Datasheet

PCF2129 All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2014. All rights reserved.
Product data sheet Rev. 7 — 19 December 2014 13 of 86
NXP Semiconductors
PCF2129
Accurate RTC with integrated quartz crystal for industrial applications
8.3.1 Temperature compensated crystal oscillator
The frequency of tuning fork quartz crystal oscillators is temperature-dependent. In the
PCF2129, 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 15
).
8.3.1.1 Temperature measurement
The PCF2129 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 OTP refresh
Each IC is calibrated during production and testing of the device. The calibration
parameters are stored on EPROM cells called One Time Programmable (OTP) cells. It is
recommended to process an OTP refresh once after the power is up and the oscillator is
operating stable. The OTP refresh takes less than 100 ms to complete.
To perform an OTP refresh, bit OTPR has to be cleared (set to logic 0) and then set to
logic 1 again.
8.3.3 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.
Table 14. Temperature measurement period
TCR[1:0] Temperature measurement period
00
[1]
4min
01 2 min
10 1 min
11 30 seconds