ST STM32F301 6 Series Reference Manual page 611

RM0366
The WUTF flag must then be cleared by software.
When the periodic wake-up interrupt is enabled by setting the WUTIE bit in the RTC_CR
register, it can exit the device from low-power modes.
The periodic wake-up flag can be routed to the RTC_ALARM output provided it has been
enabled through bits OSEL[1:0] of RTC_CR register. RTC_ALARM output polarity can be
configured through the POL bit in the RTC_CR register.
System reset, as well as low-power modes (Sleep, Stop and Standby) have no influence on
the wake-up timer.
24.3.7 RTC initialization and configuration
RTC register access
The RTC registers are 32-bit registers. The APB interface introduces two wait states in RTC
register accesses except on read accesses to calendar shadow registers when
BYPSHAD = 0.
RTC register write protection
After system reset, the RTC registers are protected against parasitic write access by
clearing the DBP bit in the PWR_CR register (refer to the power control section). DBP bit
must be set in order to enable RTC registers write access.
After Backup domain reset, all the RTC registers are write-protected. Writing to the RTC
registers is enabled by writing a key into the Write Protection register, RTC_WPR.
The following steps are required to unlock the write protection on all the RTC registers
except for RTC_TAFCR, RTC_BKPxR and RTC_ISR[31:8].
1. Write '0xCA' into the RTC_WPR register.
2. Write '0x53' into the RTC_WPR register.
Writing a wrong key reactivates the write protection.
The protection mechanism is not affected by system reset.
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 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.
the RTC_ISR register, and the wake-up counter is automatically reloaded with its
reload value (RTC_WUTR register value).
RM0366 Rev 5
Real-time clock (RTC)
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