AN3133 datasheet(15/43 Pages) STMICROELECTRONICS

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AN3133

Real-time clock overview

Doc ID 16956 Rev 5

15/43

RTC_ALARM output connected to the wakeup unit

When the wakeup downcounting timer reaches 0, the wakeup flag is set to ‘1’. If this flag is

selected as source for the RTC_ALARM output (OSEL[1:0] bits set to ‘11’ in RTC_CR3

as the flag is not cleared.

Figure 10.

Periodic wake-up routed to RTC_ALARM pinout

1.8

RTC security aspects

1.8.1

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.

1.8.2

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 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 Free-running mode.

The INIT bit of the RTC_ISR1 register allows to switch from one mode to another, while 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_TRx

and RTC_DRx) and the prescaler registers (RTC_SPRERx and RTC_APRER). This is done

by setting the INIT bit and waiting until the RTC_ISR1_INITF flag is set.

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

This is done by resetting the INIT bit.

ai17266

Periodic wakeup flag

Alarm flag

Wakeup unit

16-bit autoreload

timer

RTC_ALARM

output

RTC_CR3/OSEL[1:0]

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