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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