Why is zero reported timing violations insufficient for readiness?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Zero violations only tells you the paths that were actually analyzed, in the scenarios that were actually active, came back clean — it says nothing about paths the tool never looked at. A clean report can hide missing clock definitions, exceptions that were never written (so paths are silently over- or under-constrained), tied-off constants that eliminate paths from analysis, inactive modes that weren't checked, or entire absent paths that should exist but don't.
Technical Explanation
- This is the STA equivalent of "the test suite passed" when half your code paths have no tests — passing coverage you have is meaningless without knowing what coverage you don't have.
- Before trusting a "zero violations" result, actively verify scenario/mode coverage, clock completeness, and exception completeness — the same readiness-check discipline used elsewhere in floorplan/placement handoff verification.
- The practical danger is that "zero violations" reads as reassuring to non-STA stakeholders, so it's worth explicitly communicating what was and wasn't covered whenever you report it.
What To Check
- Warning sign: WNS is positive while coverage reports show missing or removed analysis.
- Inspect: Keep at least two possible causes open, then use one named object and the active mode or scenario to separate them.
- Correct: Use coverage and intent reports as the gate, restore missing analysis, and only then interpret slack.
Command Checks & Actions
check_timing: finds missing or inconsistent timing setup- report_scenarios: shows the MCMM scenario matrix and analysis status
- report_clocks: shows clock definitions and relationships
- report_exceptions: shows timing exceptions and their scope
report_case_analysis: shows constants applied for the active mode
Run the commands in order. Each line answers a separate part of the check.
Healthy, Suspicious & Hard-stop Results
- Expected: Slack covers only analysed paths in active scenarios; missing clocks, exceptions, constants, inactive modes, or absent paths can make the result artificially clean.
- 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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