PCF2131 datasheet(14/85 Pages) NXP

14 / 85 page

NXP Semiconductors

PCF2131

Nano-power highly accurate RTC with integrated quartz crystal

Bits labeled as T are unused and return 0 when read.

Bit

Symbol

Value

Description

Reference

7 to 6

TCR[1:0]

see Table 17

temperature measurement period

no OTP refresh

OTPR

OTP refresh performed

Section 7.3.2

unused [1]

unused

2 to 0

COF[2:0]

see Table 18

CLKOUT frequency selection

Section 7.3.3

Table 16. CLKOUT_ctl - CLKOUT control register (address 13h) bit description

[1]

programming this bit to 1 may lead to a decrease of timing accuracy.

7.3.1 Temperature compensated Real Time Clock

The frequency of tuning fork quartz crystal oscillators is temperature-dependent. In the PCF2131, the frequency

deviation caused by temperature variation is corrected by adjusting the RTC frequency divider with digital

method.

The load capacitance is trimmed to the required value at 25 °C in order to compensate the frequency deviation

over process variation.

The frequency accuracy at 25 °C 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) can lead

to inaccurate measurements. Accurate frequency measurement occurs when fCLKOUT = 16.384 kHz or lower is

selected (see Table 18).

The temperature compensated frequency input for the Real Time Clock cannot be observed at the CLKOUT pin

but can be evaluated by following these steps.

• Set Second Interrupt, bit SI in register Control_1 to 1

• Set bit TI_TP in register Watchdg_tim_ctl to 1 for a pulsed interrupt signal

• Set bit SIA in register INTA_MASK_1 to 0 to direct the Second Interrupt to pin INTA for a 1 Hz pulse output.

The RTC temperature compensation works by adding or deleting pulses at the 32.768 kHz level. These

correction pulses are spaced evenly over a sufficiently long period of time to reach the required resolution and

accuracy. Every second corrections with a resolution of about 30.5 ppm (1/32768) can be generated by the

temperature compensation engine. If for instance a 10 ppm correction is called for, the correction pulses will be

generated approximately once every 3 seconds, for a 50 ppm correction every 0.6 s and so on.

The 1 Hz interrupt output signal can be measured with a counter and by selecting an appropriate gating time the

measurement resolution can be set to the desired level. A gating time of 100 s for instance will determine the

averaged 1 second period with a resolution of 0.3 ppm.

The feature of temperature compensation can be turned off for ultra low power consumption by first performing

a software reset (SR) followed by setting TC_DIS to ‘1’ within 5 seconds.

7.3.1.1 Temperature measurement

The PCF2131 has a temperature sensor circuit used to perform temperature compensation of the clock input to

the RTC. 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. During the first approximately 60 s after

start-up the compensation will be inactive, after this period the temperature compensation is active.

PCF2131

All information provided in this document is subject to legal disclaimers.

Product data sheet

Rev. 2.0 — 5 October 2023

14 / 85

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