VLSI QnA: Digital Design Interview Questions

Q.1) Implement inverter using NAND Gate?

Answer) The key to solving such type of questions is, first draw the truth table of the gate given and then the table for what you want to achieve.

Truth table for AND and INVERTER Gate

From the truth table,

when both inputs => 0, output => 1 (same as inverter)

when both inputs => 1, output => 0 (same as inverter)

Implementation of Inverter using NAND Gate.

Inverter using NAND gate

Another Implementation :

Inverter using NAND gate

NAND Expression : Y = not(A) or not (B)

If B => 1

Y = not(A) or not(1) => Y = not (A) or '0'

Y = not (A) ----> inverter

Q.2) Implement inverter using only NOR gate.

Answer) NOR Expression: Y = not(A) and not(B)

When both inputs => '0' , output => '1'

When both inputs => '1' , output => '0'

Another Implementation:

NOR Expression : Y = not(A) and not(B)

If B => '0'

Y = not(A) and not(0)

Y = not(A) and '1'

Y = not(A) ---> inverter

Inverter using NOR gate

Q.3) Implement inverter using 2-to-1 multiplexer.

Answer) Multiplexer Equation :
Out = S1.S + S0.not(S)
If S1 => '0' ; S0 => '1'
Out = 0.(S) + 1.not(S),
Out= not(S) ---> inverter

Inverter using multiplexer

Q.4) Implement AND gate using 2-to-1 multiplexer.

Answer) Multiplexer Equation :
Out = S1.S + S0.not(S)
If S0 => '0'
Out = S1.S
S1 => 'A'
Out = A.S ---> AND gate

AND gate using multiplexer

Q.5) Implement NAND gate using 2-to-1 multiplexer.

Answer) Multiplexer Equation :
Out = S1.S + S0.not (S)
S0 => '1';
S1 => not(A);
Out = not(A).S + not(S) ---> NAND gate
Simplifying it further,
Out = (not(S) + S)(not(A) + not(S))
Out = 1.(not(A) + not(S))
Out = not(A) + not(S) ----> NAND gate