PCF2123TS datasheet(32/54 Pages) NXP

32 / 54 page

PCF2123_1

Product data sheet

Rev. 01 — 19 November 2008

32 of 54

NXP Semiconductors

PCF2123

SPI Real time clock/calendar

Operation example:

1. Set EXT_TEST test mode (register Control_1, bit EXT_TEST = 1).

2. Set STOP (Control_1, bit STOP = 1).

3. Clear STOP (Control_1, bit STOP = 0).

4. Set time registers to desired value.

5. Apply 32 clock pulses to pin CLKOUT.

6. Read time registers to see the ﬁrst change.

7. Apply 64 clock pulses to pin CLKOUT.

8. Read time registers to see the second change.

Repeat 7 and 8 for additional increments.

9.13 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 9.10).

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).

Fig 20. STOP bit

001aai556

OSC

OSC STOP

DETECTOR

F13

RES

F14

RES

512 Hz

16384 Hz

8192 Hz

1 Hz tick

stop

CLKOUT source

oscillator stop flag

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