Clock signal

From Freepedia

In electronics and synchronous digital circuits, such as most computers, a clock signal is a signal used to coordinate the actions of two or more circuits. A clock signal oscillates between a high and a low state, normally with a 50% duty cycle, and is usually a square wave. The circuits using the clock signal for synchronization may become active at either the rising or falling edge, or both (see for example DDR SDRAM), of the clock signal.

Some sensitive mixed-signal circuits, such as precision analogue-to-digital converters, use sine waves rather than square waves as their clock signals, because square waves contain high-frequency harmonics that can interfere with the analogue circuitry and cause noise. Such sine wave clocks are often differential signals, because this type of signal has twice the slew rate, and therefore half the timing uncertainty, of a single-ended signal with the same voltage range.

Most integrated circuits of sufficient complexity require a clock signal in order to synchronize different parts of the chip and to account for propagation delays. As chips get more complex, the problem of supplying accurate and synchronized clocks to all the circuits becomes more and more difficult. The preeminent example of such complex chips is the microprocessor, the central part of modern computers.

A clock signal might also be gated, i.e. combined with a controlling signal, in order to enable or disable the clock signal for a certain part of a circuit. This technique is often used to save power by shutting down parts of a digital circuit currently not being used.

The speed of a clock signal in a computer is called the clock rate or clock frequency.