Two equal-area aspect ratios disagree on timing and congestion. Which wins?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
" It's entirely normal for the two shapes to disagree in opposite directions: one aspect ratio shortens critical paths (better timing) while concentrating pins into a smaller region (worse congestion), and the other does the reverse.
Technical Explanation
- "
- It's entirely normal for the two shapes to disagree in opposite directions.
- The right method is a scorecard using identical definitions and identical measurement methods for both candidates — row usability, macro legality, channel widths, pin access, PG reservation, congestion, timing, and change risk, evaluated the same way for each.
- Pick the shape that best matches actual project priorities given that scorecard — a chip chasing max frequency weighs timing differently than one chasing area/cost, and that weighting should be explicit, not implicit.
- For each metric that differs between the two candidates, trace the physical cause of the delta (why is congestion worse — pin concentration? ) and assign a risk owner — a scorecard without causal explanation is just numbers without a decision.
Formula Or Decision Rule
Decision rule: no hard stop, then compare weighted project risks with evidence.
What To Check
- Warning sign: The choice is made from WNS alone or the prettiest outline.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Align the experiments, document tradeoffs, and choose or iterate with owners.
Command Checks & Actions
report_design-floorplan: reports the floorplan state understood by the toolreport_congestion-mode summary: reports routing demand, capacity, overflow, or hot spots- estimate_timing: performs early virtual timing estimation
report_timing: reports timing paths in the active analysis context
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.
Healthy, Suspicious & Hard-stop Results
- Healthy: Use a scorecard with identical definitions and methods. Choose the shape whose combined row usability, macro legality, channels, pin access, PG reservation, congestion, timing, and change risk best matches project priorities.
- 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 “Two equal-area aspect ratios disagree on timing and congestion. ” 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 choice is made from WNS alone or the prettiest outline. ”
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