GATE Overflow Test Series | Mixed Subjects | Test 2 | Question: 19

Question

In the sequential circuit shown below, if the initial value of the output $Q_1Q_0$ is $11,$ what are the next four values of $Q_1Q_0?$

• A. $11,01,10,00$
• B. $10,11,01,00$
• C. $10,00,01,11$
• D. $11,10,01,00$

Answer 1

The second flipflop will toggle its output only when the first flipflop output becomes $1.$ For each clock, the first FF output - $Q_0$ toggles.

Thus after $1$ clock, we get $Q_0 = 1,Q_1 = 1.$

After second clock, $Q_0 = 0, Q_1 = 1.$

After third clock, $Q_0 = 1, Q_1 = 0.$

After fourth clock, $Q_0 = 0, Q_1 = 0.$

Correct Answer: D.

Comments

If the initial value is 11, why after the first clock is it still 11? shouldn’t it be 10?

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Flip flops are positive edge triggered as we can see in the diagram.

After 1 clock cycle, Q0 moves 1 to 0. Therefore, no positive edge. Q1 stays at 1

State: 10

Q0 moves 0 to 1. Therefore positive edge and Q1 should change from 1 to 0

But due to gate delay, we have 2 states:

11 and  01

Now, Q0 moves from 1 to 0. No positive edge. So Q1 stays at 0

New state: 00

Therefore ans assuming positive edge triggered: 10,11,01,00

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none of the option is correct..
ans should be 10 01 00 11

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@gatecse In this question of the same test, we ignored the initial state and counted from the next state, whereas in the above question, we are also taking the initial state into consideration. Is that something to do with the wording of of the questions? And if that is the case, could you kindly point out the specific words/phrases that produce this difference in interpretation?