How do sites, rows, and routing tracks differ?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
" A site is the smallest legal placement increment — think of it as the grid unit that every standard-cell origin must snap to.
Technical Explanation
- "
- A site is the smallest legal placement increment — think of it as the grid unit that every standard-cell origin must snap to.
- A row is a horizontal strip built from repeated sites, sized and oriented to match the standard-cell library's height and power-rail requirements — this is literally where cells get placed.
- A routing track is a completely separate concept living in the technology/layer definitions — it defines candidate wire centerlines based on layer pitch, offset, preferred direction, and width, and has nothing to do with cell placement legality.
- The key trap to avoid: a row is not a track, and being aligned with one doesn't say anything about the other. A cell can sit perfectly legally in a row while its pins land off any usable routing track.
- Rows depend on the standard-cell library's site and rail conventions; tracks depend entirely on the technology file's per-layer pitch, offset, and direction — two independent grids that both have to be satisfied.
Visual Verification
Placement sites define discrete transistor width steps, rows define legal vertical standard-cell bounding boxes, and metal tracks define routing centerlines on interconnect layers.
Formula Or Decision Rule
Decision rule: cell origins must satisfy the site/row system while pins and wires need legal access to routing tracks.
What To Check
- Warning sign: Rows exist but use the wrong site, or pins touch the boundary without usable tracks.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Correct the row/site or track definition at its source and rerun floorplan and pin checks.
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
- create_track -layer <layer> -space <pitch> -coord <offset> -dir <X_or_Y>: creates a routing-track pattern
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.
Healthy, Suspicious & Hard-stop Results
- Healthy: A site defines legal placement increments, a row repeats sites for standard cells, and a routing track defines candidate wire centre lines. A row is not a track.
- 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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