In the M451 series, why the read RTC_TIME and RTC_CAL values are incorrect after VDD is powered on when VBAT and VDD are

[a_ziliu]

Since VBAT and VDD are powered independently, the across power domain and synchronization issue exists. If RTC_TIME and RTC_CAL registers are read immediately after power on, the RTC_TIME and RTC_CAL values may be incorrect.

This is because both the RTC_TIME and RTC_CAL registers are triggered and updated by the RTC Counter. Please refer to the following figure. The RTC Counter is updated once per second.

Thus, to solve the power-on synchronization issue, wait a second, until the RTC Counter updates RTC_TIME and RTC_CAL, and then read the value of the register.