Page 717 - Engineering Digital Design

CHAPTER 14

Asynchronous State Machine

Design and Analysis:

Basic Concepts

14.1 INTRODUCTION

In Chapters 10 through 13 the emphasis was directed toward synchronous sequential ma-
chine design. These chapters developed a rather thorough understanding of the concepts
necessary for the meaningful and reliable design of these machines. Now, it is necessary
to move on to another type of sequential machine — the asynchronous FSM. In Fig. 14.1
is presented an overview of the various types of digital machines. Observe that combina-
tional machines are classified as asynchronous because they operate in the absence of a
clock signal, but they do not have feedback. Combination logic machines were the subjects
of Chapters 6 through 8. As is indicated in Fig. 14.1, all sequential machines must have
feedback, but they can be divided into two categories, synchronous and asynchronous.
The major aim of this chapter is, of course, to develop a working-level understanding of
asynchronous FSMs, their design and analysis, and to design state machines that operate at
speeds exceeding those possible for their synchronous FSM counterparts. But the mission
of this chapter is really broader than that. In the course of the various discussions, the reader
will develop a better understanding of those concepts involved in synchronous machine
design and analysis. In fact, an understanding of asynchronous sequential machines is
required before synchronous sequential machines can be fully understood.
So why has the subject of asynchronous machine concepts and methodologies been
delayed to this point? The answer is simple. The study of asynchronous FSMs forces one
to deal with the complexities of sequential machines in greater depth than was required for
the simpler synchronous machines. Putting it another way, the study of synchronous FSMs
permitted the reader to develop capabilities sufficient to design and analyze large systems
without having to deal with the intricacies of asynchronous machine design. Remember
that all digital machines can eventually be broken down into their component asynchronous
parts. For example, the synchronous FSMs, studied in Chapters 10 through 13, use memory
elements (flip-flops) that are themselves asynchronous machines but that are designed to
operate in a clock-driven environment.

683