Digital Design Using VHDL A Systems Approach

Asynchronous sequential circuits have state that is not synchronized with a clock. Like the synchronous sequential circuits we have studied up to this point, they are realized by adding state feedback to combinational logic that implements a next-state function. Unlike synchronous circuits, the state variables of an asynchronous sequential circuit may change at any point in time. This asynchronous state update – from next state to current state – complicates the design process. We must be concerned with hazards in the next-state function, since a momentary glitch may result in an incorrect final state. We must also be concerned with races between state variables on transitions between states whose encodings differ in more than one variable.

In this chapter we look at the fundamentals of asynchronous sequential circuits. We start by showing how to analyze combinational logic with feedback by drawing a flow table. The flow table shows us which states are stable, which are transient, and which are oscillatory.We then show how to synthesize an asynchronous circuit from a specification by first writing a flow table and then reducing the flow table to logic equations. We see that state assignment is quite critical for asynchronous sequential machines, since it determines when a potential race may occur. We show that some races can be eliminated by introducing transient states.

After the introduction given in this chapter, we continue our discussion of asynchronous circuits in Chapter 27 by looking at latches and flip-flops as examples of asynchronous circuits.