J-K versus S-R flip-flop: What is the key advantage of the J-K flip-flop when compared to the S-R latch/flip-flop?
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AThe J-K flip-flop is much faster.
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BThe J-K flip-flop does not have propagation delay problems.
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CThe J-K flip-flop has a toggle state.
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DThe J-K flip-flop has two outputs.
Answer
Correct Answer: The J-K flip-flop has a toggle state.
Explanation
Introduction / Context:The S-R latch has an invalid input condition when both S and R are HIGH simultaneously. The J-K flip-flop extends this behavior to allow a defined toggle action when both inputs are asserted, making it more versatile.
Given Data / Assumptions:
- J-K is edge-triggered in most logic families.
- Inputs J = 1 and K = 1 cause toggling on each active clock edge.
- S-R with S = R = 1 is invalid or forbidden.
Concept / Approach:The toggle state enables divide-by-2 counters and flexible state machines without additional gating to avoid illegal states, simplifying designs.
Step-by-Step Solution:
1) Map inputs: J=1, K=0 → set; J=0, K=1 → reset; J=K=0 → hold; J=K=1 → toggle.2) Compare to S-R: S=R=1 is undefined; hardware must prevent it.3) Conclude advantage: J-K provides a clean, useful toggle mode.Verification / Alternative check:Construct a divide-by-2 using a single J-K with J=K=1; observe Q toggling each clock.
Why Other Options Are Wrong:
- Much faster: Speed depends on implementation, not a fundamental advantage.
- No propagation delay problems: All real devices have propagation delays.
- Two outputs: Many flip-flops provide Q and not-Q; this is not unique to J-K.
Common Pitfalls:Assuming J-K eliminates all illegal transitions automatically; race-around must be mitigated by edge triggering or master-slave design.
Final Answer:The J-K flip-flop has a toggle state.