Flip-flop timing integrity: What is a race condition in the context of an edge-triggered flip-flop and its input timing?

Difficulty: Medium

Correct Answer: The inputs to a trigger device are changing at the same time as the active trigger edge.

Explanation:


Introduction / Context:
A race condition in sequential logic occurs when signals that should be stable during the sampling event change too close to the active clock edge, potentially violating setup or hold requirements and causing unpredictable outputs or metastability.


Given Data / Assumptions:

  • Edge-triggered flip-flop with defined setup time ts and hold time th.
  • Inputs intended to be stable around the sampling edge.
  • Clock with finite transition time.


Concept / Approach:
When inputs transition during the aperture window around the clock edge, internal latches can capture inconsistent states, leading to raced or metastable outcomes. Ensuring ts and th margins avoids races.


Step-by-Step Solution:

1) Identify the aperture: from (edge − ts) to (edge + th).2) If input changes inside this window, the device may not resolve deterministically.3) Design practice: constrain combinational delays and synchronize asynchronous signals.4) Add proper timing constraints and CDC synchronizers where needed.


Verification / Alternative check:
Static timing analysis should show positive slack for ts and th; oscilloscope capture reveals glitches if violated.


Why Other Options Are Wrong:

  • Changing slightly before the edge: If it is earlier than ts, it is acceptable.
  • Changing slightly after the edge: If it is later than th, it is acceptable.
  • Static and unchanged: This is the desired condition, not a race.


Common Pitfalls:
Feeding asynchronous inputs directly to synchronous logic without synchronization, and neglecting clock skew while budgeting ts/th.


Final Answer:
The inputs to a trigger device are changing at the same time as the active trigger edge.

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