ST STM32F405 Reference Manual page 804

Real-time clock (RTC)
Calendar initialization and configuration
To program the initial time and date calendar values, including the time format and the
prescaler configuration, the following sequence is required:
1. Set INIT bit to 1 in the RTC_ISR register to enter initialization mode. In this mode, the
calendar counter is stopped and its value can be updated.
2. Poll INITF bit of in the RTC_ISR register. The initialization phase mode is entered when
INITF is set to 1. It takes from 1 to 2 RTCCLK clock cycles (due to clock
synchronization).
3. To generate a 1 Hz clock for the calendar counter, program first the synchronous
prescaler factor in RTC_PRER register, and then program the asynchronous prescaler
factor. Even if only one of the two fields needs to be changed, 2 separate write
accesses must be performed to the RTC_PRER register.
4. Load the initial time and date values in the shadow registers (RTC_TR and RTC_DR),
and configure the time format (12 or 24 hours) through the FMT bit in the RTC_CR
register.
5. Exit the initialization mode by clearing the INIT bit. The actual calendar counter value is
then automatically loaded and the counting restarts after 4 RTCCLK clock cycles.
When the initialization sequence is complete, the calendar starts counting.
Note: After a system reset, the application can read the INITS flag in the RTC_ISR register to
check if the calendar has been initialized or not. If this flag equals 0, the calendar has not
been initialized since the year field is set at its backup domain reset default value (0x00).
To read the calendar after initialization, the software must first check that the RSF flag is set
in the RTC_ISR register.
Daylight saving time
The daylight saving time management is performed through bits SUB1H, ADD1H, and BKP
of the RTC_CR register.
Using SUB1H or ADD1H, the software can subtract or add one hour to the calendar in one
single operation without going through the initialization procedure.
In addition, the software can use the BKP bit to memorize this operation.
Programming the alarm
A similar procedure must be followed to program or update the programmable alarm (Alarm
A or Alarm B):
1. Clear ALRAE or ALRBIE in RTC_CR to disable Alarm A or Alarm B.
2. Poll ALRAWF or ALRBWF in RTC_ISR until it is set to make sure the access to alarm
registers is allowed. This takes 1 to 2 RTCCLK clock cycles (due to clock
synchronization).
3. Program the Alarm A or Alarm B registers (RTC_ALRMASSR/RTC_ALRMAR or
RTC_ALRMBSSR/RTC_ALRMBR).
4. Set ALRAE or ALRBIE in the RTC_CR register to enable Alarm A or Alarm B again.
Note: Each change of the RTC_CR register is taken into account after 1 to 2 RTCCLK clock cycles
due to clock synchronization.
804/1749 RM0090 Rev 18
RM0090