PCF2123TS datasheet(27/55 Pages) NXP

27 / 55 page

PCF2123_2

Product data sheet

Rev. 02 — 4 December 2009

27 of 55

NXP Semiconductors

PCF2123

SPI Real time clock/calendar

9.10 Clock output

A programmable square wave is available at pin CLKOUT. Operation is controlled by the

COF[2:0] bits in the register Timer_clkout. Frequencies of 32.768 kHz (default) down to

1 Hz can be generated for use as a system clock, microcontroller clock, input to a charge

pump, or for calibration of the oscillator.

Pin CLKOUT is an open-drain output and enabled at power-on. When disabled the output

is high-impedance.

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

clock generation, all will be 50 : 50 except the 32.768 kHz frequencies.

The STOP bit function can also affect the CLKOUT signal, depending on the selected

frequency. When the STOP bit is set to logic 1, the CLKOUT pin will generate a

continuous LOW for those frequencies that can be stopped. For more details of the STOP

bit function see Section 9.13.

[1]

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

[2]

1 Hz clock pulses will be affected by offset correction pulses.

9.10.1 CLKOE pin

The CLKOE pin can be used to block the CLKOUT function and force the CLKOUT pin to

an high-impedance state. The effect is the same as setting COF[2:0] = 111.

9.11 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.

Table 35.

CLKOUT frequency selection

Bits COF[2:0]

CLKOUT frequency (Hz) Typical duty cycle[1]

Effect of STOP bit

000

32768

60 : 40 to 40 : 60

no effect

001

16384

50 : 50

no effect

010

8192

50 : 50

no effect

011

4096

50 : 50

CLKOUT = LOW

100

2048

50 : 50

CLKOUT = LOW

101

1024

50 : 50

CLKOUT = LOW

110

1[2]

50 : 50

CLKOUT = LOW

111

CLKOUT = high-Z

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