Digital Logic Design, 4th Edition

7

Counters and registers

7.1 Introduction

A counting circuit composed of memory elements, such as flip-flops and electronic gates, is the simplest form of sequential circuit available. All sequential circuits are of two types, (1) synchronous (clock driven) and (2) asynchronous (event driven). In synchronous circuits, changes in the circuit state are synchronised to the normally periodic clock pulses, whereas in event driven circuits state changes are governed by events such as, for example, the occurrence of a system fault.

Counting circuits can be in either of the two categories described above. All counter circuits count clock pulses and store the number received in an array of memory elements. In the case of synchronous counters the flip-flops ...