Resetting The Rtc - ST STM32F413 Reference Manual

Real-time clock (RTC)
After waking up from low power mode (Stop or Standby), RSF must be cleared by software.
The software must then wait until it is set again before reading the RTC_SSR, RTC_TR and
RTC_DR registers.
The RSF bit must be cleared after wakeup and not before entering low power mode.
Note: After a system reset, the software must wait until RSF is set before reading the RTC_SSR,
RTC_TR and RTC_DR registers. Indeed, a system reset resets the shadow registers to
their default values.
After an initialization (refer to
wait until RSF is set before reading the RTC_SSR, RTC_TR and RTC_DR registers.
After synchronization (refer to
reading the RTC_SSR, RTC_TR and RTC_DR registers.
When the BYPSHAD control bit is set in the RTC_CR register (bypass shadow
registers)
Reading the calendar registers gives the values from the calendar counters directly, thus
eliminating the need to wait for the RSF bit to be set. This is especially useful after exiting
from low power modes (STOP or Standby), since the shadow registers are not updated
during these modes.
When the BYPSHAD bit is set to 1, the results of the different registers might not be
coherent with each other if an RTCCLK edge occurs between two read accesses to the
registers. Additionally, the value of one of the registers may be incorrect if an RTCCLK edge
occurs during the read operation. The software must read all the registers twice, and then
compare the results to confirm that the data is coherent and correct. Alternatively, the
software can just compare the two results of the least-significant calendar register.
Note: While BYPSHAD=1, instructions which read the calendar registers require one extra APB
cycle to complete.
25.3.7

Resetting the RTC

The calendar shadow registers (RTC_SSR, RTC_TR and RTC_DR) and some bits of the
RTC status register (RTC_ISR) are reset to their default values by all available system reset
sources.
On the contrary, the following registers are resetted to their default values by a backup
domain reset and are not affected by a system reset: the RTC current calendar registers,
the RTC control register (RTC_CR), the prescaler register (RTC_PRER), the RTC
calibration registers (RTC_CALIBR or RTC_CALR), the RTC shift register (RTC_SHIFTR),
the RTC timestamp registers (RTC_TSSSR, RTC_TSTR and RTC_TSDR), the RTC tamper
and alternate function configuration register (RTC_TAFCR), the RTC backup registers
(RTC_BKPxR), the wakeup timer register (RTC_WUTR), the Alarm A and Alarm B registers
(RTC_ALRMASSR/RTC_ALRMAR and RTC_ALRMBSSR/RTC_ALRMBR).
When a backup domain reset occurs, the RTC is stopped and all the RTC registers are set
to their reset values.
25.3.8 RTC synchronization
The RTC can be synchronized to a remote clock with a high degree of precision. After
reading the sub-second field (RTC_SSR or RTC_TSSSR), a calculation can be made of the
precise offset between the times being maintained by the remote clock and the RTC. The
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Calendar initialization and
Section 25.3.8): the software must wait until RSF is set before
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configuration): the software must
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