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Overview of the STM32 advanced RTC AN3371

26/45 Doc ID 018624 Rev 5

The RTC keeps working in reset mode and its registers are only reset by a VDD or VBAT

power-on, if both supplies have previously been powered off or the Backup Domain is reset

on STM32F2xx devices.

Registers are only reset by a power-on reset. RTC register values are not lost after a reset

and the calendar keeps the correct time and date.

After a system reset or a power-on reset, the STM32 operates in Run mode. In addition, the

device supports five low power modes to achieve the best compromise between low power

consumption, short startup time and available wakeup sources.

The RTC peripheral can be active in the following low power modes:

● Sleep mode

● Low power Run mode (only for ULPM and ULPH density devices)

● Low power Sleep mode (only for ULPM and ULPH density devices)

● Standby mode

● Stop mode

Refer to the low power modes section of the STM32 reference manuals for more details

about low power modes.

1.11 Alternate function RTC outputs

The RTC peripheral has two outputs:

● RTC_CALIB, used to generate an external clock.

● RTC_ALARM, a unique output resulting from the multiplexing of the RTC alarm and

wakeup events.

1.11.1 RTC_CALIB output

The RTC_CALIB output is used to generate a variable-frequency signal. Depending on the

user application, this signal can play the role of a reference clock to calibrate an external

device, or be connected to a buzzer to generate a sound.

The signal frequency is configured using the 7 LSB bits (PREDIV_A [6:0]) of the

asynchronous prescaler PREDIV_A[7:0].

RTC_CALIB is the output of bit 4 of the 7-bit asynchronous prescaler PREDIV_A. If

PREDIV_A[5]=0, no signal is output on RTC_CALIB.

Setting 512 Hz as the output signal

1. Select LSE “32768 Hz” as RTC clock source.

2. Set the asynchronous prescaler to the default value “128“.

3. Enable the output calibration by setting “COE” to ‘1’.

4. Select 512 Hz as the calibration output by setting CALSEL to ‘0’.

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Ultra Start 23xx User Manual Page 26