When should macros be edge-biased versus placed internally?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Edge placement can simplify external access and preserve central rows; internal placement can shorten central dataflow but creates more channel and blockage interactions.
Technical Explanation
The better strategy depends on where pins, logic, and routes need to go. Compare macro pin sides, I/O affinity, timing paths, row continuity, PG access, and escape capacity. Avoid turning the entire core boundary into a pin wall.
Formula Or Decision Rule
Decision rule: choose the strategy with the stronger combined timing, routability, and change-risk evidence.
What To Check
• Warning sign: All macros are pushed to edges by habit, creating long internal buses or corner congestion. • Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause. • Correct: Test an alternative with the same area and measurement method, then document the winning tradeoff.
Command Checks & Actions
• set_macro_constraints -style <on_edge_or_freeform> <macro_collection>: records allowed macro or I/O placement constraints
• create_placement -floorplan -congestion: runs early floorplan placement when -floorplan is used
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.Healthy, Suspicious & Hard-stop Results
• Healthy: Edge placement can simplify external access and preserve central rows; internal placement can shorten central dataflow but creates more channel and blockage interactions. • Suspicious: All macros are pushed to edges by habit, creating long internal buses or corner congestion. • Hard stop: required legality or physical feasibility is missing or unexplained. Test an alternative with the same area and measurement method, then document the winning tradeoff.
Common Mistake
Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change. All macros are pushed to edges by habit, creating long internal buses or corner congestion.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “When should macros be edge-biased versus placed internally?” 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 all macros are pushed to edges by habit, creating long internal buses or corner congestion. I would use the named objects and reports to decide whether to iterate, waive, or return the issue.”
Practical Example
Symptom: All macros are pushed to edges by habit, creating long internal buses or corner congestion. Corrective path: Test an alternative with the same area and measurement method, then document the winning tradeoff.
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