Interfacing PIC Microcontrollers

internal clock driver circuit, to generate the clock. The time constant (product

R ⫻ C) determines the clock period. A variable resistor can be used to give a

manually adjustable frequency, although it is not very stable or accurate.

CRYSTAL OSCILLATOR

If greater precision is required, especially if the program uses the hardware

timers to make accurate measurements or generate precise output signals, a

crystal (XTAL) oscillator is needed. Normally, it is connected across the clock

pins with a pair of small capacitors (15 pF) to stabilise the frequency. The

crystal acts as a self-contained resonant circuit, where the quartz or ceramic

crystal vibrates ...