Positive edge-triggered D flip-flop capture A positive edge-triggered D flip-flop will store a logic 1 under which timing condition?

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:

  • Device: positive edge-triggered D flip-flop.
  • Input: D must be stable around the sampling edge.
  • Clock: the active edge is rising (LOW → HIGH).


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:

Ensure D = 1 during the setup interval before the rising edge.Apply a LOW-to-HIGH clock transition.Maintain D stable through the hold interval immediately after the edge.Observe Q transitioning to 1 after t_pd (propagation delay) and remaining latched until a later edge.


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:

  • HIGH-to-LOW: Describes negative-edge devices.
  • Clock LOW or HIGH (steady levels): Edge-triggered devices do not capture simply due to level; they sample on transitions.


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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