ST STM32L4+ Series Reference Manual page 1604

Real-time clock (RTC) applied to STM32L4P5xx and STM32L4Q5xx only
The update of the RTC calendar can be synchronized to a reference clock, RTC_REFIN,
which is usually the mains frequency (50 or 60 Hz). The precision of the RTC_REFIN
reference clock should be higher 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 RTC_REFIN 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_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 RTC_REFIN detection is enabled, PREDIV_A and PREDIV_S must be set to their
default values:
•
PREDIV_A = 0x007F
•
PREVID_S = 0x00FF
Note: RTC_REFIN clock detection is not available in Standby mode.
47.3.14 RTC smooth digital calibration
The RTC frequency can be digitally calibrated with a resolution of about 0.954 ppm with a
range from -487.1 ppm to +488.5 ppm. The correction of the frequency is performed using
series of small adjustments (adding and/or subtracting individual ck_cal pulses).
If LPCAL=0: ck_cal = RTCCLK
If LPCAL=1: ck_cal = ck_apre
These adjustments are fairly well distributed so that the RTC is well calibrated even when
observed over short durations of time.
Calibration ultra-low-power mode
The calibration consumption can be reduced by setting the LPCAL bit in the RTC calibration
register (RTC_CALR). In this case, the calibration mechanism is applied on ck_apre instead
of RTCCLK. The resulting accuracy is the same, but the calibration is performed during a
calibration cycle of about 2
when LPCAL=0.
1604/2301
20
x PREDIV_A x RTCCLK pulses instead of 2
RM0432 Rev 6
RM0432
20
RTCCLK pulses