Real-time clock (RTC) RM0440
1548/2126 RM0440 Rev 4
1. Set INIT bit to 1 in the RTC_ICSR 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_ICSR register. The initialization phase mode is entered
when INITF is set to 1. It takes around 2 RTCCLK clock cycles (due to clock
synchronization).
3. To generate a 1 Hz clock for the calendar counter, program both the prescaler factors
in 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_ICSR 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_ICSR 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 alarms. The
procedure below is given for alarm A but can be translated in the same way for alarm B.
1. Clear ALRAE in RTC_CR to disable alarm A.
2. Program the alarm A registers (RTC_ALRMASSR/RTC_ALRMAR).
3. Set ALRAE in the RTC_CR register to enable alarm A again.
Note: Each change of the RTC_CR register is taken into account after around 2 RTCCLK clock
cycles due to clock synchronization.
Programming the wakeup timer
The following sequence is required to configure or change the wakeup timer auto-reload
value (WUT[15:0] in RTC_WUTR):
1. Clear WUTE in RTC_CR to disable the wakeup timer.
2. Poll WUTWF until it is set in RTC_ICSR to make sure the access to wakeup auto-
reload counter and to WUCKSEL[2:0] bits is allowed. This step must be skipped in
calendar initialization mode. It takes around 2 RTCCLK clock cycles (due to clock
synchronization).
3. Program the wakeup auto-reload value WUT[15:0], and the wakeup clock selection
(WUCKSEL[2:0] bits in RTC_CR). Set WUTE in RTC_CR to enable the timer again.
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