On Fri, 27 Jun 2008 11:12:56 -0600, "Leslie, Shawn J" said: > 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. Hi Shawn, I've had some experience with 32KHz problems on another chip, and there are a few things that come to mind that may help you diagnose the problem: If you can raise the power supply voltage 1/2 volt and retest, if the oscillator works better then it suggests the lower impedance and higher gain of the oscillator driver is having some efect. If you have no problem with units before potting it could be moisture being potted along with the circuit. Using a very low load scope probe you can see if you need to offset the capacitance to one side of the oscillator or the other. Scoping it quickly you can see if there's DC bias, that could suggest board leakage. I have a manufacturer's document somewhere that may have some useful tidbits... let me look here and see if I can find it on the interwebs... nope, I'll have to look for a reference to it on my computer at home. I'll post another message tonight with a link. Cheerful regards, Bob -- http://www.fastmail.fm - Send your email first class -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist