTCs)
3.0 879 KB
AN4641: Integrating TIMEKEEPER® and real-time clock supervisor functions
1.0 479 KB
AN923: Managing century information using serial real-time clocks and TIMEKEEPER® NVRAMs
3.0 195 KB
AN934: How to use the digital calibration feature in TIMEKEEPER® and serial real-time clock (RTC) products
4.1 123 KB

HW Model & CAD Libraries

HW Model & CAD Libraries

Description Version Size
Ibis model
2.1 15 KB

Related Tools and Software

Related Tools and Software

Part Number Description
STSW-SRTC001 I2C code for Serial RTC
STSW-SRTC002 SPI code -Serial RTC

Publications and Collaterals


Description Version Size
Serial real-time clock ICs
827 KB

Quality & Reliability

Product Certification

Description Version Size
Products Underwriter Laboratories (UL) Information
1.0 548 KB

Sample & Buy

Part Number Marketing StatusPackagePacking TypeOperating Temperature (min) Operating Temperature (max) NVRAM sizeAutomotive GradeOrder From STUnit Price (US$)*
@ 1000
Distributor AvailabilityRoHS Compliance GradeDownload
Material Declaration**
M41ST87WMX6TRActiveSO28 WITH EMBEDDED CRYSTALTape And Reel-4085128_Free Samples4.12Distributor reported inventory date: 2016-02-06
Distributor NameRegionStockMin. order
ARROWOrder NowAMERICA13821000
M41ST87WSS6FActiveSSOP 20 7.2x5.3Tape And Reel-4085128_Free Samples3.03Distributor reported inventory date: 2016-02-06
Distributor NameRegionStockMin. order

(*) Suggested Resale Price per unit (USD) for BUDGETARY USE ONLY. For quotes, prices in local currency, please contact your local ST Sales Office or our Distributors
(**) The Material Declaration forms available on may be generic documents based on the most commonly used package within a package family. For this reason, they may not be 100% accurate for a specific device. Please contact our sales support for information on specific devices.
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