ITPEC FE Morning October 2018 Question 22

Topic: Hardware

There is a logic circuit shown below. When the input values are A = 1, B = 0, and C = 1, which of the following is the combination of the values of E and Y?

ABCEY

Explanation

E = 1, Y = 1 — evaluate the XOR gate first, then feed its output into the AND gate with C.

  • •E is the XOR of A and B: the inputs 1 and 0 differ, so E = 1
  • •Y is the AND of E and C: both are 1, so Y = 1

Answer (d)

Why not others:

  • •(a) E = 0, Y = 0 and (b) E = 0, Y = 1 — E = 0 would need equal inputs to the XOR, but A = 1 and B = 0 differ; (b) is also impossible for any input, since an AND gate cannot output 1 when one input is 0
  • •(c) E = 1, Y = 0 — gets the XOR right but not the AND: with E = 1 and C = 1 both inputs are 1, so Y is 1

Key rule: Work a multi-gate circuit stage by stage from the inputs, writing down each intermediate output before using it in the next gate.

Hardware · AI-written, may contain errors

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