Synchronous Circuits

Circuits that react immediately to the stimulus of the input are called asynchronous . This term is a combination of the greek words meaning “without regard to time”. In digital systems it is important that outputs change at precise points in time. Circuits that operate in this manner are called synchronous . Digital circuits often use time reference signals called clocks . A clock signal is nothing more than a square wave that has a precise known period. The clock will be the timing reference that synchronizes all circuit activity and tells the device when it should execute its function. Thus the clock signal is the signal that causes things to happen at regularly spaced intervals. In particular, operations in the system are made to take place at times when the clock signal is making a transition from 0 to 1 or from 1 to 0. These transitions are pointed out in the figure. The 0-to-1 transition is called the rising edge or positive-going edge of the clock signal. The synchronous action of the clock signal is the result of using clocked latches, which are designed to change states on either (but not both) the rising edge or the falling edge of the clock signal. In other words, the clocked latches will change states at the appropriate clock transition and will rest between successive clock pulses. The frequency of the clock pulses is generally determined by how long it takes the latches and gates to respond to the level changes by the clock pulse, that is, the propagation delays of the various logic circuits.