Difficulty: Easy
Correct Answer: the D input is HIGH and the clock transitions from LOW to HIGH
Explanation:
Introduction / Context:
Edge-triggered flip-flops sample inputs only at a specific clock transition. For a positive edge-triggered device, the LOW-to-HIGH transition is the critical instant. Understanding this sampling instant is essential for meeting setup and hold time constraints and avoiding metastability.
Given Data / Assumptions:
Concept / Approach:
At the rising edge of the clock, the flip-flop captures the logic level present on D (if setup/hold requirements are satisfied). If D = 1 at that instant, Q becomes 1 after the device propagation delay and remains 1 until the next active edge (unless asynchronously overridden by preset/clear signals, if present).
Step-by-Step Solution:
Verification / Alternative check:
Timing diagrams show the sampling window centered on the rising edge, with setup/hold slacks specified. Simulations using HDL primitives confirm that Q follows D only on that edge, not during steady clock levels.
Why Other Options Are Wrong:
Common Pitfalls:
Violating setup/hold times, which may cause metastability or incorrect capture, and assuming level-sensitive behavior instead of edge-triggered.
Final Answer:
the D input is HIGH and the clock transitions from LOW to HIGH
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