How do you audit false paths without masking real timing?

Short Answer

A false path declaration is a powerful tool — and like any powerful tool, it's easy to misuse as a way to make an inconvenient timing violation disappear rather than to correctly describe a functionally impossible path. Start by checking exact membership: does the -from/-through/-to specification match precisely the path that's actually functionally impossible, or is it broader than intended and quietly swallowing real paths along with it?

Technical Explanation

• Check mode scope — is this false path declaration active in every mode, or only the mode where the path is genuinely unreachable? A false path that's true in test mode but not in functional mode needs to be scoped accordingly, not applied blanket.
• Check precedence against other exceptions on the same path — a false path can silently override a multicycle or other exception you intended to keep, depending on how SDC exception precedence resolves in your tool.
• Count how many paths the exception actually affects — if a set_false_path declaration you expected to hit one specific path is instead matching hundreds, that's a strong signal your selector is too broad.
• Most important: for every false path in the audit, be able to state the functional reason it can never happen — a tied-off mux select, a mode-exclusive signal, mutually exclusive control states. "It was violating timing" is never a valid reason on its own.

What To Check

• Warning sign: A wildcard false path removes unrelated synchronous paths and makes timing appear clean.
• Inspect: Compare one named object across the related reports; the same object should tell a consistent story.
• Correct: Narrow or remove the exception and validate representative paths with the STA/functional owner.

Healthy, Suspicious & Hard-stop Results

• Expected: Inspect exact from/through/to membership, mode scope, precedence, affected path count, and functional reason.
• 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. ”