PCF8523TSLASH1 datasheet(31/75 Pages) NXP

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PCF8523

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Product data sheet

Rev. 4 — 5 July 2012

31 of 74

NXP Semiconductors

PCF8523

Real-Time Clock (RTC) and calendar

8.8.3 Offset calibration workflow

The calibration offset has to be calculated based on the time. Figure 18 shows the

workflow how the offset register values can be calculated:

Fig 18. Offset calibration calculation workflow

Measure the frequency on pin CLKOUT:

fmeas

Convert to time:

tmeas = 1 / fmeas

Calculate the difference to the ideal

period of 1 / 32768.00:

Dmeas = 1 / 32768 - tmeas

Calculate the ppm deviation compared

to the measured value:

Eppm = 1000000 × Dmeas / tmeas

Calculate the offset register value:

Mode = 0 (low power):

Offset value = Eppm / 4.34

Mode = 1 (fast correction)

Offset value = Eppm / 4.069

013aaa683

Example

32768.48 Hz

30.5171 µs

0.000447 µs

14.6484 ppm

3.375

3 correction pulses

are needed

3.600

4 correction pulses

are needed

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