GATE CSE 1989 | Question: 4-ix

Question

Explain the behaviour of the following logic circuit with level input $A$ and output $B$.

Comments

Please, someone, explain this question in brief.

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@abbas, he's accounted for feedback too

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@Abbas2131 Including B refers to that feedback is being accounted

Answer 1

This is a sequential circuit (whose output depends not only on the present value of its input signals but on the sequence of past inputs) not a combinational one (whose output depends only on the present inputs), therefore solving using just input variable does not yields correct output.

First we need to simplify the circuit.

The two $\textsf{NOT}$ gates at the input end of the $\textsf{NOR}$ gate can be combined with the gate to get: $(A'+B')' = AB$

Now, since we have two variables we will have $4$ combinations $00\; 01\; 10\; 11.$

On analyzing each we will see that for every combination where

• $A = 0$ we have the stable output of $0$
• $A=1$ we will have a RACE condition

Comments

For A=1, we should say that circuit is unstable as output continually changes between 0 and 1. This is different from race condition. Race condition is defined as certain unwanted spikes at the output due to different path delays in the circuits.

 

Race condition can also occur in combinational circuits whereas instability is issue of only sequential circuits.

Answer 2

There is a NOT after NAND i,e. It is AND.

So it will be A.B

and other side : $\bar{A}$

after NORING $\overline{AB+\overline{A}}$

$\overline{AB} + \overline{\overline{A}}$

$\bar{A} + \bar{B} + A = 1$

So Answer is 1.

Comments

Same mistake as above.

Treating a sequential ckt as combinational. Your answer is correct if it had been a combinational ckt just dependent on inputs for all cycles.