AN3371 datasheet(29/45 Pages) STMICROELECTRONICS

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AN3371

Overview of the STM32 advanced RTC

Doc ID 018624 Rev 5

29/45

Figure 20. Periodic wakeup routed to RTC_ALARM pinout

1.12

RTC security aspects

1.12.1

RTC register write protection

To protect RTC registers against possible parasitic write accesses after reset, the RTC

registers are automatically locked. They must be unlocked to update the current calendar

time and date.

Writing to the RTC registers is enabled by programming a key in the Write protection

The following steps are required to unlock the write protection of the RTC register:

Write

0xCA into the RTC_WPR register.

Write

0x53 into the RTC_WPR register.

Writing an incorrect key automatically reactivates the RTC register write access protection.

1.12.2

Enter/exit initialization mode

The RTC can operate in two modes:

●

Initialization mode, where the counters are stopped.

●

Free-running mode, where the counters are running.

The calendar cannot be updated while the counters are running. The RTC must

consequently be switched to the Initialization mode before updating the time and date.

When operating in this mode, the counters are stopped. They start counting from the new

value when the RTC enters the Free-running mode.

The INIT bit of the RTC_ISR register enables you to switch from one mode to another, and

the INITF bit can be used to check the RTC current mode.

The RTC must be in Initialization mode to program the time and date registers (RTC_TR

and RTC_DR) and the prescalers register (RTC_PRER). This is done by setting the INIT bit

and waiting until the RTC_ISR_INITF flag is set.

To return to the Free-running mode and restart counting, the RTC must exit the Initialization

mode. This is done by resetting the INIT bit.

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

the shadow registers that are read by the application. They are updated again when the

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

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

MS19536V1

RTC_ALARM

output

OSEL[1:0]=11

Wakeup unit

16-bit autoreload

timer

Periodic

wakeup flag

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