In order to keep the computer moving from one state to another in a timely fashion, the control unit has a clock which emits a regular series of pulses. These pulses are sent through various delay gates and used by the hardware DFA to make transitions to the next state after a measured amount of time. The bus and other devices, like I/O controllers, have their own clocks that are not linked into the main computer's clock.

When a computer's clock speed is advertised, such as a 120 MHz Pentium, it is the speed of the controlling DFA clock which is being touted. Another name for cycle is Hertz, in honor of a German physicist, Heinrich Rudolf Hertz (1857-1894) who worked on electromagnetic phenomena. One Hertz, abbreviated as Hz, is one cycle per second, so 120 MHz (pronounce "megahertz") is 120,000,000 cycles every second. Of course, one complete machine instruction may require 100 cycles, so the computer is really doing 1,200,000 instructions per second. To speed up the computer, either increase the clock speed or shorten the instruction so that fewer cycles are needed to perform each machine instruction.

Clocks in computers are created by applying electric current to tiny quartz crystals which vibrate with a regular frequency when current is applied to them. This vibration can be used to raise and lower voltages in wires.