Page 449 - Engineering Digital Design

CHAPTER 10

Introduction to Synchronous State

Machine Design and Analysis

10.1 INTRODUCTION

Up to this point only combinational logic machines have been considered, those whose
outputs depend solely on the present state of the inputs. Adders, decoders, MUXs, PLAs,
ALUs, and many other combinational logic machines are remarkable and very necessary
machines in their own right to the field of logic design. However, they all suffer the same
limitation. They cannot perform operations sequentially. A ROM, for example, cannot
make use of its present input instructions to carry out a next-stage set of functions, and an
adder cannot count sequentially without changing the inputs after each addition. In short,
combinational logic devices lack true memory, and so lack the ability to perform sequential
operations. Yet their presence in a sequential machine may be indispensable.
We deal with sequential devices all the time. In fact, our experience with such devices is
so commonplace that we often take them for granted. For example, at one time or another
we have all had the experience of being delayed by a modem four-way traffic control light
system that is vehicle actuated with pedestrian overrides and the like. Once at the light we
must wait for a certain sequence of events to take place before we are allowed to proceed.
The controller for such a traffic light system is a fairly complex digital sequential machine.
Then there is the familiar elevator system for a multistory building. We may push the
button to go down only to find that upward-bound stops have priority over our command.
But once in the elevator and downward bound, we are likely to find the elevator stopping
at floors preceding ours in sequence, again demonstrating a sequential priority. Added to
these features are the usual safety and emergency overrides, and a motor control system that
allows for the carrier to be accelerated or decelerated at some reasonable rate. Obviously,
modern elevator systems are controlled by rather sophisticated sequential machines.
The list of sequential machines that touch our daily lives is vast and continuously growing.
As examples, the cars we drive, the homes we live in, and our places of employment all use
sequential machines of one type or another. Automobiles use digital sequential machines
to control starting, braking, fuel injection, cruise control, and safety features. Most homes
have automatic washing machines, microwave ovens, sophisticated audio and video devices
of various types, and, of course, computers. Some homes have complex security, energy,

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