PCF2127ATSLASH1 datasheet(12/80 Pages) NXP

12 / 80 page

PCF2127A_2

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Product data sheet

Rev. 02 — 7 May 2010

12 of 80

NXP Semiconductors

PCF2127A

Integrated RTC, TCXO and quartz crystal

8.3 Register CLKOUT_ctl

8.3.1 Temperature compensated crystal oscillator

The frequency of tuning fork quartz crystal oscillators is temperature-dependent. In the

PCF2127A the frequency drift caused by temperature variation is corrected by adjusting

the load capacitance of the crystal oscillator.

The load capacitance is changed by switching between two load capacitance values using

a modulation signal with a programmable duty cycle. Every chip is calibrated in order to

produce, at the measured temperature, the correct duty cycle which compensates for the

frequency drift.

The frequency accuracy can be evaluated by measuring the frequency of the square

wave signal available at the output pin CLKOUT. However, the selection of

fCLKOUT = 32.768 kHz (default value) leads to inaccurate measurements. The most

accurate frequency measurement occurs when fCLKOUT = 1 Hz is selected (see Table 10).

8.3.1.1

Temperature measurement

The PCF2127A has a temperature sensor circuit used to perform the temperature

compensation of the frequency. The temperature is measured immediately after power-on

and then periodically with a period set by the temperature conversion rate TCR[1:0] in the

[1]

Default value.

8.3.2 Clock output

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

COF[2:0] control bits in register CLKOUT_ctl. 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.

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 but the 32.768 kHz frequencies will be 50 : 50.

Table 8.

CLKOUT_ctl - CLKOUT control register (address 0Fh) bit description

Bit

Symbol

Value

Description

7 to 6 TCR[1:0]

see Table 9

temperature measurement period

5 to 3 -

unused

2 to 0 COF[2:0]

see Table 10 CLKOUT frequency selection

Table 9.

Temperature measurement period

TCR[1:0]

Temperature measurement period

[1]

4min

2 min

1 min

30 seconds

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