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Digital circuits

Most integrated circuits (ICs) of sufficient complexity utilize a clock signal in order to synchronize different parts of the circuit and to account for propagation delays. 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.

Clock signal characteristics

Real-life clock signal.

• The most important charactersitic of a clock signal is its frequency or period. The frequency is represented by a letter F and measured in Hertz. The period is represented by a letter T and measured in seconds.

$T={\frac {1}{F}}$

• A period can be divided into a portion during which the signal is high, designated as T_high or T_h and a portion during which the signal is low - T_low or T_l. Relation between high and low times is called duty cycle or DC in short. Duty cycle is measured in percents.

$DC[\%]={\frac {T_{high}}{T_{high}+T_{low}}}*100$

Ideal clock signal is a square wave. In real life a clock signal is disturbed and may require additional characterization parameters.

• Transition from low level to high level does not occur immediately, but takes time. This time is called Rise time T_rise or T_r. The opposite transition is called Fall time T_fall or T_f. These paramters are extremely important in clock signal analysis since they determine the signal bandwidth.

• Often an overshoot appears after a transition, which is very important for the signal integrity analysis.

A clock signal might also be gated, that is, combined with a controlling signal that enables or disables the clock signal for a certain part of a circuit. This technique is often used to save power by effectively shutting down portions of a digital circuit when they are not in use.

In some early microprocessors such as the National Semiconductor IMP-16 family, a multi-phase clock was used. In the case of the IMP-16, the clock had four phases, each 90 degrees apart, in order to synchronize the operations of the processor core and its peripherals. Most modern microprocessors and microcontrollers use a single-phase clock, however.

Many modern microcomputers utilize a "clock multiplier" which multiplies a lower frequency external clock to the appropriate clock rate of the microprocessor. This allows the CPU to operate at a much higher frequency than the rest of the computer, which affords performance gains in situations where the CPU does not need to wait on an external factor (like memory or I/O).