BEE Digital Circuits 2 — Questions and Answers
Question 1: A JK flip-flop with J=1 and K=1 is in the SET state. After one clock pulse, what is the output?
- Remains SET
- Goes to RESET
- Toggles to RESET (Correct answer)
- Goes to invalid state
Correct answer: Toggles to RESET
When J=K=1, the JK flip-flop toggles its output on each clock pulse, so it moves from SET (Q=1) to RESET (Q=0).
Question 2: Which hazard occurs in combinational circuits when a variable and its complement arrive at a gate at different times?
- Race condition
- Static hazard (Correct answer)
- Dynamic hazard
- Glitch propagation
Correct answer: Static hazard
A static hazard is a momentary unwanted output transition caused by unequal propagation delays of a signal and its complement.
Question 3: What is the minimum number of NAND gates required to implement a 2-input XOR function?
- 3
- 4 (Correct answer)
- 5
- 6
Correct answer: 4
A 2-input XOR gate can be built using 4 two-input NAND gates with the standard NAND-only XOR topology.
Question 4: In a ripple counter with 4 flip-flops, what is the maximum count before the counter resets to zero?
- 8
- 15 (Correct answer)
- 16
- 31
Correct answer: 15
A 4-bit ripple counter counts from 0 to 2⁴−1 = 15 before rolling over to 0.
Question 5: Which logic family has the highest noise margin compared to its supply voltage?
- TTL
- ECL
- CMOS (Correct answer)
- DTL
Correct answer: CMOS
CMOS has noise margins of approximately 30–45% of VDD on each rail, giving it superior noise immunity relative to supply voltage.
Question 6: A full adder has inputs A=1, B=1, Cin=1. What are the Sum and Cout outputs?
- Sum=0, Cout=0
- Sum=1, Cout=1 (Correct answer)
- Sum=0, Cout=1
- Sum=1, Cout=0
Correct answer: Sum=1, Cout=1
1+1+1 = 3 in decimal = 11 in binary, so Sum=1 and Cout=1.
Question 7: Which statement best describes a tri-state buffer?
- It has three voltage levels: low, high, and mid
- It can drive a bus to 0, 1, or disconnect (high-Z) (Correct answer)
- It is a buffer with three inputs
- It implements a 3-variable Boolean function
Correct answer: It can drive a bus to 0, 1, or disconnect (high-Z)
A tri-state buffer can output logic 0, logic 1, or present a high-impedance (high-Z) state, allowing multiple drivers to share a bus.
BEE Digital Circuits 2
A JK flip-flop with J=1 and K=1 is in the SET state.
After one clock pulse, what is the output?