The oscillator is a circuit that produces an output
waveform without any external signal source. The only
input to an oscillator is the dc power supply. As such, the
oscillator can be viewed as a signal generator. There are
many types of the oscillators. Crystal oscillator is the one of
the most widely used oscillator in microcontroller and
microprocessor based projects. It is called the crystal
oscillator because it generates extremely stable output wave
forms. The most communication applications require the
use of oscillators that have extremely stable output signals.
Crystal oscillators are oscillators where the primary
frequency determining element is a quartz crystal.