AN3371 datasheet(30/45 Pages) STMICROELECTRONICS

30 / 45 page

Overview of the STM32 advanced RTC

AN3371

30/45

Doc ID 018624 Rev 5

calendar year field is set to 0x00 (power-on reset value), meaning that the calendar must be

initialized.

1.12.3

RTC clock synchronization

When the application reads the calendar, it accesses shadow registers that contain a copy

of the real calendar time and date clocked by the RTC clock (RTCCLK). The RSF bit is set in

the RTC_ISR register each time the calendar time and date shadow registers are updated

with the real calendar value. The copy is performed every two RTCCLK cycles,

synchronized with the system clock (SYSCLK). After a system reset or after exiting the

initialization mode, the application must wait for RSF to be set before reading the calendar

shadow registers.

When the system is woken up from low power modes (SYSCLK was off), the application

must first clear the RSF bit, and then wait until it is set again before reading the calendar

registers. This ensures that the value read by the application is the current calendar value,

and not the value before entering the Low power mode.

By setting the “BYPASHAD” bit to ‘1’ in the RTC_CR register, the calendar values are taken

directly from the calendar counters instead of reading the shadow register. In this case, it is

not mandatory to wait for the synchronization time, but the calendar registers consistency

must be checked by the software. The user must read the required calendar field values.

The read operation must then be performed again. The results of the two read sequences

are then compared. If the results match, the read result is correct. If they do not match, the

fields must be read one more time, and the third read result is valid.

Note:

After resetting the BYPASHAD bit, the shadow registers may be incorrect until the next

synchronization. In this case, the software should clear the “RSF” bit then wait for the

synchronization (“RSF” should be set) and finally read the shadow registers.

AN3371 Datasheet(PDF) 30 Page - STMicroelectronics