It provides an accurate and stable periodic clock signal to a processor
The clock period or cycle time, Tc, is the time between rising edges of a repetitive clock signal. Its reciprocal, fc = 1/Tc, is the clock frequency. All else being the same, increasing the clock frequency increases the work that a digital system can accomplish per unit time. Frequency is measured in units of Hertz (Hz), or cycles per second: 1 megahertz (MHz) = 106 Hz, and 1 gigahertz (GHz) = 109 Hz.
In the three decades from when one of the authors families bought an Apple II+ computer to the present time of writing, microprocessor clock frequencies have increased from 1 MHz to several GHz, a factor of more than 1000. This speedup partially explains the revolutionary changes computers have made in society.
Figure 3.38(a) illustrates a generic path in a synchronous sequential circuit whose clock period we wish to calculate. On the rising edge of the clock, register R1 produces output (or outputs) Q1. These signals enter a block of combinational logic, producing D2, the input (or inputs) to register R2. The timing diagram in Figure 3.38(b) shows that each output signal may start to change a contamination delay after its input changes and settles to the final value within a propagation delay after its input settles. The gray arrows represent the contamination delay through R1 and the combinational logic, and the blue arrows represent the propagation delay through R1 and the combinational logic. We analyze the timing constraints with respect to the setup and hold time of the second register, R2.