Hi,

1.  Get a crystal impedence meter and measure "good" 
and "bad" crystals.  Compare against spec.  Verify that 
the parameters don't shift when heat is applied (such 
as frequency jumps, no oscillation, capacitance 
changes, etc).

2. Get network analyzer and plot crystal response. 
Look for spurs.  Heat crsytal and see if anything 
shifts.

3. Create loop and measure crystal current.

4. Use small series resistors and calculate safety 
factor.


- Ken

----- Original Message ----- 
From: "Leslie, Shawn J" <sjlesli@sandia.gov>
To: <piclist@mit.edu>
Sent: Friday, June 27, 2008 1:12 PM
Subject: [PIC] Timer 1 Oscillator Failure at high 
temperature


>I am using a PIC18LF4320I/ML. I am using Timer 1 and 
>the oscillator as an RTC. I am driving the oscillator 
>with a Epson FC-135 series 32.768KHz 12.5pF crystal 
>good to 85C. I am hanging 27pF caps off the crystal. 
>After 2 years of successful builds we are suddenly 
>having this circuit fail at high temp. The oscillator 
>fails at 60-70C and a few even do not work at room 
>temperature. I am having 40% fallout at temp. If you 
>lower the temperature to <40C it starts to work while 
>power is applied. The problem is made more difficult 
>since the units are potted. We cannot get at the 
>components to troubleshoot. We are trying to test 
>unpotted units over temperature. In the meantime I 
>have tried to find a threshold of failure on a working 
>unpotted unit by increasing and decreasing the 27pF 
>caps - amazingly the circuit works at room temperature 
>with 47pF caps on the high end to 6pF on the low end. 
>With 6pF I can make it fail if I raise the temperature 
>to 70C, but as it cools it begins to wor!
> k. I have wrote Epson for crystal ideas and sent them 
> my layout - any ideas from you guys? What have you 
> all used? Maybe someone has a successful crystal 
> oscillator design I could use.
>
> Cheers,
> Shawn
>
>
>
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