AN3371

Overview of the STM32 advanced RTC

Doc ID 018624 Rev 5

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Setting 1 Hz as the output signal

1.

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

2.

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

3.

Set the synchronous prescaler to the default value “256“.

4.

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

5.

Select 1 Hz as the calibration output by setting CALSEL to ‘1’.

Figure 18. RTC_CALIB clock sources

Maximum and minimum RTC_CALIB 512 Hz output frequency

The RTC can output the RTCCLK clock divided by a 7-bit asynchronous prescaler. The

divider factor is configured using bits PREDIV_A[6:0] of the RTC_PRER register.

RTC_CALIB maximum and minimum frequencies are

31.250 kHz and 500 Hz, respectively.

Table 14.

RTC_CALIB output frequency versus clock source

RTC clock source

RTC_CALIB output frequency

Minimum

(PREDIV_A[6:0] = 111 111b)

(div64)

Maximum

(div32)

1.

PREDIV_A[5] must be set to ‘1’ to enable the RTC_CALIB output signal generation. If PREDIV_A[5] bit is zero, no signal

is output on RTC_CALIB.

HSE_RTC = 1MHz

15,625 kHz

31.250 KHz

LSE = 32768 Hz

512 Hz

(default output frequency)

1.024 KHz

2.

For STM32L1xx, LSI = 37 KHz.

500 Hz

1 KHz

3.

For STM32F2xx and STM32F4xx, LSI = 32 KHz.

578.125 Hz

1156.25 Hz

MS30116V1

512 Hz

AFO_CALIB

1 Hz

CALSEL

Calendar

Synchronous

prescaler

Asynchronous

prescaler

Shadow registers

(RTC_TR and

RTC_DR)

Synchronous 15-bit

prescaler (default=256)

Asynchronous 7-bit

prescaler (default=128)

Ck_Spre

RTC Clock