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A counter is a special kind of register, designed to count the number of clock pulses arriving at its input. The input to this counter is a rectangular waveform called clock. Each time the clock signal changes state from low to high, the counter will add one (1) to the number stored in its flip flop.

The counter can be used as an instrument for measuring time and therefore frequency. There are basically two different types of counters Synchronous and Asynchronous.

  1. Synchronous counters: In a ripple counter each flip - flop has a delay time and these delay times are additive. Therefore the total “settling” time for the counter is equal to the delay time multiplied by total number of flip - flops. This problem can be overcome by the use of a synchronous or parallel counter. Here every flip-flop is triggered in synchronism with the clock.


Fig. : Synchronous counter

  1. Asynchronous counter (ripple counter): When the output of a flip-flop is used as the clock input for the next flip-flop we call the counter as a ripple counter or asynchronous counter.


Fig. : Asynchronous counter


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