Two engineers report different results for the same floorplan. How do you isolate the cause?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Compare floorplan write-out, checkpoint, tool version, app options, scenarios, libraries, constraints, placement seed, PG/blockage assumptions, and report settings before comparing numbers.
Technical Explanation
“Same screenshot” does not mean same analysis state. Use provenance and one named object/path/hot spot to find the first divergence. Reproduce from a clean checkpoint.
Formula Or Decision Rule
Decision rule: do not choose an alternative until both runs share the same controlled experiment.
What To Check
• Warning sign: One result is accepted because it looks more reasonable. • Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause. • Correct: Align inputs and settings, rerun, and archive the reproducer.
Command Checks & Actions
• report_design -floorplan: reports the floorplan state understood by the tool
• report_congestion -mode summary: reports routing demand, capacity, overflow, or hot spots
• report_timing: reports timing paths in the active analysis context
• write_floorplan -output <output_dir>: writes reproducible floorplan information to files
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.Healthy, Suspicious & Hard-stop Results
• Healthy: Compare floorplan write-out, checkpoint, tool version, app options, scenarios, libraries, constraints, placement seed, PG/blockage assumptions, and report settings before comparing numbers. • Suspicious: One result is accepted because it looks more reasonable. • Hard stop: required legality or physical feasibility is missing or unexplained. Align inputs and settings, rerun, and archive the reproducer.
Common Mistake
Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change. One result is accepted because it looks more reasonable.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “Two engineers report different results for the same floorplan. How do you isolate the cause?” to measurable physical evidence and an owned correction.
Follow-up Question & Model Response
Follow-up probe: What would make you change your decision? Model response: “I would change it if the same controlled rerun shows that one result is accepted because it looks more reasonable. I would use the named objects and reports to decide whether to iterate, waive, or return the issue.”
Practical Example
Symptom: One result is accepted because it looks more reasonable. Corrective path: Align inputs and settings, rerun, and archive the reproducer.
Continue practising