Floorplan Design Methodology

Chip Floorplanning is the early physical design stage that determines the die size, the spatial arrangement of major functional blocks (macros, memory arrays, analog blocks, I/O ring), and the top-level power/ground grid structure — where decisions made during floorplanning propagate through the entire implementation flow, making a well-optimized floorplan the single most impactful factor in achieving timing closure, power delivery integrity, and routability in the final chip.

Floorplanning Objectives

The floorplanner must simultaneously optimize multiple competing objectives:
- Minimize die area: Directly reduces manufacturing cost. Target: place blocks as compactly as possible with minimal wasted space.
- Minimize total wirelength: Place blocks that communicate heavily close to each other. Total wirelength correlates with timing, power, and routability.
- Ensure routability: Leave sufficient routing channels between macros for signal and power wires.
- Power delivery: Position power pads/bumps and plan the power ring/strap structure to meet IR drop and electromigration requirements.
- Thermal balance: Distribute high-power blocks across the die to avoid thermal hotspots.

Floorplan Components

• Core Area: The central region containing standard cell logic and embedded macros. Bounded by the I/O ring or pad frame.
• I/O Ring: Pad cells arranged around the periphery (wire bond) or distributed across the surface (flip-chip). I/O placement determines package pin assignment and signal routing topology.
• Power Ring: Wide metal straps (M_top-1, M_top) forming a ring around the core, connecting to power pads. Power stripes extend from the ring into the core at regular intervals.
• Macro Placement: SRAM arrays, ROM, analog blocks are placed considering: data flow (proximity to connected logic), pin orientation (face pins toward the core), routing channels (leave space between macros), and power rail alignment.

Die Size Estimation

Before detailed floorplanning:
1. Cell Area: Sum of all standard cell areas × utilization factor (typically 0.65-0.80).
2. Macro Area: Sum of all hard macro areas × macro utilization factor (typically 0.80-0.90, accounting for halos).
3. Total Core Area: (Cell Area + Macro Area) / target utilization.
4. Die Area: Core Area + I/O ring + seal ring + scribe lane.

Floorplan Iteration

Modern flows iterate between floorplanning and placement/routing:
1. Initial floorplan → trial placement → congestion analysis → refine floorplan.
2. Power grid design → IR drop analysis → adjust power strap density → re-evaluate area.
3. Timing estimation → identify critical paths → adjust macro/block locations to reduce critical path wirelength.

Chip Floorplanning is the architectural blueprint that determines the chip's physical fate — a well-crafted floorplan enables timing closure in days while a poor floorplan creates congestion, IR drop, and timing problems that no amount of downstream optimization can resolve.