The basics of crystals and crystal oscillators application note from Microchip, link here (PDF!)
This Application Note will not make you into an oscillator designer. It will only explain the operation of an oscillator in simplified terms in an effort to convey the concepts that make an oscillator work.
The goal of this Application Note is to assist the product design engineer in selecting the correct crystal and external capacitors required for the rfPICTM or PICmicro® device. In order to do this the designer needs a clear understanding of the interrelationship of the various circuits that make up an oscillator circuit. The product design engineer should also consult with the crystal manufacturer about the needs of their product design.
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