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Serial I²C bus real-time clock (RTC) with battery switchover
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The M41T8x are low-power serial I2C real-time clocks (RTCs) with a built-in 32.768 kHz oscillator (external crystal-controlled for the QFN16 and SO8 packages, embedded crystal for the SOX18 package). Eight bytes of the register map (see Table 2 on page 23) are used for the clock/calendar function and are configured in binary-coded decimal (BCD) format. An additional 17 bytes of the register map provide status/control of the two alarms, watchdog, 8-bit counter, and square wave functions. An additional seven bytes are made available as user SRAM.

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 M41T8x 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 battery when a power failure occurs.

Functions available to the user include a non-volatile, time-of-day clock/calendar, two alarm interrupts, watchdog timer, programmable 8-bit counter, and square wave outputs. 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. The M41T83 is supplied in either a QFN16 or an SOX18, 300 mil SOIC which includes an embedded 32 KHz crystal. The SOX18 package requires only a user-supplied battery to provide non-volatile operation. The M41T82 is available only in an SO8 package.

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Errata Sheet
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Key Features

  • Ultra-low battery supply current of 365 nA
  • Factory-calibrated accuracy of ±5 ppm typical after 2 reflows (SOX18)
  • Much better accuracies achievable using built-in programmable analog and digital calibration circuits
  • 2.0 V to 5.5 V clock operating voltage
  • Counters for tenths/hundredths of seconds, seconds, minutes, hours, day, date, month, year, and century
  • Automatic switchover and reset output circuitry (fixed reference)
  • M41T83S: VCC= 3.00 V to 5.50 V
  • M41T83R: VCC= 2.70 V to 5.50 V
  • M41T83Z: VCC= 2.38 V to 5.50 V
  • Serial interface supports I2C bus (400 kHz protocol)
  • Programmable alarm with interrupt function (valid even during battery backup mode)
  • Optional 2ndprogrammable alarm available
  • Square wave output defaults to 32 KHz on power-up (M41T83 only)
  • RESET (RST) output
  • Watchdog timer
  • Programmable 8-bit counter/timer
  • 7 bytes of battery-backed user SRAM
  • Battery low flag
  • Low operating current of 80 μA
  • Oscillator stop detection
  • Battery or SuperCap™ backup
  • Operating temperature of –40 °C to 85 °C
  • Package options
  • a 16-lead QFN (M41T83)
  • an 8-lead SOIC (M41T82) or
  • an 18-lead embedded crystal SOIC (M41T83)
  • RoHS compliance: lead-free components are compliant with the RoHS directive

Circuit Diagram

Design Resources


Technical Documentation

Product Specifications

Description Version Size
DS4858: Serial I2C bus real-time clock (RTC) with battery switchover
18.0 2,228 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
AN2678: Extremely accurate timekeeping over temperature using adaptive calibration
2.1 214 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

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**
M41T82RM6FActiveSO-8Tape And Reel-408512_Free Samples1.2Distributor reported inventory date: 2016-02-13
Distributor NameRegionStockMin. order
ARROWOrder NowAMERICA24302500
Ecopack1 (*)PDF
M41T82SM6FActiveSO-8Tape And Reel-408512_Free Samples1.2Distributor reported inventory date: 2016-02-13
Distributor NameRegionStockMin. order
ARROWOrder NowAMERICA57352500
Ecopack1 (*)PDF
M41T82ZM6FActiveSO-8Tape And Reel-408512_Free Samples1.2Distributor reported inventory date: 2016-02-13
Distributor NameRegionStockMin. order
Ecopack1 (*)PDF

(*) 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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