Why does a macro-dominated block need different area reasoning?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Macro dimensions and channels often determine the outline before standard-cell area does. Dividing total area by one utilisation target can hide geometric infeasibility.
Technical Explanation
Large fixed rectangles cannot be poured into whitespace like sand. Build candidate tilings from real macro dimensions, legal orientations, pin sides, and required channels; then fit usable row area around them.
Formula Or Decision Rule
Decision rule: satisfy macro geometry and access first, then compute effective standard-cell capacity in the remaining legal rows.
What To Check
• Warning sign: The arithmetic area is enough but no legal macro arrangement leaves continuous rows. • Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause. • Correct: Change aspect ratio, macro strategy, or hierarchy boundary; do not simply increase a percentage allowance.
Command Checks & Actions
• report_design -floorplan: reports the floorplan state understood by the tool
• check_design -checks {dp_pre_macro_placement}: runs named design-planning check groups
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.Healthy, Suspicious & Hard-stop Results
• Healthy: Macro dimensions and channels often determine the outline before standard-cell area does. Dividing total area by one utilisation target can hide geometric infeasibility. • Suspicious: The arithmetic area is enough but no legal macro arrangement leaves continuous rows. • Hard stop: required legality or physical feasibility is missing or unexplained. Change aspect ratio, macro strategy, or hierarchy boundary; do not simply increase a percentage allowance.
Common Mistake
Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change. The arithmetic area is enough but no legal macro arrangement leaves continuous rows.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “Why does a macro-dominated block need different area reasoning?” 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 arithmetic area is enough but no legal macro arrangement leaves continuous rows. I would use the named objects and reports to decide whether to iterate, waive, or return the issue.”
Practical Example
Symptom: The arithmetic area is enough but no legal macro arrangement leaves continuous rows. Corrective path: Change aspect ratio, macro strategy, or hierarchy boundary; do not simply increase a percentage allowance.
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