NXP Semiconductors PCA2125, PCA8565, PCF2123, PCF85x3 - page 23

NXP Semiconductors UM10301
User Manual PCF85x3, PCA8565 and PCF2123, PCA2125
UM10301_1 © NXP B.V. 2008. All rights reserved.
User manual Rev. 01 — 23 December 2008 23 of 52
Fig 9. Oscillator tuning
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.