A critical path detours around a macro despite acceptable centre-to-centre distance. What should you inspect?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Inspect actual macro pin sides, route corridor, blockages, voltage areas, feedthrough options, hierarchy crossings, congestion, and buffering sites. Centre distance can hide a long legal path.
Technical Explanation
Timing follows routable geometry, not the ruler line between macro centres. Compare full estimated paths and the floorplan route opportunity in both alternatives with identical constraints.
Formula Or Decision Rule
Decision rule: reduce the legal route and preserve access, not merely the geometric distance.
What To Check
• Warning sign: The macro is moved closer while its pins still face away. • Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause. • Correct: Reorient or reposition it, open the correct corridor or feedthrough, and rerun timing plus 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 context
• route_global -floorplan true -congestion_map_only true: runs an early floorplan global-route estimate
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.Healthy, Suspicious & Hard-stop Results
• Healthy: Inspect actual macro pin sides, route corridor, blockages, voltage areas, feedthrough options, hierarchy crossings, congestion, and buffering sites. Centre distance can hide a long legal path. • Suspicious: The macro is moved closer while its pins still face away. • Hard stop: required legality or physical feasibility is missing or unexplained. Reorient or reposition it, open the correct corridor or feedthrough, and rerun timing plus congestion.
Common Mistake
Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change. The macro is moved closer while its pins still face away.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “A critical path detours around a macro despite acceptable centre-to-centre distance. What should you inspect?” 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 macro is moved closer while its pins still face away. I would use the named objects and reports to decide whether to iterate, waive, or return the issue.”
Practical Example
Symptom: The macro is moved closer while its pins still face away. Corrective path: Reorient or reposition it, open the correct corridor or feedthrough, and rerun timing plus congestion.
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