Real-time clock

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Dallas semiconductor realtime clock from an older PC. This version also contains a battery backed SRAM.



The term is used to avoid confusion with ordinary hardware clocks which are only signals that govern digital electronics, and do not count time in human units. RTC should not be confused with real-time computing, which shares its three-letter acronym, but does not directly relate to time of day.



  • Low power consumption [2] (important when running from alternate power)
  • Frees the main system for time-critical tasks
  • Sometimes more accurate than other methods

A GPS receiver can shorten its startup time by comparing the current time, according to its RTC, with the time at which it last had a valid signal.[3] If it has been less than a few hours, then the previous ephemeris is still usable.

Power source[edit]



Most RTCs use a crystal oscillator,[7][8] but some use the power line frequency.[9] In many cases the oscillator's frequency is 32.768 kHz.[7] This is the same frequency used in quartz clocks and watches, and for the same reasons, namely that the frequency is exactly 215 cycles per second, which is a convenient rate to use with simple binary counter circuits.


This chip, labeled ODIN, is a generic equivalent to a particular Dallas RTC.

Many integrated circuit manufacturers make RTCs, including Epson, Intersil, IDT, Maxim, NXP Semiconductor, Texas Instruments and STMicroelectronics. The RTC was introduced to PC compatibles by the IBM PC/AT in 1984, which used a MC146818 RTC. Later Dallas Semiconductor made compatible RTCs, which was often used in older personal computers, and are easily found on motherboards because of their distinctive black battery cap and silkscreened logo. In newer systems the RTC is integrated into the southbridge chip.[10]

Some microcontrollers have a real-time clock built in, generally only the ones with many other features and peripherals.

See also[edit]


  1. ^ Ala-Paavola, Jaakko (2000-01-16). "Software interrupt based real time clock source code project for PIC microcontroller". Retrieved 2007-08-23. 
  2. ^ Enabling Timekeeping Function and Prolonging Battery Life in Low Power Systems, NXP Semiconductors, 2011 
  3. ^ US 5893044  Real time clock apparatus for fast acquisition or GPS signals
  4. ^ New PCF2123 Real Time Clock Sets New Record in Power Efficiency, futurlec 
  5. ^ Application Note 3816, Maxim/Dallas Semiconductor, 2006 
  6. ^ Torres, Gabriel (24 November, 2004). "Introduction and Lithium Battery". Replacing the Motherboard Battery. Retrieved June 20, 2013. 
  7. ^ a b Application Note 10337, ST Microelectronics, 2004, p. 2 
  8. ^ Application Note U-502, Texas Instruments, 2004, p. 13 
  9. ^ Application Note 1994, Maxim/Dallas Semiconductor, 2003 
  10. ^ "ULi M1573 Southbridge Specifications". Retrieved 2007-08-23.