AN3133 datasheet(10/43 Pages) STMICROELECTRONICS

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Real-time clock overview

AN3133

10/43

Doc ID 16956 Rev 5

Figure 4.

Typical crystal accuracy plotted against temperature (and different

values of K)

In Figure 4, accuracy = K x (T – TO)

where, TO = 25° C ±5° C and K = –0.036 ppm/°C

2 ± 0.006 ppm/°C2

RTC clock smooth digital calibration consists in masking N (configurable) 32 kHz clock

pulses that are fairly well distributed in a configurable window (8s, 16s or 32s).

The number of masked or added pulses is configured by bits CALP and CALM[8:0] in the

RTC_CALRH and RTC_CALRL registers. By default, the window is 32s. It can be reduced

to 8s or 16s by setting bits CALW8 or CALW16 in the RTC_CALRH register. Reducing the

calibration window allows to test the calibration result in a lesser time, which can be useful

for factory tests. As a drawback, the digital calibration resolution is decreased when the

window size is smaller.

The calibration range is from –487.1 ppm to +488.5 ppm, which corresponds to a correction

of approximately ±0.05%.

A 1 Hz output is provided to measure the quartz crystal frequency and calibration results.

The calibration value can be changed on the fly so that it can be changed when a

temperature change is detected.

The measurement window must be multiple of the calibration window.

Table 2.

Calibration window description

Calibration window

Accuracy

Calibration step

8 s

±1.91 ppm

3.81 ppm

16 s

±0.95 ppm

1.91 ppm

32 s

±0.48 ppm

0.95 ppm

–50

–100

–150

–200

–40

–30

–20

–10

–35 ppm

+35 ppm

Temperature (

°C)

Maximum K at 25

°C

Typical K at 25

°C

Minimum K at 25

°C

AI02498

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