Page 95 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Digital Circuits and Logic Design Sandeep Kumar, Lovely Professional University

Notes Unit 6: Implementation of Combinational Logic Circuit

CONTENTS
Objectives
Introduction
6.1 Multiplexers
6.1.1 Synthesis of Logic Functions Using Multiplexers
6.1.2 Multiplexer Synthesis Using Shannon’s Expansion
6.2 Demultiplexer

6.3 Code Converters
6.4 Comparators
6.5 Summary
6.6 Keywords
6.7 Review Questions
6.8 Further Readings

Objectives

After studying this unit, you will be able to:
• Discuss the multiplexer
• Describe the demultiplexer
• Explain the code converters
• Describe the comparators

Introduction

Combinational logic (sometimes also referred to as combinatorial logic) is a type of digital logic
which is implemented by Boolean circuits, where the output is a pure function of the present
input only. This is in contrast to sequential logic, in which the output depends not only on the
present input but also on the history of the input. In other words, sequential logic has memory
while combinational logic does not. Combinational logic is used for building circuits where certain
outputs are desired, given certain inputs. The construction of combinational logic is generally
done using one of two methods: a sum-of-products, or a product-of-sums.

This unit introduces several combinational circuits that are frequently used by digital designers,
including a data selector (also called a multiplexor or just “mux”), a binary decoder, a seven-
segment decoder, an encoder, and a shifter. Each of these circuits can be used by themselves in
the solution of some simpler logic problems, but they are more often used as building blocks in
the creation of larger, more complex circuits. In this module, these circuits will be developed
from first principles following a general design procedure that will serve as a model for all later
designs. In later modules, these circuits will be used as modular (or “macro”) building blocks in
larger designs.

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