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?

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:

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