How do you verify macro and block-grid alignment?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
"Inside the core boundary" and "on a legal coordinate" are two different things — a macro can sit entirely within the die and still be parked on an illegal manufacturing-grid or block-grid location. Verification means checking the macro's origin, its boundary, its orientation, its designated alignment point, and the applicable grid (manufacturing grid, or a stricter block/FinFET grid) all together — checking any one alone isn't enough.
Technical Explanation
- Movement, rotation, mirroring, and snapping are genuinely separate operations in the tool — doing one doesn't automatically fix the others, so verify grid legality again after each transform, not just once at the end.
- Advanced-node macros and macros that get instantiated many times across the design often carry stricter alignment relationships than an ordinary standard cell does — don't assume the same casual check that works for standard cells is sufficient here.
- This is exactly the kind of thing that passes a casual visual review and fails signoff DRC — treat grid-alignment verification as its own explicit checklist item.
Formula Or Decision Rule
Decision rule: all required coordinates and dimensions satisfy the owning grid after final orientation.
What To Check
- Warning sign: A legal orientation becomes off-grid after a manual move.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Snap to the approved grid, recheck boundary clearance and pins, and record the final coordinates.
Command Checks & Actions
- set_macro_constraints -alignment_grid <grid_name> -alignment_point <align_point> <macro_collection>: records allowed macro or I/O placement constraints
check_design-checks {dp_pre_macro_placement}: runs named design-planning check groups
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.
Healthy, Suspicious & Hard-stop Results
- Healthy: Check macro origin, boundary, orientation, alignment point, and applicable block or FinFET grid. Movement, rotation, mirroring, and snapping are separate operations.
- 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 a legal orientation becomes off-grid after a manual move. ”
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