Digital Counters

Digital Counters, as the name suggests, are digital circuits used for counting occurrence of any event. For any digital system like a computer, counters are most useful and versatile subsystem. A flip-flop can be used as a counter to count two states since Flip flop has two states either 0 or 1. Similarly, two flip-flops can be connected together to count 4 states (00, 01, 10, 11). Thus, with n flip-flops, we can make a counter that can count maximum 2ⁿ stages . Digital counters have following properties:

1)      Asynchronous or Synchronous – Asynchronous counters are one in which output of one flip-flop drives succeeding FF. Only first FF needs clock. All other Flops will be driven by its preceding FF. While in synchronous counters All FF are clocked by same external clock pulse.    

2)      Maximum number of counts – Also known as Modulus of counter. Modulus of counter is equal to total number of distinct stages through which counter progresses. For example:  A 4 Flip flop counter is often referred to as mod-16 counter i.e it can count 16 stages.

3)      Up or down counter – It dictates whether counter counts in upward or downward direction. For exp . 0 ->1->2->3->4->5->6->7 is up counter. 7->6->5->4->3->2->1->0 is down counter. The counter which can count in up or down direction depending upon some external signal(e.g.  if up(some external signal) is 1, count in upward direction else in downward) is called up-down counter.

4)      Free running or self stopping – if counter begins counting again after reaching its maximum stage,counter is called free running and if it stops after reaching maximum stage value it is called self stopping. Free running counters are also known as blind running counters.

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