GATE CSE 2024 | Set 1 | Question: 18

Question

​​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?

• A. With no propagation delays, the output $Y$ is always logic Zero
• B. With no propagation delays, the output $Y$ is always logic One
• C. With propagation delays, the output $Y$ can have a transient logic One after $X$ transitions from logic Zero to logic One
• D. With propagation delays, the output $Y$ can have a transient logic Zero after $X$ transitions from logic One to logic Zero 

Comments

From above graph, we can observe C is True and D is False.

Y = X.X’ = 0, hence A is True and B is False.

Answer - A, C

Answer 1

Option - A,C

Case 1 : when there is no propogation delay, this circuit is a simple combinational circuit XX' which is always false or Logic 0. 

Case 2 : If we consider the delay in the Not gate, When the signal X transitions from 0 to 1, And if the delay of the And gate is larger than the not gate momentarily the Not gate will be outputing the older (incorrect) signal, because of this we will have a transient (also called temporary) logical 1  output from the circuit.

Comments

Just have a doubt why D cannot be an answer?

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You don't have transient 0. you get a unchanging 0. when output is changing from 1 to 0, both ports at the end gate will be having logic 0 and 0and0 is 0. only in the case of chaning from 0 to 1 we can have momentary 1. 

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can you explain  about option D