Positive edge-triggered D flip-flop operation Which statement best describes how a positive edge-triggered D-type flip-flop transfers data from D to Q?
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AIf both inputs are HIGH, the output will toggle.
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BThe output will follow the input on the leading edge of the clock.
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CWhen both inputs are LOW, an invalid state exists.
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DThe input is captured on the leading edge and only passed to the output on the trailing edge.
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EThe output updates continuously while the clock is HIGH.
Answer
Correct Answer: The output will follow the input on the leading edge of the clock.
Explanation
Introduction / Context:Edge-triggered flip-flops are the building blocks of synchronous digital systems. A positive edge-triggered D flip-flop samples its data input D precisely at the rising edge of the clock and immediately updates its output Q accordingly, holding that value until the next active edge. This behavior supports precise timing and state sequencing in registers and pipelines.
Given Data / Assumptions:
- Positive edge-triggered topology (rising-edge sensitive).
- Setup and hold times around the clock edge are satisfied.
- Single data input D with complementary outputs Q and Q̄.
Concept / Approach:
Unlike level-sensitive latches, an edge-triggered D flip-flop is insensitive to D during the rest of the clock cycle (beyond setup/hold windows). At the instant of the rising edge, the value on D is captured and immediately appears at Q; it remains held until the next rising edge.
Step-by-Step Explanation:
Clock low → no change in Q, regardless of D (outside setup/hold).Approach the rising edge → D must be stable for setup/hold timing.At the rising edge → Q takes the current D value.Afterwards → Q holds that value until the next rising edge.Verification / Alternative check:
Datasheet timing diagrams show a vertical transfer at the rising edge. Simulation waveforms confirm Q updates only on the leading edge and stays constant otherwise.
Why Other Options Are Wrong:
- (a) Toggle behavior is characteristic of T or J–K devices under specific inputs, not D flip-flops.
- (c) There is no invalid “both inputs LOW” condition for a single-input D device.
- (d) Immediate update occurs at the rising edge; there is no delayed transfer to the trailing edge.
- (e) Continuous updating describes a transparent latch, not an edge-triggered flip-flop.
Common Pitfalls:
- Violating setup/hold times, which can cause metastability.
- Confusing level-sensitive latches with edge-triggered flip-flops in mixed-clock designs.
Final Answer:
The output will follow the input on the leading edge of the clock.