How do you diagnose a row rail-orientation problem?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Compare row orientation and alternation with the standard-cell library’s legal orientations and power-rail convention.
Technical Explanation
A row direction that looks regular can still flip VDD and VSS incorrectly for the library. Check the first-row setting, alternate-row setting, multiple-height pattern, boundary cells, and voltage-area supplies together.
Formula Or Decision Rule
Decision rule: every legal cell orientation must connect to the intended rails in its assigned row.
What To Check
• Warning sign: Rows alternate by habit but violate the library rail pattern. • Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause. • Correct: Recreate the row pattern from library requirements and verify representative cells and boundary cells.
Command Checks & Actions
• create_site_array -name <array> -site <site> -boundary <boundary> -flip_first_row <true_or_false> -flip_alternate_row <true_or_false>: creates a bounded site array
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.Healthy, Suspicious & Hard-stop Results
• Healthy: Compare row orientation and alternation with the standard-cell library’s legal orientations and power-rail convention. • Suspicious: Rows alternate by habit but violate the library rail pattern. • Hard stop: required legality or physical feasibility is missing or unexplained. Recreate the row pattern from library requirements and verify representative cells and boundary cells.
Common Mistake
Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change. Rows alternate by habit but violate the library rail pattern.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “How do you diagnose a row rail-orientation problem?” 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 rows alternate by habit but violate the library rail pattern. I would use the named objects and reports to decide whether to iterate, waive, or return the issue.”
Practical Example
Symptom: Rows alternate by habit but violate the library rail pattern. Corrective path: Recreate the row pattern from library requirements and verify representative cells and boundary cells.
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