How do you audit false paths without masking real timing?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Inspect exact from/through/to membership, mode scope, precedence, affected path count, and functional reason.
Technical Explanation
A false path is functionally impossible and is deliberately removed from timing analysis. It is not a tool for hiding a path that is merely difficult to meet.
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.
Command Checks & Actions
• report_exceptions: shows timing exceptions and their scope
• report_exceptions -ignored: shows timing exceptions and their scope
Run the commands in order. Each line answers a separate part of the check.Healthy, Suspicious & Hard-stop Results
• Expected: Inspect exact from/through/to membership, mode scope, precedence, affected path count, and functional reason. • Investigate: A wildcard false path removes unrelated synchronous paths and makes timing appear clean. • Stop before floorplanning when required logic or timing coverage is missing or unexplained. Narrow or remove the exception and validate representative paths with the STA/functional owner.
Common Mistake
Do not assume the check passed just because ICC2 continued: A wildcard false path removes unrelated synchronous paths and makes timing appear clean. Fix or narrowly justify the named objects, then rerun the same command.
What The Interviewer Is Testing
Be ready to explain why this matters before floorplanning: A wildcard false path removes unrelated synchronous paths and makes timing appear clean.
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: Narrow or remove the exception and validate representative paths with the STA/functional owner. Then I would reopen the clean checkpoint and rerun the same evidence.”
Practical Example
Symptom: A wildcard false path removes unrelated synchronous paths and makes timing appear clean. Corrective path: Narrow or remove the exception and validate representative paths with the STA/functional owner.
Continue practising