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.
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.
Although keeping time can be done without an RTC, 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
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. If it has been less than a few hours, then the previous ephemeris is still usable.
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, because they are rechargeable and can be soldered. The alternate power source can also supply power to battery backed RAM.
Most RTCs use a crystal oscillator, but some use the power line frequency. In many cases the oscillator's frequency is 32.768 kHz. 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.
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.
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