What design rules and site constraints does Placement Legalization verify?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
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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