Real Time Clock

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

A real-time clock (RTC) is a computer clock (most often in the form of an integrated circuit) that keeps track of the current time. Although the term often refers to the devices in personal computers, servers and embedded systems, RTCs are present in almost any electronic device which needs to keep accurate time.

1 Terminology
2 Purpose
3 Power source
4 Timing
5 Examples
6 See also
7 References

Terminology

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.

Purpose

Although keeping time can be done without an RTC^[1], using one has benefits:

Low power consumption (important when running from alternate power)
Frees the main system for time-critical tasks
Sometimes more accurate than other methods (although personal computer RTC's are often inaccurate)

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.^[2] If it has been less than a few hours then the previous ephemeris is still usable.

Power source

RTCs often have an alternate source of power, so they can continue to keep time while the primary source of power is off or unavailable. This alternate source of power is normally a lithium battery in older systems, but some newer systems use a supercapacitor^[3], because they are rechargeable and can be soldered. The alternate power source can also supply power to battery backed RAM.

Timing

Most RTCs use a crystal oscillator^[4]^[5], but some use the power line frequency^[6] . In many cases the oscillator's frequency is 32.768 kHz.^[4] 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.

Examples

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

Many integrated circuit manufacturers make RTCs, including Intersil, Maxim, Philips, Texas Instruments and STMicroelectronics. RTCs in older personal computers were often made by Dallas Semiconductor, now a subsidiary of Maxim, 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.^[7]