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Consider the circuit shown below where the gates may have propagation delays. Assume that all signal transitions occur instantaneously and that wires have no delays. Which of the following statements about the circuit is/are CORRECT?

1. With no propagation delays, the output Y is always logic Zero
2. With no propagation delays, the output Y is always logic One
3. With propagation delays, the output Y can have a transient logic One after X transitions from logic Zero to logic One
4. With propagation delays, the output Y can have a transient logic Zero after X transitions from logic One to logic Zero

Correct answer: With propagation delays, the output Y can have a transient logic One after X transitions from logic Zero to logic One

Solution

In hazard-prone combinational circuits, the steady-state output may be correct, but propagation delays can create a temporary glitch during input transitions. Here, the correct statement is that a transient logic 1 can appear when X changes from 0 to 1.

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