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Sample Digital Questions Asked in Interviews

1. What is the output of AND gate in the circuit below, when A and B are as in waveform? Tp is the gate delay of respective gate.

2. Identify the circuit below, and its limitation.

3. What is the current through the resistor R1 (Ic) ?

4. Referring to the diagram below, briefly explain what will happen if the propagation delay of the clock signal in path B is much too high compared to path A. How do we solve this problem if the propagation delay in path B can not be reduced ?

5. What is the function of a D flip-flop, whose inverted output is connected to its input ?

6. Design a circuit to divide input frequency by 2.

7. Design a divide-by-3 sequential circuit with 50% duty cycle.

8. Design a divide-by-5 sequential circuit with 50% duty cycle.

9. What are the different types of adder implementations ?

10. Draw a Transmission Gate-based D-Latch.

11. Give the truth table for a Half Adder. Give a gate level implementation of it.

12. What is the purpose of the buffer in the circuit below, is it necessary/redundant to have a buffer ?

13. What is the output of the circuit below, assuming that value of 'X' is not known ?

14. Consider a circular disk as shown in the figure below with two sensors mounted X, Y and a blue shade painted on the disk for an angle of 45 degree. Design a circuit with minimum number of gates to detect the direction of rotation.

15. Design an OR gate from 2:1 MUX.

16. Design an XOR gate from 2:1 MUX and a NOT gate

17. What is the difference between a LATCH and a FLIP-FLOP ?
  • Latch is a level sensitive device while flip-flop is an edge sensitive device.
  • Latch is sensitive to glitches on enable pin, whereas flip-flop is immune to glitches.
  • Latches take less gates (also less power) to implement than flip-flops.
  • Latches are faster than flip-flops.

18. How can you generate random sequences in digital circuits?
19. Design a D Flip-Flop from two latches.

20. Design a 2 bit counter using D Flip-Flop.

21. What are the two types of delays in any digital system ?

22. Design a Transparent Latch using a 2:1 Mux.

23. Design a 4:1 Mux using 2:1 Muxes and some combo logic.

24. What is metastable state ? How does it occur ?

25. What is metastability ?

26. Design a 3:8 decoder

27. Design a FSM to detect sequence "101" in input sequence.

28. Convert NAND gate into Inverter, in two different ways.

29. Design a D and T flip flop using 2:1 mux; use of other components not allowed, just the mux.

30. Design a divide by two counter using D-Latch.

31. Design D Latch from SR flip-flop.

32. Define Clock Skew , Negative Clock Skew, Positive Clock Skew.

33. What is Race Condition ?

34. Design a 4 bit Gray Counter.

35. Design 4-bit Synchronous counter, Asynchronous counter.

36. Design a 16 byte Asynchronous FIFO.

37. What is the difference between an EEPROM and a FLASH ?

38. What is the difference between a NAND-based Flash and a NOR-based Flash ?

39. You are given a 100 MHz clock. Design a 33.3 MHz clock with and without 50% duty cycle.

40. Design a Read on Reset System ?

41. Which one is superior: Asynchronous Reset or Synchronous Reset ? Explain.

42. Design a State machine for Traffic Control at a Four point Junction.

43. What are FIFO's? Can you draw the block diagram of FIFO? Could you modify it to make it asynchronous FIFO ?

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