GATE | GATE-CS-2003 | Question 47

Consider the following circuit composed of XOR gates and non-inverting buffers.

The non-inverting buffers have delays d1 = 2 ns and d2 = 4 ns as shown in the figure. Both XOR gates and all wires have zero delay. Assume that all gate inputs, outputs and wires are stable at logic level 0 at time 0. If the following waveform is applied at input A, how many transition(s) (change of logic levels) occur(s) at B during the interval from 0 to 10 ns ?

(A) 1
(B) 2
(C) 3
(D) 4


Answer: (D)

Explanation: As we draw wave forms of different inputs:

1. The 2ns buffer input is shifted form of A
2. The output of first EXOR gate is a single beat of 1 through 3
3. While this is shifted by the 4ns buffer
4. Which in-turn results in two beats output from the second EXOR gate

Hence 4 transitions, so option (D) is correct.

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