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Combinational and Sequential Circuits

GATE CSE 2023 · Q44

MCQ2 marksMedium

Time

A Boolean digital circuit is composed using two 4-input multiplexers (M1 and M2) and one 2-input multiplexer (M3) as shown in the figure. X0–X7 are the inputs of the multiplexers M1 and M2 and could be connected to either 0 or 1. The select lines of the multiplexers are connected to Boolean variables A, B and C as shown.

Which one of the following set of values of (X0, X1, X2, X3, X4, X5, X6, X7) will realise the Boolean function A‾+A‾.C‾+A.B‾.C\overline{\text{A}} + \overline{\text{A}}.\overline{\text{C}} + \text{A}.\overline{\text{B}}.\text{C}?

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Solution

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

Combinational and Sequential Circuits

To find the values of (X0,X1,X2,X3,X4,X5,X6,X7)(X_0, X_1, X_2, X_3, X_4, X_5, X_6, X_7), let us simplify the given Boolean function FF:

F=A‾+A‾⋅C‾+A⋅B‾⋅CF = \overline{A} + \overline{A}\cdot\overline{C} + A\cdot\overline{B}\cdot C

Using absorption law A‾+A‾⋅C‾=A‾\overline{A} + \overline{A}\cdot\overline{C} = \overline{A}:

F=A‾+A⋅B‾⋅C=A‾+B‾⋅CF = \overline{A} + A\cdot\overline{B}\cdot C = \overline{A} + \overline{B}\cdot C

Now, let us determine the minterms of F(A,B,C)F(A, B, C):

  • When A=0A = 0, F=1F = 1 for all combinations of (B,C)(B, C), which corresponds to (A,B,C)∈{000,001,010,011}(A, B, C) \in \{000, 001, 010, 011\} (m0,m1,m2,m3m_0, m_1, m_2, m_3).

  • When A=1A = 1, F=1F = 1 only when B‾⋅C=1\overline{B}\cdot C = 1, i.e., B=0,C=1B = 0, C = 1, which corresponds to (A,B,C)=101(A, B, C) = 101 (m5m_5).

  • For all other combinations (m4=100,m6=110,m7=111m_4 = 100, m_6 = 110, m_7 = 111), F=0F = 0.

From the multiplexer circuit:

  • Multiplexer M3 is controlled by select line S=BS = B:

    • When B=0B = 0, Q=QM1Q = Q_{M1} (output of M1)

    • When B=1B = 1, Q=QM2Q = Q_{M2} (output of M2)

  • For M1 and M2, select lines are S1=AS_1 = A and S0=CS_0 = C.

Evaluating the inputs for each combination of (A,B,C)(A, B, C):

  1. For B=0B = 0 (M1 selected):

    • (A,C)=(0,0)  ⟹  (A,B,C)=(0,0,0)  ⟹  X0=F(0,0,0)=1(A, C) = (0, 0) \implies (A, B, C) = (0, 0, 0) \implies X_0 = F(0, 0, 0) = 1

    • (A,C)=(0,1)  ⟹  (A,B,C)=(0,0,1)  ⟹  X1=F(0,0,1)=1(A, C) = (0, 1) \implies (A, B, C) = (0, 0, 1) \implies X_1 = F(0, 0, 1) = 1

    • (A,C)=(1,0)  ⟹  (A,B,C)=(1,0,0)  ⟹  X2=F(1,0,0)=0(A, C) = (1, 0) \implies (A, B, C) = (1, 0, 0) \implies X_2 = F(1, 0, 0) = 0

    • (A,C)=(1,1)  ⟹  (A,B,C)=(1,0,1)  ⟹  X3=F(1,0,1)=1(A, C) = (1, 1) \implies (A, B, C) = (1, 0, 1) \implies X_3 = F(1, 0, 1) = 1

  2. For B=1B = 1 (M2 selected):

    • (A,C)=(0,0)  ⟹  (A,B,C)=(0,1,0)  ⟹  X4=F(0,1,0)=1(A, C) = (0, 0) \implies (A, B, C) = (0, 1, 0) \implies X_4 = F(0, 1, 0) = 1

    • (A,C)=(0,1)  ⟹  (A,B,C)=(0,1,1)  ⟹  X5=F(0,1,1)=1(A, C) = (0, 1) \implies (A, B, C) = (0, 1, 1) \implies X_5 = F(0, 1, 1) = 1

    • (A,C)=(1,0)  ⟹  (A,B,C)=(1,1,0)  ⟹  X6=F(1,1,0)=0(A, C) = (1, 0) \implies (A, B, C) = (1, 1, 0) \implies X_6 = F(1, 1, 0) = 0

    • (A,C)=(1,1)  ⟹  (A,B,C)=(1,1,1)  ⟹  X7=F(1,1,1)=0(A, C) = (1, 1) \implies (A, B, C) = (1, 1, 1) \implies X_7 = F(1, 1, 1) = 0

Thus, (X0,X1,X2,X3,X4,X5,X6,X7)=(1,1,0,1,1,1,0,0)(X_0, X_1, X_2, X_3, X_4, X_5, X_6, X_7) = (1, 1, 0, 1, 1, 1, 0, 0), which corresponds to Option (C).

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This Combinational and Sequential Circuits previous year question is Q44 of the GATE CSE 2023 paper. Find more GATE CSE PYQs on Combinational and Sequential Circuits in Digital Logic, or browse all GATE CSE PYQs.

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GateAI solution cardGATE CSE 2023 · Q44 · MCQ

A Boolean digital circuit is composed using two 4-input multiplexers (M1 and M2) and one 2-input multiplexer (M3) as shown in the figure. X0–X7 are the inputs of the multiplexers M1 and M2 and could be connected to either 0 or 1. The select lines of the multiplexers are connected to Boolean variables A, B and C as shown. Which one of the following set of values of (X0, X1, X2, X3, X4, X5, X6, X7) will realise the Boolean function A + A. C + A. B.C?

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