Logic circuits

Check the basics of logic circuits — logic gates, De Morgan's laws and flip-flops — with multiple-choice questions.

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Logic circuits: key points

Digital circuits are built from basic gates such as AND, OR and NOT, and their combinations such as NAND, NOR and XOR. Truth tables, Boolean algebra and De Morgan's laws let you simplify logic expressions and replace gates with others.

Also understand the difference between combinational circuits, whose outputs depend only on their inputs, and sequential circuits, which hold state in elements such as flip-flops.

All Logic circuits questions (50)

  1. When is the output of a two-input NAND gate 0?
  2. According to De Morgan's laws, which expression is equal to NOT(A AND B)?
  3. Which correctly describes the operation of a D flip-flop?
  4. When is the output of a two-input XOR (exclusive OR) gate 1?
  5. What is the binary number 1011 in decimal?
  6. Which correctly describes a sequential circuit?
  7. In a 1-bit half adder, which logic produces the sum (S) and carry (C)?
  8. What is the hexadecimal number FF in decimal?
  9. Which bit pattern represents −1 in 4-bit two's complement?
  10. Which correctly describes the function of a multiplexer?
  11. What does a two-input NAND gate do when its two inputs are tied together to form a single input?
  12. What is the main use of a Karnaugh map?
  13. Which correctly describes the setup time of a flip-flop?
  14. How many states can a 4-bit binary counter represent?
  15. Which is a characteristic of Gray code?
  16. Which correctly describes the operation of a D latch?
  17. What is the state called when a flip-flop receiving an asynchronous signal violates setup or hold time and its output is temporarily neither 0 nor 1?
  18. Which type of sequential circuit (state machine) has outputs determined only by the current state?
  19. Which represents decimal 59 in BCD (binary-coded decimal)?
  20. When is the output of a three-input majority circuit 1?
  21. Which correctly lists the inputs of a full adder?
  22. Which circuit activates exactly one of eight outputs according to a 3-bit input?
  23. What is a typical use of a shift register, which chains flip-flops in series and moves data one bit to the next on each clock?
  24. What happens to the value of a binary number when it is shifted left by one bit (assuming no overflow)?
  25. When an even parity bit is added to the 7-bit data 1011001, what is the parity bit?
  26. When is the output of a two-input NOR gate 1?
  27. What is the simplified result of the expression A + A·B?
  28. What is an advantage of a synchronous circuit, in which all flip-flops operate on a common clock?
  29. In a synchronous circuit, what determines the maximum clock frequency at which it can operate?
  30. In a combinational circuit, what is the unwanted pulse that appears briefly at the output when an input changes, due to different delays along different paths?
  31. When is the output of a 2-input AND gate 1?
  32. When is the output of a 2-input OR gate 0?
  33. Which correctly describes what a NOT gate (inverter) does?
  34. Which is the decimal number 13 in binary?
  35. How many rows does the truth table of a 3-input logic circuit have?
  36. How many bits are in one byte?
  37. What is the hexadecimal number 1A in decimal?
  38. What does a JK flip-flop do when a clock edge arrives with J = 1 and K = 1?
  39. A T flip-flop whose output toggles on every clock edge produces an output at what multiple of the clock frequency?
  40. Which circuit outputs, in binary, the number of the highest-priority input when several inputs are 1 at the same time?
  41. What is the drawback of a ripple-carry adder made by chaining full adders?
  42. Which correctly describes the hold time of a flip-flop?
  43. Which correctly describes clock skew in a synchronous circuit?
  44. What is the third state that the output of a tri-state buffer can take?
  45. What range of integers can be represented in 8-bit two's complement?
  46. Why is a carry-lookahead adder fast?
  47. When adding two's complement numbers, which condition indicates that an overflow has occurred?
  48. How many states does a Johnson (twisted-ring) counter made of n flip-flops go through?
  49. What is a typical use of a linear feedback shift register (LFSR), which feeds the XOR of some outputs of a shift register back to its input?
  50. When several open-drain outputs are connected to one signal line with a pull-up resistor, how does the line behave?

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