Nxp Semiconductors PCA2125 User Manual

 

 

UM10301_1 

© NXP B.V. 2008. All rights reserved.

 

 

Accuracy: 

In order to be able to adjust the clock with accuracy better than 1 s/day, the frequency 
counter used to check the output at CLKOUT should have at least an 8 digit reading with 
an accuracy of 1 ppm. 

Given a nominal frequency of 32.768 kHz, 1 ppm = 32.728 mHz (milli Hertz). Therefore 
+1 ppm = 32768.0327 Hz,  -1 ppm = 32767.9673 Hz. Tune the oscillator while it is at the 
average operating temperature of the application. 

The PCF8573 can be tuned by monitoring the 128 Hz signal at the FSET output. 

Tuning the PCF8583/93 is somewhat more difficult since no buffered clock signal is 
available. There are four different options, all with their own drawbacks: 

•  Measure the period of the 1 s output signal (countdown timer). This however is time 

consuming; 

•  Attach the frequency counter probe to the OSCO-pin. This adds capacity to the 

OSCO-pin and detunes (lowers) the oscillator frequency by ∆f. The frequency 
adjustment now needs to be lower by the same ∆f in order for the clock to run at the 
correct speed after the probe has been removed again. Obviously the difficulty here 
lies in determining how big ∆f is; 

•  In the watch industry the frequency is coupled out acoustically. A sensitive 

microphone is placed near the crystal. The signal is then fed as input to the tuning 
gear.