How should pre-floorplan clock uncertainty be reviewed?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Separate jitter, modelling margin, and setup/hold uncertainty according to methodology; ensure it is not duplicated by other margins.
Technical Explanation
Clock uncertainty reserves time for jitter and modelling margin. It should be traceable to the methodology rather than chosen simply to make a report look conservative.
What To Check
• Warning sign: A large generic value hides weak setup or creates meaningless hold pessimism. • Inspect: Compare one named object across the related reports; the same object should tell a consistent story. • Correct: Apply approved per-mode/per-check values and document their source.
Command Checks & Actions
• report_clocks: shows clock definitions and relationships
• report_clock_settings: shows latency, transition, uncertainty, and clock-analysis settings
Run the commands in order. Each line answers a separate part of the check.Healthy, Suspicious & Hard-stop Results
• Expected: Separate jitter, modelling margin, and setup/hold uncertainty according to methodology; ensure it is not duplicated by other margins. • Investigate: A large generic value hides weak setup or creates meaningless hold pessimism. • Stop before floorplanning when required logic or timing coverage is missing or unexplained. Apply approved per-mode/per-check values and document their source.
Common Mistake
Do not assume the check passed just because ICC2 continued: A large generic value hides weak setup or creates meaningless hold pessimism. 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 large generic value hides weak setup or creates meaningless hold pessimism.
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: Apply approved per-mode/per-check values and document their source. Then I would reopen the clean checkpoint and rerun the same evidence.”
Practical Example
Symptom: A large generic value hides weak setup or creates meaningless hold pessimism. Corrective path: Apply approved per-mode/per-check values and document their source.
Continue practising