AN3371 datasheet(19/45 Pages) STMICROELECTRONICS

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AN3371

Overview of the STM32 advanced RTC

Doc ID 018624 Rev 5

19/45

A smooth calibration can be performed on the fly so that it can be changed when the

temperature changes or if other factors are detected.

Checking the smooth calibration

The smooth calibration effect on the calendar clock (RTC Clock) can be checked by:

●

Calibration using the AFO_CALIB (512 Hz or 1 Hz).

●

Calibration using the sub-second alarms.

●

Calibration using the Wakeup timer.

1.5

Synchronizing the RTC

The RTC calendar can be synchronized to a more precise clock, “remote clock”, using the

RTC shift feature. After reading the RTC sub-second field, a calculation of the precise offset

between the time being maintained by the remote clock and the RTC can be made. The

RTC can be adjusted by removing this offset with a fine adjustment using the shift register

control.

Figure 12. RTC shift register

It is not possible to check the “Synchronization” Shift function using the AFO_CALIB output

since the shift operation has no impact on the RTC clock, other than adding or subtracting a

few fractions from the calendar counter.

Correcting the RTC calendar time

If the RTC clock is advanced compared to the remote clock by n fractions of seconds, the

offset value must be written in SUBFS, which will be added to the synchronous prescaler’s

counter. As this counter counts down, this operation effectively subtracts from (delays) the

clock by:

Delay (seconds) = SUBFS / (PREDIV_S + 1)

If the RTC is delayed compared to the remote clock by n fractions of seconds, the offset

value can effectively be added to the clock (advancing the clock) when the ADD1S function

is used in conjunction with SUBFS, effectively advancing the clock by:

Advance (seconds) = (1 - (SUBFS / (PREDIV_S + 1))).

MS30111V1

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

Shift

(RTC_SHIFTER)

delay

advance

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