Rtc Reference Clock Detection; Rtc Coarse Digital Calibration - ST STM32F412 Reference Manual

Real-time clock (RTC)
22.3.9

RTC reference clock detection

The RTC calendar update can be synchronized to a reference clock RTC_REFIN, usually
the mains (50 or 60 Hz). The RTC_REFIN reference clock should have a higher precision
than the 32.768 kHz LSE clock. When the RTC_REFIN detection is enabled (REFCKON bit
of RTC_CR set to 1), the calendar is still clocked by the LSE, and RTC_REFIN is used to
compensate for the imprecision of the calendar update frequency (1 Hz).
Each 1 Hz clock edge is compared to the nearest reference clock edge (if one is found
within a given time window). In most cases, the two clock edges are properly aligned. When
the 1 Hz clock becomes misaligned due to the imprecision of the LSE clock, the RTC shifts
the 1 Hz clock a bit so that future 1 Hz clock edges are aligned. Thanks to this mechanism,
the calendar becomes as precise as the reference clock.
The RTC detects if the reference clock source is present by using the 256 Hz clock
(ck_apre) generated from the 32.768 kHz quartz. The detection is performed during a time
window around each of the calendar updates (every 1 s). The window equals 7 ck_apre
periods when detecting the first reference clock edge. A smaller window of 3 ck_apre
periods is used for subsequent calendar updates.
Each time the reference clock is detected in the window, the asynchronous prescaler which
outputs the ck_apre clock is forced to reload. This has no effect when the reference clock
and the 1 Hz clock are aligned because the prescaler is being reloaded at the same
moment. When the clocks are not aligned, the reload shifts future 1 Hz clock edges a little
for them to be aligned with the reference clock.
If the reference clock halts (no reference clock edge occurred during the 3 ck_apre window),
the calendar is updated continuously based solely on the LSE clock. The RTC then waits for
the reference clock using a large 7 ck_apre period detection window centered on the
ck_spre edge.
When the reference clock detection is enabled, PREDIV_A and PREDIV_S must be set to
their default values:
• PREDIV_A = 0x007F
•
PREDIV_S = 0x00FF
Note: The reference clock detection is not available in Standby mode.
Caution: The reference clock detection feature cannot be used in conjunction with the coarse digital
calibration: RTC_CALIBR must be kept at 0x0000 0000 when REFCKON=1.
22.3.10

RTC coarse digital calibration

Two digital calibration methods are available: coarse and smooth calibration. To perform
coarse calibration refer to
The two calibration methods are not intended to be used together, the application must
select one of the two methods. Coarse calibration is provided for compatibly reasons. To
perform smooth calibration refer to
Section 22.6.16: RTC calibration register (RTC_CALR)
The coarse digital 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).
Positive and negative calibration are selected by setting the DCS bit in RTC_CALIBR
register to '0' and '1', respectively.
626/1163
Section 22.6.7: RTC calibration register
Section 22.3.11: RTC smooth digital calibration
RM0402 Rev 6
RM0402
(RTC_CALIBR).
and to