The majority of clock sources for microcontroller can be grouped into two types: those based on mechanical resonant devices, such as crystals and ceramic resonators, and those based on electrical phase-shift circuits such as RC (resistor, capacitor) oscillators. Silicon oscillators are typically a fully integrated version of the RC oscillator with the added benefits of current sources, matched resistors and capacitors, and temperature compensation circuits for increased stability. Two examples of clock source are illustrated in the following figure. Figure (a) shows a Pierce oscillator configuration suitable for use with mechanical resonant device like crystals and ceramic resonators, while figure (b) shows a simple RC feedback oscillator.

The various clock sources are compared below:

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|'''Clock Source'''

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