AN3371 datasheet(20/45 Pages) STMICROELECTRONICS

20 / 45 page

Overview of the STM32 advanced RTC

AN3371

20/45

Doc ID 018624 Rev 5

1.6

RTC reference clock detection

The reference clock (at 50 Hz or 60 Hz) should have a higher precision than the 32.768 kHz

LSE clock. This is why the RTC provides a reference clock input (RTC_50Hz pin) that can

be used to compensate the imprecision of the calendar frequency (1 Hz).

The RTC_50Hz pin should be configured in input floating mode.

This mechanism enables the calendar to be as precise as the reference clock.

The reference clock detection is enabled by setting REFCKON bit of the RTC_CR register.

When the reference clock detection is enabled, PREDIV_A and PREDIV_S must be set to

their default values: PREDIV_A = 0x007F and PREVID_S = 0x00FF.

When the reference clock detection is enabled, 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. The update window is 3 ck_calib periods (ck_calib is the output of the

coarse calibration block).

If the reference clock halts, the calendar is updated continuously based solely on the LSE

clock. The RTC then waits for the reference clock using a detection window centered on the

Synchronous Prescaler output clock (ck_spre) edge. The detection window is 7 ck_calib

periods.

The reference clock can have a large local deviation (for instance in the range of 500 ppm),

but in the long term it must be much more precise than 32 kHz quartz.

The detection system is used only when the reference clock needs to be detected back after

a loss. As the detection window is a bit larger than the reference clock period, this detection

system brings an uncertainty of 1 ck_ref period (20 ms for a 50 Hz reference clock) because

we can have 2 ck_ref edges in the detection window. Then the update window is used,

which brings no error as it is smaller than the reference clock period.

We assume that ck_ref is not lost more than once a day. So the total uncertainty per month

would be 20 ms *1* 30 = 0.6 s, which is much less than the uncertainty of a typical quartz

(1.53 minute per month for 35 ppm quartz).

Figure 13. RTC reference clock detection

Note:

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

The reference clock calibration is the best (ensures a high calibrated time) if the 50 Hz is

always available. If the 50 Hz input is lost, the LSE can be used.

The reference clock detection cannot be used in Vbat mode.

The reference clock calibration can only be used if you provide a precise 50 or 60 Hz input.

MS19545V1

CK_Spre

Calendar

Shadow registers

(RTC_TR

RTC_DR)

Autodetection

of 50 or 60 Hz

Voltage

adaptor 220V

to 5V or 3.3V

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