Brian Scearce wrote: > > I don't think this is specifically PIC-related, so please feel free > to reply to me privately if you think the list wouldn't be interested > in your answer. > > I was having a problem the other day where my 20MHz 16C63/JW seemed > to be running at more like 6-7MHz. I wrote up a description of > the problem and almost submitted it to the list, but then I decided > to spend $0.75 of my own money buying another 20MHz crystal first. > Sure enough, the new crystal works fine. Sounds to me that the first crystal was actually a third-overtone one. These are commonly used for higher frequencies because otherwise the crystal would be too thin. By using an LC filter, the oscillator can filter out the fundamental frequency and actually oscillate on three times the fundamental. The same trick can be used to use higher overtones (5th,7th, etc) for even higher frequencies. The Pierce oscillator that's used in the PICs could probably be modified with such a filter, but it is cheaper to get 'real' 20MHz crystals. Frank ------------------------------------------------------------------------ Frank A. Vorstenbosch Phone: +44-181-636 3391 Electronics & Software Engineer or: +44-181-636 3000 Eidos Technologies Ltd., Wimbledon, London Mobile: +44-976-430 569