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Serial real-time clock (RTC) with alarm
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The M41T66 is a low-power serial real-time clock (RTC) with a built-in 32.768 kHz oscillator (external crystal controlled). Eight registers are used for the clock/calendar function and are configured in binary coded decimal (BCD) format. An additional 8 registers provide status/control of alarm, square wave, calibration, and watchdog 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.

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

The M41T66 is supplied in a 16-pin QFN.

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Errata Sheet
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Key Features

  • Counters for tenths/hundredths of seconds, seconds, minutes, hours, day, date, month, year, and century
  • 32 KHz crystal oscillator integrating load capacitance and high crystal series resistance operation
  • Oscillator stop detection monitors clock operation
  • Serial interface supports I2C bus (400 kHz)
  • 525 nA timekeeping current at 3 V
  • Low operating current of 35 μA (at 400 kHz)
  • Timekeeping down to 1.0 V
  • 1.3 V to 4.4 V I2C bus operating voltage
  • Allows use in lithium ion rechargeable applications
  • 32 KHz square wave on power-up to drive a microcontroller in low-power mode
  • Programmable (1 Hz to 32 KHz) square wave
  • Programmable alarm with interrupt function
  • Accurate programmable watchdog (from 62.5 ms to 31 min)
  • Software clock calibration to compensate deviation of crystal due to temperature
  • Automatic leap year compensation
  • Operating temperature of –40 to 85 °C
  • Lead-free 16-pin QFN package

Circuit Diagram

Design Resources


Technical Documentation

Product Specifications

Description Version Size
DS5956: Serial real-time clock with alarms
2.0 412 KB

Application Notes

Description Version Size
AN1011: Battery technology used in NVRAM and real-time clock (RTC) products from ST
4.0 248 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
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

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**
M41T66Q6FActiveVFQFPN 16 3x3x0.9Tape And Reel-4085_Free Samples.95Distributor reported inventory date: 2016-02-10
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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