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. A real time clock is a clock that keeps track of the time even when the system is turned off. In contrast, clocks that are not real-time do not function when the system is off. Most of the real-time clocks operate at 32.768KHz.
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.
 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, 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 the Americas it is typically 60 Hz). In many cases the oscillator's frequency is 32.768KHz. 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. You can check Equivalent Circuit and Paramenters of Crystal for more details.
-  http://en.wikipedia.org/wiki/Real-time_clock
-  http://www.st.com/stonline/products/literature/an/10337.htm
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