Early timing improves while congestion worsens. How do you choose the next move?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Last updated:
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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