Clocking

The clock signal serving as the heartbeat of a processor 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 with 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 using the piezoelectric effect. Quartz crystals resonate at precise frequencies, which leads to their use as timing elements in computers, wristwatches, and other digital devices.

Although crystal oscillators are more accurate time references than alternative timing references...