In this post I am writing some frequently asked Digital Design Interview Questions
Q1. The minimum number of flip-flops that can be used to construct a mod-5 counter is.
• 3 flip flop are required.
Q2. Design an inverter and buffer using a XOR gate.
Q3. Design a 4:1 MUX using 2:1 multiplexers?
Q4. What is the difference between a flip-flop and a latch?
• Latch continuously check its inputs and changes its output correspondingly. Latches are level sensitive. Latches based on enable signal.
• Flip flop continuously check its inputs and changes its output correspondingly only at times determined by clocking signal. Flip flop are edge sensitive. Flip flop based on clock signal.
Q5. Implement D flip-flop using 2:1 MUX.

Q6. Convert a JK flip-flop to D Flip-flop.
Q7. Difference between a synchronous counter and an asynchronous counter?
• In synchronous counters, the clock inputs of all flip-flops are connected to a common Clock signal and hence all flip-flops changes synchronously. Synchronous are faster. Synchronous counters also called ring counter.
• In asynchronous counters, the clock input is connected only to the first flip-flop and the Output for first flip-flop is connected to the clock input of second flip-flop and similarly every other flip-flop is clocked by the output of previous flip-flop. Asynchronous counters also called ripple counter. Asynchronous are slower.
Q8. What is the difference between synchronous and asynchronous reset?

Q9. What is the difference between a Moore and a Mealy finite state Machine?
• A Moore Machine is a finite state machine whose output depends only on the present state.
• A Mealy Machine is a finite state machine whose output depends on the present state as well as the present input.
Q10. How can you implement two input NOR and NAND gates using a 2:1 MUX?
Q11. What is transparent latch?
• D-Latch is called transparent Latch. It transfers the data as it is to the output on enable.
Q12. How to convert S-R Latch to transparent latch?
Q13. What is race-around condition?
Q14. Draw the circuit for a D flip flop with Synchronous Reset?
Q15. Design a 4-bit binary counter using TFFs?
Q16. How many unused states will be there in a Johnson’s counter with N flip-flops?
• For N-flops, the total possible states = 2^N.
• The number of states of a Johnson counter = 2N
• The number of unused states = 2^N – 2N.
Q17. If each flip flop has a clock-to-Q delay of 10ns, how much time will it take for output to change to its next state(a) 4-bit Ripple Counter (b) 4-bit Synchronous counter?
• (a) 4x10= 40ns
• (b) 10ns
Q18. Convert the Gray code number 11001 to binary code?
• Binary number is 10001.
Q19. Y = F(A,B,C,D) = Σ (0,1,4,5,7,9,12).

Q20. Design half subtractor using NOR gates. (Cadence 2019)
Q21. Implement a full adder using two 4:1 Muxes?

Q22. . Design a frequency divide-by-2 circuit using D flip flop and external gates which gives (a) 50% duty cycle (b) 25% duty cycle?
Q23. What is the output frequency of a 4-bit binary counter for an input clock of 160 MHz.
• The output of last flip flop of a 4-bit counter is equal to the input clock/16.
• So output frequency = 160MHz/16 = 10MHz.
Q24. The figure shows a binary counter with synchronous clear input. With the decoding logic shown, the counter works as a

Q25. The gate delay of an NMOS inverter is dominated by charge time rather than discharge time because.
• The load transistor has smaller W/L ratio as compared to driver transistor.
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