Real-time clock (RTC) RM0033
570/1381 RM0033 Rev 9
22 Real-time clock (RTC)
This section applies to the whole STM32F20x and STM32F21x family, unless otherwise
specified.
22.1 Introduction
The real-time clock (RTC) is an independent BCD timer/counter. The RTC provides a time-
of-day clock/calendar, two programmable alarm interrupts, and a periodic programmable
wakeup flag with interrupt capability. The RTC also includes an automatic wakeup unit to
manage low-power modes.
Two 32-bit registers contain the seconds, minutes, hours (12- or 24-hour format), day (day
of week), date (day of month), month, and year, expressed in binary coded decimal format
(BCD). .
Compensations for 28-, 29- (leap year), 30-, and 31-day months are performed
automatically. Daylight saving time compensation can also be performed.
Additional 32-bit registers contain the programmable alarm seconds, minutes, hours, day,
and date.
A digital calibration feature is available to compensate for any deviation in crystal oscillator
accuracy.
After backup domain reset, all RTC registers are protected against possible parasitic write
accesses.
As long as the supply voltage remains in the operating range, the RTC never stops,
regardless of the device status (Run mode, low-power mode or under reset).
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