Introduction
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A real-time clock (RTC) is a computer clock that keeps track of the current time. RTCs are also present in almost any electronic device that requires accurate time keeping. The real-time clock (RTC) embedded in STM32 microcontrollers acts as an independent BCD timer/ counter.

Clock Source
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LSI RC : 32 kHz
LSE OSC : 32.768 kHz
HSE OSC : 4-26 MHz