AN3371 datasheet(17/45 Pages) STMICROELECTRONICS

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AN3371

Overview of the STM32 advanced RTC

Doc ID 018624 Rev 5

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Summary of timebase/wakeup period extrema

When RTCCLK= 32768 Hz, the minimum and maximum period values, depending on the

configuration, are listed in Table 10.

These values are calculated when RTCCLK = 32768 Hz

1.4

RTC digital calibration

1.4.1

RTC coarse calibration

The digital coarse calibration can be used to compensate crystal inaccuracy by adding

(positive calibration) or masking (negative calibration) clock cycles at the output of the

asynchronous prescaler (ck_apre).

A negative calibration can be performed with a resolution of about 2 ppm, and a positive

calibration can be performed with a resolution of about 4 ppm. The maximum calibration

ranges from -63 ppm to 126 ppm.

Figure 10. Coarse calibration block

You can calculate the clock deviation using AFO_CALIB, then update the calibration block. It

is not possible to check the calibration result, as the 512 Hz output is before the calibration

block. You can check the calibration result with certain products, as the 1 Hz CK_Spre

output is after the coarse calibration block. Refer to Table 15: Advanced RTC features.

Note:

The calibration settings can only be changed during initialization.

The full calibration cycle lasts 64 minutes.

The calibration is done during the first minutes (from 0 to 62 min depending on the

configuration) of the calibration cycle.

We recommend the use of coarse calibration for static correction only. Due to the points

listed in note 1, changing the calibration settings brings errors:

- Entering initialization mode stops the calendar and reinitializes the prescalers

- The calibration change rate must be very much smaller than the calibration window size in

order to minimize the impact of the error brought by the change on the final accuracy.

Table 10.

Min. and max. timebase/wakeup period when RTCCLK= 32768

Configuration

Minimum period

Maximum period

122.07 µs

more than 36 hours

250.06 ms

more than 48 days

MS19531V1

RTC Clock

Asynch. 7-bit prescaler

(default=128)

Asynchronous

prescaler

Synchronous

prescaler

Synchronous 13-bit

prescaler (default=256)

Shadow registers

(RTC_TR, RTC_DR)

Calendar

512 Hz

AFO_CALIB

Ck_Spre

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