Timing becomes clean after a broad false path. What is the correct response?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
" Clean timing alone doesn't tell you which. The correct response is to treat the improvement as unproven until you've verified exact path membership — pull up precisely which paths the false-path constraint removed and check each one individually, not just trust the wildcard pattern you used.
Technical Explanation
- " Clean timing alone doesn't tell you which.
- g. a tied-off mode-select signal that structurally disables one branch of a MUX)? If you can't point to specific control logic and specific disabled arcs, the false path isn't justified yet.
- A dangerously common mistake is a false-path constraint written broadly enough (an overly wide wildcard on cell/pin names) to catch paths well beyond the one specific interface it was meant to cover — this silently removes real, functionally reachable paths from analysis.
- Bottom line: "timing is clean" after a broad false path is not evidence of a good fix — it's a prompt to go verify membership and functional justification before you trust the result, because the tool did exactly what you told it to, whether or not that was what you meant.
What To Check
- Warning sign: WNS improves dramatically and path counts collapse after a wildcard exception.
- Inspect: Keep at least two possible causes open, then use one named object and the active mode or scenario to separate them.
- Correct: Report the exception and ignored/dominant interactions, narrow it with the STA owner, and restore analysis of valid paths.
Command Checks & Actions
- report_exceptions: shows timing exceptions and their scope
- report_exceptions -ignored: shows timing exceptions and their scope
report_timing: shows a path and its timing calculation
Run the commands in order. Each line answers a separate part of the check.
Healthy, Suspicious & Hard-stop Results
- Expected: Assume the improvement is unproven until exact membership and functional justification show that only impossible paths were removed.
- Stop before floorplanning when required logic or timing coverage is missing or unexplained.
Common Mistake
The Trap: Do not assume the check passed just because ICC2 continued. 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.
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. ”
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