Serial real-time clock (RTC) with alarm

The M41T81 is a low-power serial RTC with a built-in 32.768 kHz oscillator (external crystal controlled). Eight bytes of the SRAM are used for the clock/calendar function and are configured in binary-coded decimal (BCD) format. An additional 12 bytes of SRAM provide status/control of alarm, watchdog and square wave functions. 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 M41T81 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 supply when a power failure occurs.

Functions available to the user include a non-volatile, time-of-day clock/calendar, alarm interrupts, watchdog timer and programmable square wave output. 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 - valid until year 2100), 30 and 31 day months are made automatically.

The M41T81 is supplied in an 8-pin SOIC.

Key Features

  • For all new designs use M41T81S
  • Counters for tenths/hundredths of seconds, seconds, minutes, hours, day, date, month, year, and century
  • 32 KHz crystal oscillator integrating load capacitance (12.5 pF) providing exceptional oscillator stability and high crystal series resistance operation
  • Serial interface supports I2 C bus (400 kHz protocol)
  • Ultra-low battery supply current of 0.6 μA (typ at 3 V)
  • 2.0 to 5.5 V clock operating voltage
  • Automatic switchover and deselect circuitry (for 3 V application select M41T81S datasheet)
  • Power-down time stamp (HT bit) allowing determination of time elapsed in battery backup
  • Programmable alarm and interrupt function (valid even during battery backup mode)
  • Accurate programmable watchdog timer (from 62.5 ms to 128 s)
  • Software clock calibration to compensate crystal deviation due to temperature
  • Operating temperature of –40 to 85 °C
  • ECOPACK® package available

Circuit Diagram

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Technical Documentation

Product Specifications
Description Version Size
DS2149 DS2149: Serial access real-time clock with alarm 11.0 583 KB
Application Notes
Description Version Size
AN3060 AN3060: Applications guide for serial real-time clocks (RTCs) 3.0 878 KB
AN1011 AN1011: Battery technology used in NVRAM and real-time clock (RTC) products from ST 4.0 248 KB
AN934 AN934: How to use the digital calibration feature in TIMEKEEPER® and serial real-time clock (RTC) products 4.1 123 KB
AN1216 AN1216: Implementing a periodic alarm with TIMEKEEPER® and serial real-time clocks (RTCs) 2.0 192 KB
AN4641 AN4641: Integrating TIMEKEEPER® and real-time clock supervisor functions 1.0 479 KB
AN923 AN923: Managing century information using serial real-time clocks and TIMEKEEPER® NVRAMs 4.0 142 KB
AN1572 AN1572: Power-down time-stamp function in serial real-time clocks (RTCs) 2.0 171 KB
AN1336 AN1336: Power-fail comparator for NVRAM supervisory devices 2.0 127 KB
AN1012 AN1012: Predicting the battery life and data retention period of NVRAMs and serial RTCs 4.1 439 KB
AN922 AN922: Using a Super Cap to back up the M41T56, M41T00, M41T11, M41T81, M41T94, and M41ST84W (16-pin) 2.1 84 KB

HW Model, CAD Libraries & SVD

HW Model, CAD Libraries & SVD
Description Version Size
Ibis model 1.0 8 KB

Publications and Collaterals

Description Version Size
Serial real-time clock ICs 1.0 827 KB

Tools and Software

Embedded Software
Clocks and Timers Software
Support & Community

Sample & Buy

Part Number Package Packing Type Operating Temperature (°C) (min) Operating Temperature (°C) (max) Marketing Status Unit Price (US$) * Quantity NVRAM size (B) ECCN (EU) ECCN (US) Country of Origin More info Order from ST Order from Distributors
M41T81M6F SO-8 Tape And Reel -40 85 Active 1.106 1000 - NEC EAR99 - MORE INFO DISTRIBUTOR AVAILABILITY

(*) 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

Quality & Reliability

Part Number Marketing Status Package Grade RoHS Compliance Grade Material Declaration**
M41T81M6F ActiveSO-8IndustrialEcopack1 0 0

(**) 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.