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chapter
5
Combinational Logic
Design Practices
he preceding chapter described the theoretical principles used in
combinational logic design. In this chapter, we’ll build on that
foundation and describe many of the devices, structures, and
methods used by engineers to solve practical digital design
problems.
A practical combinational circuit may have dozens of inputs and out-
puts and could require hundreds, thousands, even millions of terms to
describe as a sum of products, and
billions and billions
of rows to describe in
a truth table. Thus, most real combinational logic design problems are too
large to solve by “brute-force” application of theoretical techniques.
But wait, you say, how could any human being conceive of such a com-
plex logic circuit in the first place? The key is structured thinking. A
complex circuit or system is conceived as a collection of smaller sub-
systems, each of which has a much simpler description.
In combinational logic design, there are several straightforward struc-
tures—decoders, multiplexers, comparators, and the like—that turn up quite
regularly as building blocks in larger systems. The most important of these
structures are described in this chapter. We describe each structure generally
and then give examples and applications using 74-series components,
ABEL, and VHDL.
Before launching into these combinational building blocks, we need to
discuss several important topics. The first topic is documentation standards
T
