Serial real-time clock (RTC) with alarm

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.

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 I2 C 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 I2 C 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

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Technical Documentation

Product Specifications
Description Version Size
DS5956 DS5956: Serial real-time clock with alarms 2.0 411 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

Publications and Collaterals

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

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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
M41T66Q6F VFQFPN 16 3x3x0.9 Tape And Reel -40 85 Active 0.823 1000 - NEC EAR99 MALAYSIA MORE INFO No availability reported, please contact our Sales office

(*) 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**
M41T66Q6F ActiveVFQFPN 16 3x3x0.9IndustrialEcopack2 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.