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Tuesday, May 1, 2018

Digital Electronics - Mux Based Interview Questions


As a fresher or experienced candidate, digital electronics always play an important role in VLSI Interview. Whatever be the profile, it doesn't matter. Lot of candidates call me or message me and ask
  • What type of questions can be asked in Digital electronics ?
  • Which book/s is/are good for preparation?
  • What all topics should be cover ?
and lot more such type of questions.

It's very difficult to answer such questions because Digital electronics is so vast that no one can cover it 100% with in 1 month (max time we get before any interview). So what's the best way to prepare. I am capturing few topics from Digital electronics one by one which looks to me very important. I am also trying to capture questions which can be asked by any Interviewer. If you can focus on these topics or say questions - I am sure - it will give you a good amount of confidence for any Interview.

Note: At the end, I am trying to capture few common mistakes, more than 50% candidates do during interviews. I hope, it will help all of you to get the right answer and understand the expectation of Interviewer.

Multiplexer based Interview Questions:


  1. Describe the operation of Multiplexer
  2. How many select lines are there for 12 : 1 MUX ?
  3. How many input pins are there in a 2:1 MUX ?
  4. How can we convert 4:1 Mux into 2:1 Mux ?
    1. Any other method for converting 4:1MUX to 2:1, where no need to short "Select line" ?
  5. How many 2:1 MUX require to make 16:1 MUX ?
    1. Draw diagram with proper labelling
  6. What's the difference between MUX and Encoder?
  7. Draw internal circuitry of 2:1 MUX using only NMOS, PMOS and CMOS?
  8. How many Transistor (transistor count) in a 2:1 MUX?
  9. Design an AND gate using MUX.
  10. Design a D Flip-flop using MUX.
    1. This is very common question and lot of follow-up questions on the basis of your answer. Below are few ...
    2. Mux'ed based "Positive/Negative Triggered D-Flipflop"
    3. Mux'ed based "Positive/Negative level latch"
    4. Advantage/Disadvantage of using Mux'ed based Flipflop.
  11. What's the advantage/disadvantage using MUX based AND gate over normal CMOS based AND gate?
  12. Do you know anything about Clock gating?
    1. Draw a circuit for clock gating using AND gate
    2. Draw a circuit for clock gating using OR gate
    3. Draw a circuit for clock gating using MUX
    4. Out of above 3 which one you prefer and why?
  13. Questions where Interviewer connect 1 input of 2:1 MUX with the ground and ask you the output function. e.g
    1. If in a 2 : 1 MUX, we connect D0 input with ground, what will be the output?
    2. If we interchange the signal applied at D0 & D1 in above figure, what will be the output?
  14. In Mux'ed based AND gate, if position of Inputs are Interchanged (below fig a and fig b), is there any difference from timing perspective (Delay perspective)? Justify your answer in both case (Yes/NO).

There can be more questions but unfortunately, not able to recall them. Anyone has any other questions - please help me to add here. Else, I will update this list whenever I am able to recall. :)


Common Mistakes in Different Questions:


Common Mistake Related to Question 1:
While explaining the operation of Mux, candidates generally miss "select lines". If Interviewer ask to draw the diagram of MUX, candidates draw diagram without proper labelling. This is the area where anyone can confuse you. See below symbols & try to understand why I have mentioned correct or wrong.



Common Mistake Related to Question 3:
Very common Answer is 2 which is not correct. Why? Check .lib file.

Common Mistake Related to Question 4:
Solution is very simple - Connect both the select line or say short both the select line. But common mistake - we miss to make changes in the labelling.


Common Mistake Related to Question 5
  • Candidates become confuse between Decoder method and Mux method. :)
  • Most of the Candidates confuse at the labelling part.

Common Mistake Related to Question 7
Normally, in college text books (or say basic Digital Electronics Books) - internal circuit is based on AND gate. So, we start with the AND gate based circuit of MUX and then convert respective AND gate with MOS. But actually, Interviewer need to know Pass Transistor or say Transmission gate based MUX. :)

Common Mistake Related to Question 8
Usually, we gave single answer but actually it depends how we have implemented MUX. Means using CMOS or Only NMOS/PMOS.

Common Mistake Related to Question 9
Usually we draw a single circuit, which we have learnt. But we miss to draw truth table and then implement using MUX. If you use this approach - you can implement any logic gate using MUX. Interviewer, also see your approach and need to understand whether you know the concepts or method or you have just mugged it.


Happy Learning and Best of Luck.

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