Digital Circuit Systems

Combinational Circuit Viva Questions

Combinational Circuit Viva Questions

Combinational Circuit Viva Questions, Viva Questions on Combinational Circuit, Multiplexer Viva Questions, Encoder Viva Questions, Digital Electronics Viva Questions, Combinational Logic Circuit Viva Questions, Arithmetic Viva Questions, Hazard Viva Questions, Full Adder Viva Questions, Short Questions on Combinational Circuit, Combinational Circuit Viva Questions

Logic Gates Viva Questions

Short Questions and Answers

Q.1. What are arithmetic circuits?

Ans. Arithmetic circuits are the circuits that perform arithmetic operations.

Q.2. What is a half-adder?

Ans. A half-adder is an arithmetic circuit that adds two binary digits. It has two inputs and two outputs (sum and carry).

Q.3. What is a full-adder?

Ans. A full-adder is an arithmetic circuit that adds two binary digits and a carry, i.e. three bits. It has three inputs and two outputs (sum and carry).

Q.4. What is the disadvantage of realizing a full-adder using two half-adders?

Ans. The disadvantage of realizing a full-adder using two half-adders is that, in this, the bits must propagate through several gates in succession, which makes the propagation delay greater than that of the full-adder circuit using AOI logic.

Q.5. What is a half-subtractor?

Ans. A half-subtractor is an arithmetic circuit that subtracts one binary digit from another. It has two inputs and two outputs (difference and borrow).

Q.6. What is a full-subtractor?

Ans. A full-subtractor is an arithmetic circuit that subtracts one binary digit from another considering a borrow. It has three inputs and two outputs (difference and borrow).

Q.7. Why are subtractor ICs not available?

Ans. Since subtraction is performed using adders by making use of l’s and 2’s complement methods, separate subtraction ICs are not available.

Q.8. What is a parallel adder?

Ans. A parallel adder is an arithmetic circuit that adds two numbers in parallel form.

Q.9. What is the carry propagation delay of a full-adder?

Ans. The carry propagation delay of a full-adder in a parallel adder is the time between the application of the carry-in and the occurrence of the carry-out.

Q.10. What do you mean by cascading of parallel adders? Why is it required?

Ans. Connecting the parallel adders in series, i.e. connecting the carry out of one parallel adder to the carry-in of another parallel adder is called cascading them. It is required when a large number of bits are to be added.

Q.11. How is the addition of large binary numbers accomplished?

Ans. The addition of large binary numbers can be accomplished by cascading two or more parallel adder chips.

Q.12. What is a ripple-carry-adder?

Ans. A ripple-carry-adder is a parallel adder in which the carry-out of each full adder is the carry-in to the next most significant adder.

Q.13. How does the look-ahead-carry adder speed up the addition process?

Ans. The look-ahead-carry adder speeds up the addition process by eliminating the ripple carry delay. It examines all the input bits simultaneously and also generates the carry-in bits for all the stages simultaneously.

Q.14. When is carry generated and when is carry propagated in a look-ahead-carry adder?

Ans. In a look-ahead-carry adder, a carry-out is generated when both the input bits are 1. A carry-in may be propagated by the full-adder when either or both of the input bits are 1.

Q.15. What is a serial adder?

Ans. A serial adder is a sequential circuit used to add serial binary numbers.

Q.16. Why does a serial adder require only one full adder?

Ans. A serial adder requires only one full-adder because in this the bits are added serially, i.e. one pair of bits at a time.

Q.17. What is the drawback of serial adders? For which applications are they preferred?

Ans. The drawback of the serial adders is that serial adders are slower than parallel adders. They are preferred for applications where circuit minimization is more important than speed as in pocket calculators.

Q.18. Why serial adders are slower than parallel adders?

Ans. Serial adders are slower than parallel adders because they require one clock pulse for each pair of bits added.

Q.19. What are the differences between serial and parallel adders?

Ans. The parallel adder uses registers with parallel load, whereas the serial adder uses shift registers. The number of full-adder circuits in the parallel adder is equal to the number of bits in the binary numbers, whereas the serial adder requires only one full-adder circuit and a carry flip-flop. Excluding the registers, the parallel adder is a combinational circuit, whereas the serial adder is a sequential circuit. The sequential circuit in the serial adder consists of a fun-adder and a flip-flop that stores the output carry.

Q.20. In what way is a BCD adder different from a binary adder?

Ans. While adding BCD numbers, the output is required to be corrected which is not required in the case of binary adders.

Q.21. Compare the hardware requirements of a BCD arithmetic unit and a straight binary arithmetic unit.

Ans. Because of the need for a correction circuit, the BCD arithmetic unit requires more hardware than the straight binary arithmetic unit.

Q.22. Why is decimal 6 required to be added in a BCD adder if the sum is not a valid BCD number?

Ans. 16 possible combinations are there with 4-bit numbers. In BCD only 10 of these are used and the other 6 are skipped. That is why 6 is required to be added.

Q.23. What are code converters?

Ans. Code converters are logic circuits whose inputs are bit patterns representing numbers or characters in one code and whose outputs are the corresponding representations in a different code.

Q.24. What is a parity bit generator?

Ans. A parity bit generator is a digital circuit that generators a bit called the parity bit to be added to the data bits.

Q.25. What is a comparator?

Ans. A comparator is a logic circuit that compares the magnitudes of two binary numbers.

Q.26. Which logic gate is a basic comparator?

Ans. The X-NOR gate (coincidence gate) is a basic comparator.

Q.27. When are two binary numbers equal?

Ans. Two binary numbers are equal, if and only if all their corresponding bits coincide.

Q.28. What is an encoder?

Ans. An encoder is a device whose inputs are decimal digits and/or alphabetic characters and whose outputs are the coded representations of those inputs.

Q.29. What is encoding?

Ans. Encoding is the process of converting familiar numbers or symbols into a coded format.

Q.30. What is arbitration?

Ans. In some practical applications, priority encoders may have several inputs that are routinely high at the same time, and the principal function of the encoder in those cases is to select the input with the highest priority. This function is called arbitration.

Q.31. What is a priority encoder?

Ans. A priority encoder is a logic circuit that responds to just one input, in accordance with some priority system, among those that may be simultaneously high.

Q.32. The most common priority system is based on what?

Ans. The most common priority system is based on the relative magnitudes inputs: whichever decimal digit is the largest is the one encoded.

Q.33. What is a decoder?

Ans. A decoder is a logic circuit that converts an n-input binary code into a corresponding single numeric output code. In other words, a decoder is a device that identifies or recognizes, or detects a particular code.

Q.34. Why a binary-to-octal decoder is called a 1-of-8 decoder?

Ans. A binary-to-octal decoder is called a 1-of-8 decoder because only one of the eight outputs is activated at one time.

Q.35. What are enable inputs used in a decoder?

Ans. Enable inputs are used to control the operation of the decoder.

Q.36. What is a multiplexer (MUX)?

Ans. A multiplexer or data selector is a logic circuit that accepts several data inputs and allows only one of them at a time to get through to the outputs. It is an N:1 device.

Q.37. What is multiplexing?

Ans. Multiplexing means sharing. Selecting 1-out of-N input data sources and transmitting the selected data to a single output channel is called multiplexing.

Q.38. How many types of multiplexing are there? Name them.

Ans. There are two types of multiplexing. They are time-division multiplexing and frequency division multiplexing.

Q.39. Why is a multiplexer called a data selector?

Ans. A multiplexer is called a data selector because it accepts several data inputs and allows only one of them at a time to get through to the output.

Q.40. What are the applications of multiplexers?

Ans. Multiplexers are used for data selection, data routing, operation sequencing, parallel-to-serial conversion, waveform generation, logic function generation, etc.

Q.41. What is the minimum number of selection lines required for selecting one out of n input lines?

Ans. The minimum number of selection lines required for selecting one out of n input lines is m = log n/log 2.

Q.42. What is a demultiplexer (DMUX)?

Ans. A demultiplexer is a logic circuit that depending on the status of its select inputs, channels its data input to one of the several data outputs.

Q.43. Why is a demultiplexer called a distributor?

Ans. A demultiplexer can be thought of as a distributor since it takes a single input and distributes it over several outputs.

Q.44. Compare a decoder with a demultiplexer.

Ans. A demultiplexer has one data input, m select lines, and n output lines. A decoder, on the other hand, does not have the data input, but the select lines are used as input lines.

Q.45. What is the type of display used in calculators?

Ans. The type of display used in calculators is a 7-segment LED/LCD.

Q.46. Is the display used in a digital wristwatch LED or LCD? Why?

Ans. The display used in digital wristwatches is usually LCD. It is because LCD requires significantly less power than LED.

Q.47. What is a driver IC?

Ans. A driver IC is an IC whose outputs can operate with higher current and/or voltage limits than those of a normal standard IC.

Q.48. What do you mean by a module?

Ans. Modules are integrated circuits that are fabricated in solid-state form and are referred to as IC chips.

Q.49. What is modular design?

Ans. The design using ICs is called modular design.

Q.50. What are hazards? Why do they occur?

Ans. Hazards are unwanted switching transients. They may appear at the output of a circuit because different paths exhibit different propagation delays.

Q.51. What is a glitch?

Ans. A glitch or a spurious spike is a transient caused by the hazardous behavior of the logic circuit.

Q.52. What is a hazard in a combinational circuit?

Ans. A hazard in a combinational circuit is a condition where a single variable change produces a momentary output change when no output change should occur.

Q.53. Hazards are of how many types? Name them.

Ans. Hazards are of two types. They are:

  • Static hazards
  • Dynamic hazards 

Q.54. Static hazards are of how many types? Name them.

Ans. Static hazards are of two types. They are:

  • Static 1-hazard
  • Static 0-hazard

 Q.55. What is the static-1 hazard? What is the static-0 hazard?

Ans. When only one of the input variables of a circuit changes from 0 to 1 is 1 to 0 – if the output is expected to be at 1 regardless of the changing variable, the spurious 0 levels for a short interval is called a static 1 hazard. If the output excepted to be at 0 regardless of the changing variable the spurious 1 level for a short interval is called a static 0 hazard.

Q.56. How are static hazards eliminated?

Ans. Static hazards can be eliminated using redundant gates.

Q.57. What is the dynamic hazard? When do they occur?

Ans. The change in output three or more times when it should change from 1 to 0 or 0 to 1 or is called dynamic hazard. Dynamic hazards occur only in multi-level circuits. They occur the output changes for two adjacent input combinations.

Q.58. What is a tie set hazard?

Ans. With contact networks, the static-1 hazard is called a tie set hazard.

Q.59. What is cut set hazard?

Ans. With contact networks, the static-0 hazard is called a cut set hazard.

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