Rtc Reference Clock Detection - ST STM32F205 series Reference Manual

RM0033
the RTC backup registers (RTC_BKPxR), the wakeup timer register (RTC_WUTR), the
Alarm A and Alarm B registers (RTC_ALRMAR and RTC_ALRMBR).
In addition, when the RTC is clocked by the LSE, it goes on running under system reset if
the reset source is different from a backup domain reset one. Refer to section RTC clock of
the Reset and clock controller for details about the list of the RTC clock sources that are not
affected by system reset.
When a backup domain reset occurs, the RTC is stopped and all the RTC registers are set
to their reset values.
22.3.8

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 synchronous prescaler which
outputs the ck_spre 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.
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