How do you diagnose a row rail-orientation problem?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Row orientation can look perfectly regular visually — alternating up/down like it should — and still be wrong for the library, because the thing that actually matters is whether VDD/VSS land on the correct rail for each row, not just whether the pattern alternates. A standard-cell library has a fixed power-rail convention baked into every cell (which rail is VDD on an "N" orientation vs. a flipped "FS" row); if the floorplan's row settings don't match that convention, cells will short or float their supply on abutment.
Technical Explanation
- Diagnosis means checking the first-row setting and the alternate-row setting together against the library's convention, not just eyeballing the row pattern.
- Multiple-height cell rows add another failure mode — a taller cell spanning several rows needs every one of those rows correctly oriented, or its internal rail stitching breaks.
- Boundary cells and voltage-area supply connections need to be checked in the same pass — a rail flip at a voltage-area edge is a classic place for this bug to hide.
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.
- 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
” 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 rows alternate by habit but violate the library rail pattern. ”
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