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GATE 2023 CS – Question 43

Digital Logic · Sequential Circuits · 2 marks · Multiple choice

Consider a sequential digital circuit consisting of T flip-flops and D flip-flops as shown in the figure. CLKIN is the clock input to the circuit. At the beginning, Q1, Q2 and Q3 have values 0, 1 and 1, respectively. [Figure: T flip-flop (Q1), D flip-flop (Q2) and T flip-flop (Q3) driven by CLKIN.] Which one of the given values of (Q1, Q2, Q3) can NEVER be obtained with this digital circuit?

Diagram for GATE 2023 CS question 43
  1. (0, 0, 1)
  2. (1, 0, 0)
  3. (1, 0, 1)
  4. (1, 1, 1)

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Show answer and explanation

Correct answer: (C) (1, 0, 1)

Explanation

Tracing the state sequence from (0,1,1) through the flip-flop next-state equations, the circuit visits only a subset of the eight states. (1,0,1) is never reached.