An electronic device, generally a stable oscillator, that generates a repetitive series of pulses, known as the clock signal. The pulse repetition frequency is accurately controlled.
The clock rate is the frequency, expressed in hertz, at which active transitions of a given clock signal occur. The active transition may be from a low to a high voltage level, or vice versa, but will always be followed after a fixed time by an opposite inactive transition. The clock signal is thus formed as a series of fixed-width pulses having a fixed repetition frequency (see diagram). The pulse width, t1, is often 50% of the pulse repetition period, t2, i.e. t1 = t3. The clock rate is 1/t2 hertz. A clock cycle is considered to be one complete cycle of the clock signal and will always contain one active transition of the clock. For the clock signal illustrated, a clock cycle occurs in t2 seconds.
Because of its constant rate, a clock signal is used to initiate actions within a sequential logic circuit and to synchronize the activities of a number of such circuits. These circuits are said to be clocked. The primary clock rate controls the fastest parts of a computer while slower components are timed by numerous submultiples of the basic frequency.