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A crystal oscillator is a electro-mechanical  device . It is used to keep precise timing between different electronic components and micro controller. In its internals it uses a quartz crystal , and when electricity is applied to it it generates different frequencies . It is mostly used in micro controllers and digital watches .
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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 precise frequency.