AN3133 datasheet(16/43 Pages) STMICROELECTRONICS

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Real-time clock overview

AN3133

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Doc ID 16956 Rev 5

Only a power-on reset can reset the calendar. A system reset does not affected it but resets

the shadow registers which are read by the application. They will be updated again when

the RSF bit is set. After a system reset, the application can check the INITS status flag in

RTC_ISR1 to verify if the calendar is already initialized. This flag is reset when the calendar

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

initialized.

1.8.3

Synchronization

When the application reads the calendar, it actually accesses shadow registers which

contain a copy of the real calendar time and date clocked by the RTCCLK clock. To make

sure that the shadow registers are updated with the current calendar value, the application

must check that the RSF bit is set in the RTC_ISR1 register. This bit is set by hardware each

time the calendar time and date shadow registers are updated, that is when the RTCCLK

clock is synchronized with the system clock SYSCLK. The application software must clear

the RSF bit after reading the calendar registers.

When the system is woken up from Active-halt mode (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 Active-halt mode.

On medium+ and high density devices, it is possible to directly read the calendar instead of

reading shadow registers. This is configured by setting the BYPSHAD bit in the RTC_CR1

registers consistency must be checked by SW by executing a SW vote.

The user must read the required calendar fields values. Then the read operation must be

performed again. The results of the two read sequence are then compared. If the results

match, the read result is correct. If they do not match, the fields must be read once more,

and the 3rd read result is valid.

AN3133 Datasheet(PDF) 16 Page - STMicroelectronics