All floorplan legality checks pass, but congestion is unacceptable. What do you do?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Treat legality and quality separately. Keep the legal baseline, identify the hot spot’s demand and capacity causes, change the smallest responsible physical choice, and rerun the same estimate.
Technical Explanation
A legal street map can still contain an impossible traffic junction. Hypotheses include macro pin walls, narrow channels, local cell density, PG reservation, blockages, bus concentration, or unavailable layers. Use the map and named nets to discriminate.
Formula Or Decision Rule
Decision rule: iterate until local capacity is credible; a legality pass is not a waiver for QoR risk.
What To Check
• Warning sign: The review stops because check_floorplan_rules is clean. • Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause. • Correct: Inspect hot spots and layers, assign the owning macro/pin/PG decision, repair, and compare before/after.
Command Checks & Actions
• check_floorplan_rules: reports configured floorplan-rule violations
• report_congestion -mode hot_spot: reports routing demand, capacity, overflow, or hot spots
• create_channel_congestion_map -congestion_map_only true: estimates congestion in channel regions
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.Healthy, Suspicious & Hard-stop Results
• Healthy: Treat legality and quality separately. Keep the legal baseline, identify the hot spot’s demand and capacity causes, change the smallest responsible physical choice, and rerun the same estimate. • Suspicious: The review stops because check_floorplan_rules is clean. • Hard stop: required legality or physical feasibility is missing or unexplained. Inspect hot spots and layers, assign the owning macro/pin/PG decision, repair, and compare before/after.
Common Mistake
Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change. The review stops because check_floorplan_rules is clean.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “All floorplan legality checks pass, but congestion is unacceptable. What do you do?” 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 the review stops because check_floorplan_rules is clean. I would use the named objects and reports to decide whether to iterate, waive, or return the issue.”
Practical Example
Symptom: The review stops because check_floorplan_rules is clean. Corrective path: Inspect hot spots and layers, assign the owning macro/pin/PG decision, repair, and compare before/after.
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