Digital Design Interview Questions - 1

1. How do you convert a XOR gate into a buffer and a inverter (Use only one XOR gate for each)?

2. Implement an 2-input AND gate using a 2x1 mux.

3. What is a multiplexer?

4. What is a ring counter?

5. Compare and Contrast Synchronous and Asynchronous reset.

6. What is a Johnson counter?

7. An assembly line has 3 fail safe sensors and one emergency shutdown switch.The line should keep moving unless any of the following conditions arise:
(1) If the emergency switch is pressed
(2) If the senor1 and sensor2 are activated at the same time.
(3) If sensor 2 and sensor3 are activated at the same time.
(4) If all the sensors are activated at the same time
Suppose a combinational circuit for above case is to be implemented only with NAND Gates. How many minimum number of 2 input NAND gates are required?

8. In a 4-bit Johnson counter How many unused states are present?

9. Design a 3 input NAND gate using minimum number of 2 input NAND gates.

10. How can you convert a JK flip-flop to a D flip-flop?


swathi said...

very nice..

madhu said...


marru said...

good seems nice

Anonymous said...

How to verify answer of 7th Question????

pradeep said...

4 nand gates required

sharif said...

How can we calculate the unused states in jhonsen counter

Heman said...

i think for Q.7 it has to 6. can some1 confirm pls

ellipsis said...

for the 7th i too got six NAND2 gates!!

Sandy said...

Can anyone Help me to design a Xor gate using Pass Transistor Logic, It helps in my Thesis Work.. If anyone wanna help me then Plz contact me at

Anonymous said...

A ---| |----- Y

XOR using PassGate.

Anonymous said...


XOR using PassGate

razz said...

really useful for al learners..

fresher's venture said...

great collection,keep up the work

ramesh reddy said...

A. qusn 7:
1 NAND and 1 OR gates r sufficient
s1,s2 to 1 i/p of NAND and another has s2 and inverted result is ORed with E i/p..... then in any given condition it leads o/p to 1

Anonymous said...


The question explicitly states NAND gates only, using an OR gate defeats the purpose of the question

Anonymous said...


Anonymous said...

I can't find link for answers... plz somebody help me out. .:(

Unknown said...

Q7: is it 3 NAND gates?

Anonymous said...

I think the answer of Q7 is 6, 2 Nands are used as inverter

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