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WHAT'S YOUR ENGINEERING QUOTIENT?

Problem 2—What is the output of the following circuit, assuming that all the flip-flops are reset at power-on. What is this circuit popularly known as?

Answer

At power on, Q0 = 0 and D0 = 1. Also, Q1 = 0 and D1 = 1. The output value is 00.

When the first clock edge arrives, Q0 changes to 1, and D0 changes to 0. There is no change in Q1, so now the output is 01.

During the next clock edge, Q0 changes to 0 and Q0' changes from 0 to 1. This is an active clock edge for the left-hand flip-flop and its output changes to Q1 = 1. Now the output is 10.

When the next clock edge arrives, Q0 changes to 1, and D0 changes to 0. There is no change in Q1, so now the output is 11.

Finally, during the next clock edge, Q0 changes to 0 and Q0' changes from 0 to 1. This is another active clock edge for the left-hand flip-flop and its output changes to Q1 = 0. The output is once again 00.

The overall output sequence is 00, 01, 10 and 11. This is a 2-bit binary counter. This kind of circuit is called a ripple counter.

Contributor: Naveen P N

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