Sequential Circuits

Down Counter (Reverse counting)

Here we’ll be counting in reverse order i.e. count would start from 15 to 0 and again value goes from 0 to 15. We just make a change in the circuit as we give Q bar to the CLK of next flip-flop or we use positive edged flip-flops and give Q to CLK of next flip-flop.

And the output waveform would be as:

Or

And the output waveform would be as:

In both cases we take (Q4 Q3 Q2 Q1) 2 as value of the count

Or

We can just use the same circuit as the UP counter but

Consider the following circuit

And we see that this circuit is a UP counter which count from 0 to 7 and then it is reset but the same circuit can also work as DOWN counter when we take count as combination of inverted outputs for each FF. i.e.. Hence output count of the above circuit would go from 7 to 0 and then again it is set to 7.

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