AN3371 datasheet(18/45 Pages) STMICROELECTRONICS

18 / 45 page

Overview of the STM32 advanced RTC

AN3371

18/45

Doc ID 018624 Rev 5

Consequently, the coarse calibration is not adequate for a dynamic calibration (such as the

compensation of the quartz variations due to external temperature changes).

The reference clock calibration and the coarse calibration cannot be used together.

Caution:

Digital coarse calibration may not work correctly if PREDIV_A < 6.

1.4.2

RTC smooth calibration

The RTC clock frequency can be corrected using a series of small adjustments by adding or

subtracting individual RTCCLK pulses.The RTC clock can be calibrated with a resolution of

about 0.954 ppm with a range from -487.1 ppm to +488.5 ppm.

This digital smooth calibration is designed to compensate for the inaccuracy of crystal

oscillators due to temperature, crystal aging.

Figure 11. Smooth calibration block

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

is possible to check the calibration result using calibration output 512 Hz or 1 Hz for the

AFO_CALIB signal, depending on the products. Refer to Table 15: Advanced RTC features.

Smooth calibration consists of masking and adding N (configurable) 32 kHz pulses that are

well distributed in a configurable window (8 s, 16 s or 32 s).

The number of masked or added pulses is defined using CALP and CALM in the

RTC_CALR register.

By default, the calibration window is 32 seconds. It can be reduced to 8 or 16 seconds by

setting the CALW8 bit or the CALW16 bit in the RTC_CALR register:

Example 1: Setting CALM[0] to 1, CALP=0 and using 32 seconds as a calibration window

results in exactly one pulse being masked for 32 seconds.

Example 2: Setting CALM[2] to 1, CALP=0 and using 32 seconds as a calibration window

results in exactly 4 pulses being masked for 32 seconds.

Note:

Both CALM and CALP can be used and, in this case, an offset ranging from -511 to +512

pulses can be added for 32 seconds (calibration window).

When the asynchronous prescaler is less than 3, CALP cannot be set to 1.

The formula to calculate the effective calibrated frequency (FCAL), given the input frequency

(FRTCCLK), is:

FCAL = FRTCCLK x [ 1 + (CALP x 512 - CALM) / (2

20 + CALM - CALP x 512) ].

MS30112V1

RTC

Clock

Asynch. 7-bit prescaler

(default=128)

Asynchronous

prescaler

Synchronous

prescaler

Synchronous 15-bit

prescaler (default=256)

Shadow registers

(RTC_TR, RTC_DR)

Calendar

512 Hz

AFO_CALIB

Ck_Spre

AN3371 Datasheet(PDF) 18 Page - STMicroelectronics

AN3371 Datasheet(HTML) 18 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

Datasheet Download

Link URL