Setup improves after a multicycle exception but hold becomes strange. Why?

Short Answer

This is one of the classic SDC traps: declaring a multicycle path fixes the setup check you were chasing, but if you don't also derive the matching hold relationship, hold checking silently goes wrong on the same path. A multicycle path is one that's functionally allowed more than one clock cycle to complete — but set_multicycle_path by default only shifts the setup check; the hold check still assumes the original single-cycle relationship unless you explicitly tell it otherwise with the hold-side multicycle specification.

Technical Explanation

• The root cause is treating setup and hold as independent knobs when they're actually two views of the same launch/capture edge relationship — shift one without deriving the other from the real protocol, and you get an internally inconsistent set of checks.
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• This is a good example of why SDC exceptions need to be justified from real functional intent, not tuned reactively against whatever violation is currently showing.

What To Check

• Warning sign: A copied multicycle number changes min timing unexpectedly.
• Inspect: Keep at least two possible causes open, then use one named object and the active mode or scenario to separate them.
• Correct: Re-derive setup and hold intent from the protocol, correct both constraints, and inspect full clock-expanded min and max paths.

Healthy, Suspicious & Hard-stop Results

• Expected: The functional setup-cycle shift and corresponding hold relationship were not derived consistently from launch/capture edges.
• Stop before floorplanning when required logic or timing coverage is missing or unexplained.

What The Interviewer Is Testing

Be ready to explain why this matters before floorplanning.

Follow-up Question & Model Response

Model response: “I would save the report, inspect one affected object in the correct block and scenario, and make or request this correction. ”