Clock signal

In electronics and especially synchronous digital circuits, a clock signal (historically also known as logic beat)[1] is an electronic logic signal (voltage or current) which oscillates between a high and a low state at a constant frequency and is used like a metronome to synchronize actions of digital circuits.

In a synchronous logic circuit, the most common type of digital circuit, the clock signal is applied to all storage devices, flip-flops and latches, and causes them all to change state simultaneously, preventing race conditions.

As ICs become more complex, the problem of supplying accurate and synchronized clocks to all the circuits becomes increasingly difficult.

The preeminent example of such complex chips is the microprocessor, the central component of modern computers, which relies on a clock from a crystal oscillator.

This technique is often used to save power by effectively shutting down portions of a digital circuit when they are not in use, but comes at a cost of increased complexity in timing analysis.

[8] This was particularly common among early microprocessors such as the National Semiconductor IMP-16, Texas Instruments TMS9900, and the Western Digital MCP-1600 chipset used in the DEC LSI-11.

Four phase clocks have only rarely been used in newer CMOS processors such as the DEC WRL MultiTitan microprocessor.

The vast majority of digital devices do not require a clock at a fixed, constant frequency.

Alternatively, using a sine wave clock, CMOS transmission gates and energy-saving techniques, the power requirements can be reduced.

Since this function is vital to the operation of a synchronous system, much attention has been given to the characteristics of these clock signals and the electrical networks used in their distribution.

Finally, the control of any differences and uncertainty in the arrival times of the clock signals can severely limit the maximum performance of the entire system and create race conditions in which an incorrect data signal may latch within a register.

Novel structures are currently under development to ameliorate these issues and provide effective solutions.

Adapted from Eby Friedman Archived 2014-08-12 at the Wayback Machine's column in the ACM SIGDA e-newsletter by Igor Markov Original text is available at https://web.archive.org/web/20100711135550/http://www.sigda.org/newsletter/2005/eNews_051201.html