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The low frequency saves power, while remaining above human hearing range. Being exactly 2 15 cycles per second, it is a convenient rate to use with simple binary counter circuits. The crystal frequency is usually 32.768 kHz, the same frequency used in quartz clocks and watches. Most RTCs use a crystal oscillator, but some have the option of using the power line frequency.

The alternate power source can also supply power to battery backed RAM.

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. 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. Lithium battery inside the real-time clock IC The real time clock is omitted either out of the desire to save money (as in the Raspberry Pi system architecture) or because real time clocks may not be needed at all (as in the Arduino system architecture ). Some motherboards are made without real time clocks. If it has been less than a few hours, then the previous ephemeris is still usable.

Sometimes more accurate than other methodsĪ 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.

Frees the main system for time-critical tasks.

Low power consumption (important when running from alternate power).

RTC should not be confused with real-time computing, which shares its three-letter acronym but does not directly relate to time of day.Īlthough keeping time can be done without an RTC, using one has benefits: 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. Types of hobbyist RTC modules commercially available from ChinaĪ real-time clock ( RTC) is an electronic device (most often in the form of an integrated circuit) that measures the passage of time.Īlthough 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 of day.