Early timing improves while congestion worsens. How do you choose the next move?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Identify which named paths improved and which local resources failed, then test a change that preserves the timing benefit while relieving concentrated demand.
Technical Explanation
- The fundamental physical design tradeoff between timing closure and routing congestion is driven by conflicting placement and sizing objectives.
- Optimization Mechanism: Timing engines (
place_opt) aggressively up-size cell drive strengths, clone logic, and insert repeaters to satisfy setup timing budgets (WNS >= 0). - Congestion Side-Effect: High-drive multi-finger cells and dense buffer clusters concentrate massive pin counts into localized areas, consuming all available lower-metal routing tracks (M1 to M3) for pin access.
- The False Sign-off Trap: Selecting a floorplan or placement seed purely on lowest pre-route WNS while ignoring localized global routing overflow (>100% demand) causes catastrophic post-route design rule failures, timing degradation from cross-talk delta delays, and unfixable DRC shorts.
- Triage & Recovery: Evaluate global routing congestion heatmaps alongside timing reports. Apply selective placement cell padding (
set_placement_spacing_label) on high-pin-density cells, deploy partial placement blockages (40% to 50%) in narrow macro channels, and accept minor pre-route setup slack trades to preserve physical routability.
Formula Or Decision Rule
Decision rule: close hard routing infeasibility first while protecting critical timing through measured alternatives.
What To Check
- Warning sign: The team picks the lower WNS and ignores overflow.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Adjust orientation, channel, pins, or nearby macro position and rerun both timing and congestion.
Command Checks & Actions
- estimate_timing: performs early virtual timing estimation
report_timing-path_type full_clock_expanded: reports timing paths in the active analysis contextreport_congestion-mode hot_spot: 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
- Hard stop: required legality or physical feasibility is missing or unexplained.
Common Mistake
The Trap: Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “Early timing improves while congestion worsens. ” to measurable physical evidence and an owned correction.
Follow-up Question & Model Response
"* Candidate Model Response: Model response: “I would change it if the same controlled rerun shows that the team picks the lower WNS and ignores overflow. ”
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