What design rules and site constraints does Placement Legalization verify?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
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Short Answer
Legalization snaps floating continuous cells onto discrete row site grids, eliminates all instance overlaps, enforces row orientation flipping (N/FS) for power rail abuttal, aligns multi-height cell tracks, and ensures DRC pin-access clearance.
Technical Explanation
- Placement Legalization transforms continuous, floating analytical placement coordinates into a discrete, DRC-compliant, manufactured layout on the silicon site grid.
- Site Row Snapping: Global placement treats standard cells as continuous geometric objects to minimize global Half-Perimeter Wire Length (HPWL). Legalization snaps every cell origin onto integer placement site row tracks.
- Overlap Elimination: Detects and resolves 100% of cell bounding box overlaps by shifting instances into adjacent legal whitespace while minimizing total placement perturbation.
- Power Rail Abuttal & Row Flipping: Enforces alternating row orientation rules (e.g. R0 normal / MX flipped), ensuring that VDD rails abut VDD rails and VSS rails abut VSS rails between adjacent site rows to share horizontal power straps.
- Multi-Height & Multi-Track Cell Rules: Manages modern multi-height cell libraries (1x, 2x, 3x row heights), ensuring that n-well and p-well diffusion boundaries align continuously without violating well-to-well spacing rules.
- Pin Access & DRC Validation: Verifies that all cell input and output pin ports align with preferred routing track intersections, guaranteeing clean via access without adjacent track blockage.
Common Mistake
The Trap: Ignoring legalization displacement metrics. If average cell displacement exceeds 5 um, it indicates severe local overcrowding where cells are pushed far from their optimal timing positions.
Follow-up Question & Model Response
"What command verifies that 100% of cells in your design are legally placed?"
Candidate Model Response: Run check_legality -verbose in ICC2 or checkPlace in Innovus; it must report zero overlaps, zero off-grid instances, and zero orientation errors.
Practical Example
Executing & Verifying Legalization:
# Synopsys ICC2: Legalize placement with minimal displacement
legalize_placement -effort high
# Audit legality report
check_legality -verbose
report_placement -displacementPnR Flow Mentor Guide
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