Real-time clock
computer clock in the form of an integrated circuit that keeps track of the current time
A real-time clock (RTC) is an electronic timekeeping device, most often in the form of an integrated circuit (IC).
Although the term often refers to the devices in personal computers, servers and embedded systems, RTCs are present in almost any electronic device that needs to keep accurate time of day.
Terminology
The term real-time clock 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, using one has benefits:
Reliably maintains and provides current time through disruptive system states such as hangs, sleep, reboots, or if given sufficient backup power, full shutdown and hardware reassembly, without the need to have its time set again.
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.
Some motherboards are made without RTCs. The RTC may be omitted out of a desire to save money or reduce possible sources of hardware failure.
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.
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