Asynchronous and Synchronous Counters

The Introduction of Counters

Counters are one of the most widely used logic components in digital electronic circuits. In addition to simply counting they can be used: to measure time and frequency; increment memory addresses; and divide down clock signals amongst a host of other applications. They are basically special forms of synchronous sequential circuits in which the state of the circuit is equivalent to the count held by the flip-flops used in the circuit. In this chapter we will look only at counters which count in binary sequence, although the next chapter describes how to design circuits with any required count sequence, such as Gray code for instance.

The ‘mod’ of the counter is the number of states the counter cycles through before resetting back to its initial state. So a binary mod-8 counter has eight count states, from 0002 to 111, (e.g. the mod-8 counter actually counts from 0 to 7). All of the counters we will look at use flip-flops as the storage elements that hold the count stale. Therefore, a mod-A counter will need to contain n flip-flops, where 2n>N.

Asynchronous and Synchronous Counters

To divide the counters we will look at into two types: asynchronous and synchronous. When used with respect to counters' these adjectives describe whether the flip-flops holding the states of the circuit are all clocked together (i.e. synchronously) by a master clock or rather asynchronously, with each flip-flop clocked by the one preceding it.

• Asynchronous counters;

•       are also known as ripple counters;

•       are very simple;

•       use the minimum possible hardware (logic gates);

•       employ flip-flops connected serially, with each one triggering (clocking) the next;

•     have an overall count which 'ripples1 through, meaning the overall operation is relatively slow;

•    Require virtually no design.

Synchronous counters:

• use interconnected flip-flops, but all are clocked together by the system clock; .use the outputs from the flip-flops, to determine the next states of the following flip-flops (rather than simply clocking them);

.require no settling time due to rippling (as all flip-flops are clocked synchronously)；

.need designing, to determine how the present state of the circuit must be used to determine the next state (i.e. count);

• usually need more logic gates for their implementation.

Although we will not consider such circuits it is possible to design hybrid asynchronous/synchronous counters (e.g. two synchronous four-bit counters connected asynchronously to produce an eight-bit hybrid) that possess some of the advantages of both types and which are appropriate in some applications.

Due to their universal use in logic circuits, counters are widely available as logic elements in a wide range of forms. These often include clear and preset facilities (which can be used to load a particular count state), and up-down counts available within the same device. Because the design of binary counters is largely intuitive the approach we will take is to firstly consider simple circuits that act as mod-2,f counters, and then how they must be modified to produce a general mod-N count.