In the shown ripple counter, both positive-edge-triggered T flip-flops have T…
2021
In the shown ripple counter, both positive-edge-triggered T flip-flops have T = 1, and Q0 clocks the second flip-flop. Starting from Q1Q0 = 00, what are the states of Q1Q0 after each of the next three rising edges of the external clock?
Answer: B. 11, 10, 01 — ConceptA T flip-flop toggles its output on its active clock edge when T = 1. In a ripple counter, the output of one stage acts as the clock for the next…
- A.
11, 10, 00
- B.
11, 10, 01
- C.
10, 11, 01
- D.
10, 00, 01
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Correct answer: B
Concept
A T flip-flop toggles its output on its active clock edge when T = 1. In a ripple counter, the output of one stage acts as the clock for the next stage.
Here both devices are positive-edge triggered, so Q1 toggles only when Q0 changes from 0 to 1; a 1-to-0 change of Q0 does not toggle Q1.
Application
From 00, the first external rising edge toggles Q0 from 0 to 1. That positive edge at Q0 toggles Q1 from 0 to 1, giving 11.
From 11, the second external rising edge toggles Q0 from 1 to 0. Q1 sees no positive edge, so it remains 1, giving 10.
From 10, the third external rising edge toggles Q0 from 0 to 1. That positive edge toggles Q1 from 1 to 0, giving 01.
Cross-check
Q0 changes on every external rising edge, while Q1 changes only on the 0-to-1 transitions of Q0. The resulting state sequence is therefore 11, 10, 01.
Thus, the next three values of Q₁Q₀ are 11, 10, 01.