PCF2123TS datasheet(29/63 Pages) NXP

29 / 63 page

PCF2123

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Product data sheet

Rev. 5 — 27 April 2011

29 of 63

NXP Semiconductors

PCF2123

SPI Real time clock/calendar

8.9 Offset register

The PCF2123 incorporates an offset register (address 0Dh) which can be used to

implement several functions, such as:

• Ageing adjustment

• Temperature compensation

• Accuracy tuning

The offset is made once every two hours in the normal mode, or once every hour in the

course mode. Each LSB will introduce an offset of 2.17 ppm for normal mode and

4.34 ppm for course mode. The values of 2.17 ppm and 4.34 ppm are based on a nominal

32.768 kHz clock. The offset value is coded in two’s complement giving a range of

+63 LSB to

−64 LSB.

[1]

Default mode.

The correction is made by adding or subtracting 64 Hz clock correction pulses, thereby

changing the period of a single second.

In normal mode, the correction is triggered once per two hours and then correction pulses

are applied once per minute until the programmed correction values have been

implement.

In course mode, the correction is triggered once per hour and then correction pulses are

applied once per minute up to a maximum of 60 minutes. When correction values greater

than 60 are used, additional correction pulses are made in the 59th minute (see Table 38).

Table 36.

OFFSET[6:0]

Offset value in

decimal

Offset value in ppm

Normal mode

MODE = 0

Course mode

MODE = 1

0 1 1 1 1 1 1

+63

+136.71

+273.42

0 1 1 1 1 1 0

+62

+134.54

+269.08

:::

00 00 01 0

+4.34

+8.68

00 00 00 1

+2.17

+4.34

00 00 00 0

0[1]

11 11 11 1

−1

−2.17

−4.34

11 11 11 0

−2

−4.34

−8.68

:::

10 00 00 1

−63

−136.71

−273.42

10 00 00 0

−64

−138.88

−277.76

Table 37.

Example of converting the offset in ppm to seconds

Offset in ppm

Seconds per

Day

Week

Month

Year

2.17

0.187

1.31

5.69

68.2

4.34

0.375

2.62

11.4

136

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