Problem with RTC on LPC11u67 (Oscilator fail detect)

[lukasserap]

I have 10 devices with LPC11u67 on them. 4 of these devices has failing RTC clock - I've tried resoldering RTC crystal, checked the battery with no luck. The strange thing is, that if I connect signal generator (square, about 30 kHz) and put the probe on RTC Xtal pins, it fixes the problem permenantly (well I hope so). When debugged, peripheral registers of RTC shows OFD bit set and marked as "FAIL". Clearing this bit does nothing at all. Datasheet, User Manual and Errata didn't help with this. So my question is: Is it Hardware problem? What is the meaning of OFD bit? is there any way of fixing it with firmware?

[kerryzhou · Accepted solution]

Hi Lukas,

   OFD bit set, means the RTC oscillator fail detected.

The user manual already describe it:

   This problem is caused by the external RTC crystal circuit.

   From your description:if I connect signal generator (square, about 30 kHz) and put the probe on RTC Xtal pins, it fixes the problem 

   This point also means your external RTC crystal circuit have the mismatching factor, you need to adjust the external capacitor.

   I suggest you try to change the external capacitor to a little large capacitor. 

  We also have application note about the crystal, you can refer to it:

http://www.nxp.com/documents/report/R_10074.pdf 

Wish it helps you!

Have a great day,
Jingjing

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