The quartz tuning fork of these crystals does not change size much from temperature, so temperature does not change its frequency much. 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. Power source Lithium battery inside the real-time clock IC The real time clock may be omitted out of the desire to save money. Some motherboards are made without real time clocks. If it has been less than a few hours, then the previous ephemeris is still usable.
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