Floorplan Constraints

Floorplan Constraints are the rules and guidelines that govern the placement of major functional blocks within the chip boundary — determining macro placement, power domain boundaries, IO placement, and routing channel allocation before detailed cell placement begins.

Floorplan Elements

• Die boundary: Total chip dimensions determined by target area and package constraints.
• Core area: Active logic area inside die — surrounded by IO ring.
• Power domains: Separate voltage islands for power gating (MTCMOS blocks).
• Hard macros: SRAMs, embedded memories, analog blocks — fixed size, must be placed first.
• Soft macros: Hierarchical logic blocks — can be resized during implementation.
• IO pad placement: Input/output pads arranged around core perimeter.

Hard Macro Placement Rules

• Alignment: Macros must align to site grid (typically 2x standard cell height).
• Abutment: Memory banks often abutted for shared power rails.
• Orientation: SRAMs have preferred orientation for bit-line timing.
• Channel width: Minimum routing channel between macros — prevent congestion chokepoint.
• Rule: ≥ 10μm channel for M1–M4; ≥ 20μm for full routing clearance.
• Halo (keepout): Buffer zone around macro where standard cells cannot be placed — prevents timing and DRC issues at macro boundary.

Floorplan Quality Metrics

• Utilization: Total standard cell area / core area. Target: 60–75%.
• > 80%: Routing congestion risk.
• < 50%: Wasted area, longer wires, higher power.
• Aspect ratio: Width/height. Target: 0.8–1.3 (near square). Extreme AR → long power/clock distribution.
• Macro channel congestion: Verify routing resource between macros before proceeding.

Power Domain Constraints

• Separate voltage rails for each power domain.
• Level shifter and isolation cell placement at domain boundaries.
• Power switches (MTCMOS) placed at power domain boundary rows.

Floorplanning is the most impactful decision in physical design — a poor floorplan creates timing, congestion, and power problems that cannot be fixed downstream, while a well-planned floorplan makes every subsequent step smoother and faster to close.