PCF2123TS datasheet(33/63 Pages) NXP

33 / 63 page

PCF2123

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Product data sheet

Rev. 5 — 27 April 2011

33 of 63

NXP Semiconductors

PCF2123

SPI Real time clock/calendar

8.11 STOP bit function

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

bit function will cause the upper part of the prescaler (F2 to F14) to be held in reset and

thus no 1 Hz ticks will be generated. The time circuits can then be set and will not

increment until the STOP bit is released (see Figure 21 and Table 40).

The STOP bit function will not affect the output of 32.768 kHz, 16.384 kHz, or 8.192 kHz

(see Section 8.8).

The lower two stages of the prescaler (F0 and F1) are not reset and because the SPI-bus

is asynchronous to the crystal oscillator, the accuracy of re-starting the time circuits will be

between 0 and one 8.192 kHz cycle (see Figure 21).

The first increment of the time circuits is between 0.499878 s and 0.500000 s after STOP

bit is released. The uncertainty is caused by the prescaler bits F0 and F1 not being reset

(see Table 40).

Fig 20. STOP bit functional diagram

001aai556

OSCILLATOR

OSCILLATOR STOP

DETECTOR

F13

RESET

F14

RESET

1 Hz

1024 Hz

16384 Hz

8192 Hz

1 Hz tick

stop

CLKOUT source

oscillator stop flag

Fig 21. STOP bit release timing

001aaf912

8192 Hz

stop released

μs to 122 μs

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