PCF2127ATSLASH1 datasheet(52/80 Pages) NXP

52 / 80 page

PCF2127A_2

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Product data sheet

Rev. 02 — 7 May 2010

52 of 80

NXP Semiconductors

PCF2127A

Integrated RTC, TCXO and quartz crystal

8.15 STOP bit function

The function of the STOP bit is to allow for accurate starting of the time circuits. STOP will

cause the upper part of the prescaler (F9 to F14) to be held in reset and thus no 1 Hz ticks

are generated. The time circuits can then be set and will not increment until the STOP bit

is released. STOP will not affect the CLKOUT signal but the output of the prescaler in the

range of 32 Hz to 1 Hz (see Figure 31).

The lower stages of the prescaler, F0 to F8, are not reset and because the I2C-bus and the

SPI-bus are asynchronous to the crystal oscillator, the accuracy of re-starting the time

circuits is between 0 and one 64 Hz cycle (0.484375 s and 0.500000 s), see Table 55 and

Figure 32.

[1]

F0 is clocked at 32.768 kHz.

Table 55.

First increment of time circuits after stop release

Bit

STOP

Prescaler bits[1]

F0 to F8 - F9 to F14

1Hz tick

Time

hh:mm:ss

Comment

Clock is running normally

010000111-010100

12:45:12

prescaler counting normally

STOP bit is activated by user. F0 to F8 are not reset and values cannot be predicted externally

xxxxxxxxx-000000

12:45:12

prescaler is reset; time circuits are frozen

New time is set by user

xxxxxxxxx-000000

08:00:00

prescaler is reset; time circuits are frozen

STOP bit is released by user

xxxxxxxxx-000000

08:00:00

prescaler is now running

xxxxxxxxx-100000

08:00:00

xxxxxxxxx-100000

08:00:00

xxxxxxxxx-110000

08:00:00

111111111-111110

08:00:00

000000000-000001

08:00:01

0 to 1 transition of F14 increments the time circuits

100000000-000001

08:00:01

111111111-111111

08:00:01

000000000-000000

08:00:01

100000000-000000

111111111-111110

08:00:01

000000000-000001

08:00:02

0 to 1 transition of F14 increments the time circuits

001aaj479

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