What are boundary, end-cap, and corner cells?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
) so the row doesn't end with an invalid or DRC-violating structure. Inside/outside corner cells handle the actual corners of the core or of a voltage area — an outside corner (convex, like a normal rectangle's corner) and an inside corner (concave, like the corner of an L-shaped notch) need different cells because the geometry they're closing off is different.
Technical Explanation
- I/O-ring corner cells are a completely separate family — they belong to the pad ring around the outside of the chip, not to the core boundary, and follow I/O-guide placement rules rather than core row rules.
- Even though all three sit "at a boundary," think of them as closing three different physical structures: a row edge, a core/voltage-area corner, and a pad-ring corner — mixing them up is a category error, not just a cosmetic naming issue.
- None of these substitute for tap cells (well-tie spacing), filler cells (gap filling), or decap/well-tie cells — each of those does a different job and boundary/corner/end-cap cells don't replace them.
Visual Verification
Endcap cells terminate standard-cell rows to prevent N-well and poly oxide diffusion DRC violations, while corner cells ensure continuous power ring continuity at core corners.
Formula Or Decision Rule
Decision rule: use the library-specific cell family for the boundary it was designed to complete.
What To Check
- Warning sign: A corner is empty, uses the wrong orientation, or mixes core and I/O corner concepts.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Set and report the correct rules, insert in the right context, then run the applicable 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: inserts boundary cells using the active rules; this command is deprecated in the referenced release, so verify the project-approved targeted replacement
- create_io_corner_cell -reference_cell <io_corner_lib_cell> <two_io_guides>: inserts an I/O-ring corner cell between two perpendicular I/O guides
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.
Healthy, Suspicious & Hard-stop Results
- Healthy: Core boundary cells protect and complete standard-cell row edges; inside/outside corner cells handle core or voltage-area corners; I/O-ring corner cells belong to the pad ring. They are different library functions.
- 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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