Synchronous Up-Down Counters

Synchronous Up-Down Counters are those counters which count the data in both directions that are both up and down in the same interval of time. It is used to control the direction of the counter through a certain sequence.

 

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The above-given circuit is the 3-Bit synchronous up/Down counter.

It is constructed by using the J K flip-flops configured to operate as a toggle. They can also be constructed by using the T flip-flops. This gives a maximum count of 0 (000) to 7 (111) and goes back to 0 again. Then the 3-Bit counter advances upward in sequence (0,1,2,3,4,5,6,7) or downwards in reverse sequence (7,6,5,4,3,2,1,0).

The up/down counters are incorporated into a single IC that is fully programmable to count in both an “Up” and a “Down” direction from any pre-set value producing a complete Up/Down operation Counter chip. These counters can be made to change their count directions either in the up or down direction. This is achieved by using an additional input that determines the direction of the count.

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The count sequence for the above figure is given below:

 

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If we see the above count sequence, we see that for both Up and Down sequences, Q A toggles on each given clock pulse. Likewise, for the up sequence, Q B changes state on the next clock pulse when Q A = 1. And, for the down sequence, Q B changes state on the next clock pulse when Q B =0. Again, let us see for the Up sequence. In this, Q C changes state on the next clock pulse when Q A = Q B =1. Following this, again for the down count, Q C changes state on the next clock pulse when Q A = Q B = 0.