PCF8563 RTC timer freezes

[dexter1234]

Hi all, 

I have recently encountered a problem with PCF85063AT/AAZ (Tiny Real-Time Clock). The problem is, I read data from a sensor over the week and save it in a memory chip. The timer works fine during bench testing, However, when reading data from the past week, I see the timer freezes on the same value for Hour/Minute/Second for about an hour or two, then starts counting up again from the value that it froze on. 

I tried to regenerate the problem on the bench, by removing the crystal, the timer froze, just like the issue. 

This is my crystal by the way: CM9V-T1A-32.768KHZ-7PF-100PPM-TA-QC

I had one engineer telling me to use 4 pin crystals (with VCC and GND), he said these 2 pins are not reliable. He couldn't be wrong since on Dev boards from Adafruit and Sparkfun, we see giant radial crystals. 

It could also be caused by load capacitance from the Xtal, mine is 7pF but datasheet suggests 7pF is the default while supporting 12.5pF. 

I want to redo my PCB design, I'd be grateful if someone would clarify what could be possibly wrong with my board?

Thanks,

Dex

[dexter1234]

Thank you so much Reyes. 

This is exactly what I was looking for. 

[reyes · Accepted solution]

Hi Dex,

You can use either crystal oscillator or the use 4 pin crystal oscillator, section 7 of the RTC user manual discusses the crystal selection topic: https://www.nxp.com/docs/en/user-guide/UM10301.pdf

About the load capacitance, you are correct, the PCF85063A includes integrated oscillator load capacitors and can be selected between 7pF and 12.5pF, by default, the PCF85063A select the 7pF capacitor. Make sure this is true in CAP_SEL bit (should be ‘0’) on Control Register 1.

According to the issue you mentioned, there is no other way this can happened other than a problem with the oscillator signal, maybe the crystal you are using is failing and it stops oscillating under certain conditions on the filed.

Regards,

Jose