I had almost this exact problem several years ago.  Turns out the
increased permeativity of the potting compound caused too much parasitic
capacitance.  The low power oscillator design was marginal at best, and
this extra capacitance pushed it over the edge.

Removed the potting and all was well.  You might want to try that.


Also, I have had problems with Epson crystals damaged during reflow (in
a Quad oven running the correct profile).  Seems some crystals were
marginal and would not sustain the profile spec'd by Epson.  Changed to
another manufacturer and that problem went away.

Scott F. Touchton
 


-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
Of Leslie, Shawn J
Sent: Friday, June 27, 2008 1:13 PM
To: 'piclist@mit.edu'
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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