Introduction

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

• Calendar

• Alarm

• Auto wakeup

• Timestamp

• Tamper detection

• Programmable daylight saving time

• Didigal calibration

• Synchronization by shifting

• Reference clock detection

• Backup register

• Alternative function IO

Clock Source

LSI RC : 32 kHz

LSE OSC : 32.768 kHz

HSE OSC : 4-26 MHz

.. image:: /stm32F4 RTC clock source

Prescaler

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 propagates to synchronous prescaler to update date and time by second.

One can configure them through PREDIV_A and PREDIV_S bits in RTC_PRER

Calendar

A calendar keeps track of the date (day, week, month, year) and time (hours, minutes and seconds) and even subseconds. It manages of numbers of days of mouths automaticly. Daylight saving time adjustment is programmable by software.

Binary repersentation is in binary-coded decimal (BCD) format.

Data can be read indirectly from shadow registers or directly from connters. The former method delays some clocks but ensures consistency between date and time registers; the later is opposite but this is especially useful after exiting from low power modes, since the shadow registers are not updated during these modes.

Data, time, and subsecond corresponds to RTC_DR, RTC_TR, and RTC_SSR. With BYPSHAD bit set, data can be read directly from counter.

Synchronization and Reference clock detection

RTC can be synchronized to remote clock by adding a ‘shift’ to counter continuously to delay, or vice versa. With reference clock detection, RTC shifts misaligned 1 Hz clock to align it with the nearst referenced edge (found in a given time window).

One can adjust time by configuring shift through RTC_SHIFTR.

When reference clock detection is enabled (REFCKON), must not write RTC_SHIFTR and not enable calibration and set prescalers (PREDIV_A and PREDIV_S) to default.

Configuration