6 - Counters

Abstract

This chapter presents the counters of sequential logic circuits. Counters are fundamental components of digital systems and can be used in timing, control, or sequencing applications. They appear in computer and communications circuits; they can be used for frequency division; in some cases, they count in pure binary and in other cases, there may be a nonbinary count, for example, a Gray code counter or a BCD decade counter. All sequential circuits are of two types: (i) synchronous (clock-driven) circuits and (ii) asynchronous (event-driven) circuits. In synchronous circuits, changes in circuit state are synchronized to the clock pulses, whereas in asynchronous circuits, changes in circuit state are governed by events, such as the occurrence of a system fault. In the case of a synchronous counter, the circuit counts clock pulses and stores the number counted in the memory elements, while a ripple-through counter can be used to count irregularly occurring events such as customers entering a bank and again the number counted is stored in the memory elements. The simplest possible counter is the scale-of-two counter, which has only two states, namely, 0 and 1. As the output of a flip-flop can only exist in one of two states, it is clear that this counter can be implemented with a single flip-flop. There are two ways of connecting the inputs to successive flip-flops of counters, which are serial and parallel connections. In practice, synchronous and asynchronous counters can be built from standard JK flip-flops available and the techniques described above can be used to design either ripple-through or synchronous counters.