BEE Bachelor of Electrical Engineering Digital Logic and Design Questions and Answers — Questions and Answers
Question 1: A digital circuit's output is dependent only on the present combination of its inputs. Which category does this circuit belong to?
- Sequential logic circuit
- Combinational logic circuit (Correct answer)
- Asynchronous circuit
- Finite state machine
Correct answer: Combinational logic circuit
Combinational logic circuits are defined by the property that their outputs at any given time are a function of only their current inputs. They do not have memory elements to store past states.
Question 2: An engineer is using a Karnaugh map (K-map) to simplify a 4-variable Boolean expression. What is the maximum number of adjacent '1's that can be grouped together to form a valid simplification term?
- 6
- 10
- 12
- 16 (Correct answer)
Correct answer: 16
In a K-map, groups must consist of a number of cells that is a power of two (1, 2, 4, 8, 16, etc.). For a 4-variable K-map, there are 2^4 = 16 cells. Therefore, the largest possible group would encompass all 16 cells, representing a constant function (either 1 or 0).
Question 3: In an asynchronous sequential circuit, a critical race condition occurs when:
- Two or more state variables change simultaneously but the final state is the same regardless of the order.
- A single state variable changes value too slowly due to propagation delay.
- The order in which multiple state variables change determines a different final stable state. (Correct answer)
- The circuit enters a cycle of unstable states before reaching a final stable state.
Correct answer: The order in which multiple state variables change determines a different final stable state.
A critical race condition is a hazardous situation in asynchronous circuits where two or more state variables change at the same time in response to an input change, and the final stable state the circuit settles into depends on the unpredictable order in which these variables change. This can lead to erroneous operation and must be avoided in design.
Question 4: Which of the following digital components is also known as a 'data selector' because it uses control inputs to route one of several data inputs to a single output?
- Decoder
- Demultiplexer
- Multiplexer (Correct answer)
- Encoder
Correct answer: Multiplexer
A multiplexer (MUX) is a combinational circuit that selects one of several input lines and directs it to a single output line. The selection of which input line is connected to the output is controlled by a set of select lines, hence it is often called a data selector.
Question 5: A designer needs to create a circuit that sequences through a specific series of states, but the transitions between states should only occur on the rising edge of a clock signal. Which of the following is the most fundamental building block for this type of circuit?
- An AND gate
- A flip-flop (Correct answer)
- A comparator
- A multiplexer
Correct answer: A flip-flop
Sequential circuits, which change state based on a clock signal, use memory elements to store their current state. The fundamental building block for clocked sequential circuits is the flip-flop, which is a bistable multivibrator that can store one bit of information and change its state only at a specific edge of a clock pulse.
Question 6: When representing a finite state machine (FSM), which tool provides a more visual and intuitive representation of states and the transitions between them?
- Truth Table
- Boolean Expression
- State Table
- State Diagram (Correct answer)
Correct answer: State Diagram
A state diagram is a graphical representation of a finite state machine. It uses circles or nodes to represent states and directed arrows to represent transitions between states, labeled with the input/output conditions that cause the transition. This visual format is generally more intuitive for understanding the behavior of an FSM than a tabular representation like a state table.
BEE Bachelor of Electrical Engineering Digital Logic and Design Questions and Answers
A digital circuit's output is dependent only on the present combination of its inputs.
Which category does this circuit belong to?