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.

Features
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- Calendar
- Alarm
- Auto wakeup
- Timestamp
- Tamper detection

- Programmable daylight saving time
- Didigal calibration
- Synchronization
- Reference clock detection
- Backup register
- Alternative function IO


Clock Source
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LSI  RC : 32 kHz

LSE OSC : 32.768 kHz

HSE OSC : 4-26 MHz

.. image:: /stm32F4 RTC clock source

Prescaler
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Prescaler generates the clock to update update the calendar.

To minimize power comsuption the prescaler is split into two prescalers, asynchronous and synchronous. It is recommended to configure the asynchronous prescaler to a high value to minimize consumption.

asynchronous prescaler clocks subsecond of calendar and propagate to synchronous prescaler to update date and time by second.

Configuration
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