PCF85263ATLSLASHA datasheet(33/92 Pages) NXP

33 / 92 page

NXP Semiconductors

PCF85263A

Tiny Real-Time Clock/calendar with alarm function, battery switch-over, time stamp input, and I2C-bus

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:

OFFM = 0 (low power):

Offset value = Eppm / 2.17

OFFM = 1 (fast correction)

Offset value = Eppm / 2.0345

aaa-009636

sample calculation:

32768.28 Hz

30.517320 µs

0.000260 µs

8.51975 ppm

3.926

4 correction pulses

are needed

4.188

4 correction pulses

are needed

Figure 20. Offset calibration calculation workflow

-2

-1

aaa-009637

(3)

(2)

(1)

measured/calculated

deviation 8.51975 ppm

deviation after

correction in

OFFM = 1

+0.382 ppm

deviation after

correction in

OFFM = 0

-0.160 ppm

With the offset calibration an accuracy of ±1 ppm (0.5 × offset per LSB) can be reached (see

Table 24).

±1 ppm corresponds to a time deviation of 0.0864 seconds per day.

1. 4 correction pulses in OFFM = 0 correspond to -8.680 ppm.

2. 4 correction pulses in OFFM = 1 correspond to -8.138 ppm.

3. Reachable accuracy zone.

Figure 21. Result of offset calibration

PCF85263A

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Product data sheet

Rev. 5 — 1 February 2021

33 / 92

PCF85263ATL/A Datasheet(PDF) 33 Page - NXP Semiconductors

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