Datasheet

PCF2127AT All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
Product data sheet Rev. 6 — 11 July 2013 13 of 86
NXP Semiconductors
PCF2127AT
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
PCF2127AT, the frequency deviation 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. In order to compensate the spread of
the quartz parameters every chip is factory calibrated.
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
f
CLKOUT
= 32.768 kHz (default value) leads to inaccurate measurements. Accurate
frequency measurement occurs when f
CLKOUT
= 16.384 kHz or lower is selected (see
Table 11
).
8.3.1.1 Temperature measurement
The PCF2127AT 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
register CLKOUT_ctl.
[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 system clock, microcontroller clock, charge pump
input, or for calibrating 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 9. CLKOUT_ctl - CLKOUT control register (address 0Fh) bit description
Bit Symbol Value Description
7 to 6 TCR[1:0] see Table 10
temperature measurement period
5 to 3 - - unused
2 to 0 COF[2:0] see Table 11
CLKOUT frequency selection
Table 10. Temperature measurement period
TCR[1:0] Temperature measurement period
00
[1]
4min
01 2 min
10 1 min
11 30 seconds