Sequential Circuits

Johnson Counter

While Ring counter, we have connected Q of last to D of 1st FF, but in Johnson Counter we connect Q bar of last to D of 1st FF as shown below and we also don’t need to connect preset of 1st FF. This is also called Twisted Ring counter:

And JK implementation is as follow:

And we have outputshas follow:

Clock                     Q4 Q3 Q2 Q1

Initially                  0000

1st tick                   0001

2nd                          0011

3rd                           0111

4th                           1111

5th                           1110

6th                           1100

7th                           1000

8th                           0000

We can also note that we use only 8 out of 16 possible states and in general we have used 2n states and hence we have 2n-2n unused states

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