How do you debug rows created with the wrong site?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
A "site" defines the legal placement grid a row is built on — its width, height, and which cell classes can sit there. If the row's site doesn't match what your target cells expect, the tool will refuse to legalize them even though the row visually looks fine. Start narrow: pick one cell that's failing to legalize and one row it's supposed to sit in, then directly compare their site names — this is almost always where the mismatch is hiding, rather than some deeper floorplan issue.
Technical Explanation
- Check the full chain: row site name vs. library cell's site compatibility, row height vs. cell height, row orientation vs. cell orientation expectations, and whether the row sits in the right voltage area for that cell.
- g. a core site and a special I/O or memory-adjacent site) without explicitly telling the floorplan generation which site each row segment should use.
- Once you've found the root cause, the fix is almost always upstream — regenerate the row with the correct site via your floorplan script, not by hand-patching individual cell placements, because a hand patch just hides the same bug for the next row.
- " If your report doesn't say that directly, keep narrowing until it does.
Formula Or Decision Rule
Decision rule: the site used by each row must support the cells assigned to that row class.
What To Check
- Warning sign: Cells remain unplaceable or multiple-height cells have no compatible rows.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Recreate affected rows or site arrays from the approved library definitions, then recheck.
Command Checks & Actions
- create_site_row -name <row> -site <site> -origin {<x> <y>} -site_count <count>: creates one named row from a site and origin
report_design-floorplan: reports the floorplan state understood by the tool
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 site names, library cell site compatibility, row height/width, orientation, voltage area, and the intended row pattern.
- 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 cells remain unplaceable or multiple-height cells have no compatible rows. ”
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