AN4759

RTC safety aspects

Maximum and minimum RTC_CALIB 512 Hz output frequency

The RTC_CALIB output can also be used to generate a variable-frequency signal. Depending on the user application, this signal can play the role of a source clock for an external device, or can be connected to a buzzer to generate sound.

The signal frequency is configured using the 6 LSB bits (PREDIV_A [5:0]) of the asynchronous prescaler PREDIV_A[6:0].

Table 17. RTC_CALIB output frequency versus clock source

1.PREDIV_A[5] must be set to 1 to enable the RTC_CALIB output signal generation. If PREDIV_A[5] = 0, no signal is output on RTC_CALIB.

2.12.2RTC_ALARM (RTC2)/TAMPALRM (RTC3) output

The alarm, wakeup and tamper flags can be routed to the RTC outputs (one output for RTC2 type, three for RTC3 type) thanks to OSEL[1:0] in RTC_CR.

For RTC2 type

•OSEL[1:0] = 01 roots alarm A to the RTC_ALARM output.

•OSEL[1:0] = 10 roots alarm B to the RTC_ALARM output.

•OSEL[1:0] = 11 roots wakeup flag to the RTC_ALARM output.

Once OSEL[1:0] is fixed, the output reflects the selected flag stored in RTC_ISR.

For RTC3 type

The OSEL[1:0] selection follows the same principle as for RTC2 but, when TAMPOE = 1 in RTC_CR, tamper flags are ORed with the flag already selected (alarm A, alarm B or wakeup) before the OR result to be rooted to the TAMPALRM output.

If TAMPOE = 1 and OSEL[1:0] = 00, TAMPALRM output reflects only the tamper flags.

For both RTC2 and RTC3 types, the output pin polarity is selected with POL in RTC_CR. When POL = 1, the opposite state of the flag concerned is output on RTC_ALARM (RTC2)/TAMPALRM (RTC3). The pin can also be configured as open drain or push-pull with the ALARMOUTTYPE (RTC2)/TAMPALRM_TYPE (RTC3) bit of the same register (0 for push-pull, 1 for open-drain).

For RTC3, an internal pull-up can be added to the output pin by setting TAMPALRM_PU = 1 in RTC_CR.

2.13RTC safety aspects

2.13.1RTC register write protection

To protect the RTC registers against possible unintentional write accesses after reset, the RTC registers are initially locked after a Backup domain reset. These registers must be unlocked to update the current calendar time and date.