A crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. The function of this oscillator circuit is to provide an accurate and stable periodic clock signal to a microcontroller.
Most microcontrollers include a clock driver circuit which is designed to drive a quartz crystal into oscillation. The clock driver circuitry built into the PICmicro family is very flexible and allows for four different clocking options. The clock circuit consists of capacitors C1 and C2, a quartz crystal or ceramic resonator Y1, and a series resistor, R3. The values of capacitors C1 and C2 is determined by both the clock speed at which you intend to run the PICmicro, and by the selection of a quartz crystal or a ceramic resonator as the clock source.
The following file contains detail info about design of Clock Oscillator Basic Circuit Diagram. Included in this file information about: selecting the components, building the oscillator, reset circuit, programming the clock, the power supply, and project summary.