Clocking

The clock signal serves as the heartbeat of a processor. This signal is usually a square wave signal operating at a fixed frequency. A square wave is a digital signal that oscillates between high and low states, spending equal lengths of time at the high and low levels on each cycle. Figure 2.17 shows an example of a square wave over time:

Figure 2.17: Square wave signal

The clock signal in a computer system is usually generated from a crystal oscillator providing a base frequency of a few megahertz (MHz). 1 MHz is 1 million cycles per second. A crystal oscillator relies on the resonant vibration of a physical crystal, usually made of quartz, to generate a cyclic electrical signal. The mechanical vibration of the crystal is transformed into an electrical signal by the piezoelectric effect. The piezoelectric effect is the electrical charge that accumulates in certain crystals when placed under mechanical stress. Quartz crystals resonate at precise frequencies...