26.5.6.2 UTC Mode

To update the UTC time, the RTC must be stopped by writing a 1 in RTC_CR.UPDTIM and RTC_CR.UPDCAL.

RTC_SR.ACKUPD must then be read to 1 by either polling RTC_SR or by enabling the acknowledge update interrupt by writing a 1 in RTC_IER.ACKUP. Once RTC_SR.ACKUPD is read to 1, it is mandatory to clear this flag by writing a 1 in RTC_SCCR.ACKCLR, after which the user can write to RTC_TIMR.

Once the update is finished, the user must write a 0 in RTC_CR.UPDTIM and a 0 in RTC_CR.UPCAL.

In successive update operations, the user must first check that RTC_CR.UPDTIM and RTC_CR.UPDCAL read 0 before writing a 1 in these bits.

The timing sequence of the UTC time update is described in the figure below.