Digital logic-Waveform

Question

Comments

time                           output

 0-10                                0           (XOR gate is not activated)

10-20                               1           (1 XOR 0=1 )

20-30                                1           (1 XOR 0=1)

30-40                                 1          (1 XOR 0=1)

after 10 unit of time i am getting output 1 ,but in answer output  between 30-40 is 0 how???? correct me

Answer 1

The propagation delay, or gate delay, is the length of time which starts when the input to a logic gate becomes stable and valid to change, to the time that the output of that logic gate is stable and valid to change.

So from t = 0 to 10, output of XOR gate is 0. Input A to XOR gate is 1. After t = 10, output is stable, considering another input of the XOR gate is 0, output Y is 1. The one input to XOR gate(i.e output of AND gate) destabilise at t = 30 and again stabilses, becomes 0. Output of XOR gate again stabilises to 0 after t = 40 as gate delay for XOR gate is 10 ns.

Hence option B.

Comments

initially

From 0 to 10 --- Output = 0 :: since one input = 0 i.e. coming out of AND gate (even though the AND gate is not active but initially all input is in 0 state )is 0 and other input is A = 1 and including EX-OR gate delay is 10 so output generated after 10ns and it will be 1

now from 10 to 30 output will be 1 no doubt.

But from 30 to afterward the output should be 1 only because AND gate is not going to generate output as 1 since it will always get input in form of 0 and 1 only and resultant output will be 0 only always therefore the one input to EX-OR gate is always set to 0 and the other input 1 only as A is set to 1. Now EX-OR gate property, on input 0 and 1 it will generate output as 1 only after 30ns also. So in total output should be 1 after 10ns.

Correct me if I am wrong and please tell the line written in bold letter is true of false.