How do you plan boundary cells around rows and voltage areas?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Boundary-cell planning means selecting, for each edge type your floorplan actually has, the correct library-approved cell: left end-cap, right end-cap, top/bottom boundary cells, inside-corner cells, and outside-corner cells. These cells are selected based on the actual row and voltage-area geometry you have — not a generic assumption. A rectangular core needs only 4 outside corners; a core with voltage areas carved out of it needs both outside corners (at the die boundary) and inside corners (at the voltage-area notches).
Technical Explanation
- Boundary cells finish off an edge electrically and physically (often terminating the diffusion/well structure correctly) — they are not a substitute for taps (which maintain well-tie spacing) or fillers (which just fill gaps); each has a distinct job.
- Before running automatic insertion, report/preview the applicable rules — row orientation at each location, which corner types are needed, and whether any macro boundaries also need boundary-cell treatment — so you catch a missing-cell-type problem before insertion rather than after.
- Complex floorplans (multiple voltage areas, macros embedded near the edge, notched cores) multiply the number of distinct corner/edge conditions you need to plan for — don't assume the same 4-corner pattern that works for a simple rectangular block will cover a more elaborate floorplan.
Formula Or Decision Rule
Decision rule: every exposed row or voltage-area edge has the correct library-specific boundary treatment.
What To Check
- Warning sign: Cells are missing at inside corners or inserted with a mismatched orientation.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Correct the rule or geometry, compile deliberately, and run the applicable boundary/floorplan checks.
Command Checks & Actions
- set_boundary_cell_rules -left_boundary_cell <lib_cell> -right_boundary_cell <lib_cell>: defines library-specific boundary-cell choices
- report_boundary_cell_rules: reports the active boundary-cell rules
- compile_boundary_cells -voltage_area <voltage_area>: inserts boundary cells using the active rules; this command is deprecated in the referenced release, so verify the project-approved targeted replacement
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.
Healthy, Suspicious & Hard-stop Results
- Healthy: Select library-approved left, right, top, bottom, inside-corner, and outside-corner cells for the actual row and voltage-area geometry, then report rules before insertion.
- 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 are missing at inside corners or inserted with a mismatched orientation. ”
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