Why use two capacitors with crystal to provide oscillation?
Answer:
That is because of the crystals property. The crystal will produce charge from pressure and will deformed when charge is applied. the oscillation happen this way;
On the initial power, the voltage applied across the crystal will make it to bend. This deformation of the crystal will then produce the charge in opposite direction to charge one capacitor. The crystal will then return to it's original position and at the same time, producing another charge opposite to the previous charge hence, doubling the voltage and make the crystal to overshoot the original position charging the second capacitor. The crystal will then try to go back to its original position and repeat its cycle.
The circuit then act as an oscillator and the and the frequency is defined by the physical size of the crystal and its matching capacitors.
The two capacitors shunt to ground in a PIC micro is application specific.
See this link for the explanation by Microchip:
http://ww1.microchip.com/downloads/en/de...
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