PCF2129AT datasheet(14/86 Pages) PHILIPS

14 / 86 page

PCF2129

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Product data sheet

Rev. 7 — 19 December 2014

14 of 86

NXP Semiconductors

PCF2129

Accurate RTC with integrated quartz crystal for industrial applications

[1]

Duty cycle definition: % HIGH-level time : % LOW-level time.

[2]

Default value.

[3]

The specified accuracy of the RTC can be only achieved with CLKOUT frequencies not equal to

32.768 kHz or if CLKOUT is disabled.

The duty cycle of the selected clock is not controlled, however, due to the nature of the

clock generation all but the 32.768 kHz frequencies are 50 : 50.

8.4 Register Aging_offset

8.4.1 Crystal aging correction

The PCF2129 has an offset register Aging_offset to correct the crystal aging effects2.

The accuracy of the frequency of a quartz crystal depends on its aging. The aging offset

adds an adjustment, positive or negative, in the temperature compensation circuit which

allows correcting the aging effect.

At 25

C, the aging offset bits allow a frequency correction of typically 1 ppm per AO[3:0]

value, from

7 ppm to +8 ppm.

Table 15.

CLKOUT frequency selection

COF[2:0]

CLKOUT frequency (Hz)

Typical duty cycle[1]

000

[2][3] 32768

60 : 40 to 40 : 60

001

16384

50 : 50

010

8192

50 : 50

011

4096

50 : 50

100

2048

50 : 50

101

1024

50 : 50

110

50 : 50

111

CLKOUT = high-Z

Table 16.

Aging_offset - crystal aging offset register (address 19h) bit allocation

Bit positions labeled as - are not implemented and return 0 when read.

Bit

Symbol

AO[3:0]

Reset

value

1000

Table 17.

Aging_offset - crystal aging offset register (address 19h) bit description

Bit positions labeled as - are not implemented and return 0 when read.

Bit

Symbol

Value

Description

7 to 4

unused

3 to 0

AO[3:0]

see Table 18

aging offset value

For further information, refer to the application note Ref. 3 “AN11186”.

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