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SarkariResultOutBMS · 2024

Previous Year Question

For an edge-triggered D flip-flop,
  1. a change in the state of the flip-flop can occur only at a clock pulse edge
  2. the state that the flip-flop goes to depend on the D input
  3. the output follows the input at each clock pulse
  4. all of the above

Correct answer: D

Explanation

An edge-triggered D flip-flop samples D at the active clock edge and updates Q to that sampled value; between active edges it holds its previous state. Thus the statements describing edge-only change, D-dependent next state and edge sampling are all correct, making option D correct.

Formula

At active edge: Qnext=D

Step-by-step solution

Edge-Triggered Operation explanatory diagram

Correct Answer

D — all of the above

Concept and Reasoning

An edge-triggered D flip-flop samples D at the active clock edge and updates Q to that sampled value; between active edges it holds its previous state. Thus the statements describing edge-only change, D-dependent next state and edge sampling are all correct, making option D correct.

Calculation / Key Relation

At active edge: Qnext=D

Exam Takeaway

Sample D at the edge, update Q, then hold.

Common Mistake

Do not say the output continuously follows D; that describes a level-sensitive latch.

Common mistake

Do not say the output continuously follows D; that describes a level-sensitive latch.

Exam tip

Sample D at the edge, update Q, then hold.

Quick method

Sample D at the edge, update Q, then hold.