Positive edge-triggered S–R flip-flop — timing of state sampling On a positive edge-triggered S–R flip-flop, when do the Q and Q' outputs actually reflect (sample and latch) the S and R input condition?
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Athe clock pulse is LOW
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Bthe clock pulse is HIGH
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Cthe clock pulse transitions from LOW to HIGH
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Dthe clock pulse transitions from HIGH to LOW
Answer
Correct Answer: the clock pulse transitions from LOW to HIGH
Explanation
Introduction / Context:Edge-triggered flip-flops are fundamental memory elements in synchronous digital systems. Unlike level-sensitive latches, they sample their inputs only at a fleeting instant defined by the clock transition. Understanding precisely when a positive edge-triggered S–R (Set–Reset) flip-flop latches its inputs prevents race conditions, timing violations, and misinterpretation of waveforms during debugging.
Given Data / Assumptions:
- Device type: positive edge-triggered S–R flip-flop.
- Inputs: S (set) and R (reset), assumed synchronous and valid around the sampling instant.
- Outputs: Q and Q' (complement).
- Clock polarity: responds to the LOW-to-HIGH transition.
Concept / Approach:An edge-triggered device uses a pulse transition detector internally so that input sampling occurs only during an infinitesimal window at the clock's specified edge. For a positive edge-triggered device, that window is the rising edge (LOW→HIGH). Provided setup and hold constraints are met, the state after the edge remains stable until the next active clock edge, regardless of subsequent input chatter.
Step-by-Step Solution:
1) Identify the triggering characteristic: positive edge-triggered.2) Map this to a timing event: the active instant is the clock's LOW-to-HIGH transition.3) Determine behavior: outputs reflect S/R evaluated at that instant and then hold.4) Confirm that during HIGH or LOW levels (between edges), an ideal edge-triggered FF ignores input changes (subject to setup/hold timing).Verification / Alternative check:Examine a timing diagram: vary S and R safely before the rising edge (meeting setup time), keep them stable briefly after (meeting hold time), and observe Q updates exactly at the rising edge.
Why Other Options Are Wrong:
- Clock LOW or HIGH levels: those describe latch-like behavior, not edge-triggered sampling.
- HIGH-to-LOW transition: corresponds to a negative edge-triggered device, not a positive one.
Common Pitfalls:Violating setup/hold around the edge, assuming outputs track inputs while the clock is HIGH, or confusing edge-triggered flip-flops with level-sensitive latches.
Final Answer:the clock pulse transitions from LOW to HIGH