Why do row orientation and fragments matter?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Think of standard-cell rows like bookshelves: every row has to sit in a legal orientation so a cell's power rail lines up with the row's VDD/VSS rail — flip a shelf the wrong way and books (cells) that need to sit on it simply can't. Rows commonly alternate orientation (R0/MX) from one row to the next specifically so adjacent rows can share a power rail by abutment, but that alternating pattern is a library/methodology choice, not a universal law — always check what the target library actually supports.
Technical Explanation
- "Fragments" are the leftover slivers of a row — often created near macros or blockages — and while they still count as legal row area on paper, they may be too short or too oddly shaped for any real cell to occupy, so they're bookkeeping area, not usable capacity.
- When auditing a floorplan, check the first row's orientation choice, confirm the alternating pattern is consistent, verify multi-height cell compatibility across rows, and specifically inspect fragments near macros and voltage-area boundaries — that's where wasted, unusable row area hides.
Visual Verification
Alternating row orientations (R0 and MX) allow adjacent cell rows to share continuous VDD and VSS power rails by abutment, maximizing active placement area.
Formula Or Decision Rule
Decision rule: keep only row segments that can legally accept intended cells and connect to the required supplies.
What To Check
- Warning sign: Rows flip incorrectly, extend through macros, or leave slivers too short for useful placement.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Recreate or trim rows using the approved site and row pattern; never assume one universal flip rule.
Command Checks & Actions
- create_site_array -name <array> -site <site> -boundary <boundary> -flip_alternate_row <true_or_false>: creates a bounded site array
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: Row orientation must support the library’s legal cell orientations and power-rail alignment. Tiny row fragments consume bookkeeping space but may be unusable for real cells.
- 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.
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