A macro channel routes before PG reservation but fails after straps are planned. What is the correct fix?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
The original channel capacity estimate was incomplete. Recalculate usable signal tracks after PG widths, spacing, vias, and blockages, then negotiate macro spacing, strap placement, pin access, or layer allocation.
Technical Explanation
The channel did not “become bad”; the resource model became honest. Compare demand by direction and layer before and after reservation. Check whether facing pins, shielding, and vias leave alternative layers.
Formula Or Decision Rule
Decision rule: both power feasibility and signal capacity must pass in the same geometry.
What To Check
• Warning sign: The team removes PG reservation from the congestion run. • Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause. • Correct: Keep the reservation, revise competing resources, and rerun the same map.
Command Checks & Actions
• check_design -checks {dp_pre_power_insertion}: runs named design-planning check groups
• create_channel_congestion_map -boundary <coordinates>: estimates congestion in channel regions
• report_congestion -mode summary: reports routing demand, capacity, overflow, or hot spots
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.Healthy, Suspicious & Hard-stop Results
• Healthy: The original channel capacity estimate was incomplete. Recalculate usable signal tracks after PG widths, spacing, vias, and blockages, then negotiate macro spacing, strap placement, pin access, or layer allocation. • Suspicious: The team removes PG reservation from the congestion run. • Hard stop: required legality or physical feasibility is missing or unexplained. Keep the reservation, revise competing resources, and rerun the same map.
Common Mistake
Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change. The team removes PG reservation from the congestion run.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “A macro channel routes before PG reservation but fails after straps are planned. What is the correct fix?” 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 team removes PG reservation from the congestion run. I would use the named objects and reports to decide whether to iterate, waive, or return the issue.”
Practical Example
Symptom: The team removes PG reservation from the congestion run. Corrective path: Keep the reservation, revise competing resources, and rerun the same map.
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