Pierce Oscillator Amplitude problems

peter8 · ‎2008-08-27

Posted on August 27, 2008 at 09:24

peter8 · ‎2011-05-17

Posted on May 17, 2011 at 11:52

I've been successuly using the uPSD3434 for a while now using a 24MHz xtal. I now want to use a 33MHz xtal instead. The problem is that the device just won't boot up using the faster xtal. I've scoped the xtal1 pin and it looks like the amplitude of the sine wave is out of spec. (I'm getting 1v to 2.2v at 3.3v VCC). Is this why it won't start? The xtal is a Farnell supplied part (144-8114). It should work in fundamental mode (and the scope would indicate it does...just at too low amplitude. (I've tried various caps 15pF to 56pF). Am I missing something obvious. I've tried 2 different uPSDs, same result. Can anyone recommend a 33-40MHz xtal for the purpose?

Many thanks for any help.

Peter.

peter8 · ‎2011-05-17

Posted on May 17, 2011 at 11:52

I've made a little progress. As some of you more experienced people could have predicted the reason I can't 'see' the waveform is cos the scope is loading it. Well, I know that now..

I've managed to get one board out of 3 working, but it's intermittent. I keep switching the power on and off. Half the time it boots and runs my program, half the time it doesn't. (when it doesn't boot it draws twice the current).

I've looked at...ground planes..lead lengths..stability of supply..and I can't work out what's up with it..

Anyone any ideas...or experienced the same??

TIA, Peter. :-?