floorplan power domain
**Floorplan Power Domain Partitioning** is the **strategic division of a chip's physical layout into distinct voltage domains (power domains)**, each operating at independent supply voltages or with independent power-gating capability, enabling aggressive power management while maintaining signal integrity across domain boundaries.
Modern SoCs contain dozens of power domains: CPU cores that can be individually voltage-scaled or shut down, always-on peripherals, I/O banks at different voltages, and memory arrays with retention voltage requirements. The floorplan must physically organize these domains for efficient power delivery and minimal cross-domain overhead.
**Power Domain Architecture**:
| Domain Type | Voltage | Power Control | Example |
|------------|---------|--------------|----------|
| **Always-on** | Nominal (0.75V) | None | PMU, clock gen, interrupt ctrl |
| **Switchable** | Nominal | Power gating (MTCMOS) | CPU cores, GPU |
| **Multi-voltage** | 0.5V-1.0V DVFS | Voltage scaling | CPU, DSP |
| **Retention** | Low voltage (0.5V) | State retention | SRAM, registers |
| **I/O** | 1.8V / 3.3V | Level shifting | External interfaces |
**UPF/CPF Specification**: Power intent is captured in Unified Power Format (UPF/IEEE 1801) or Common Power Format (CPF). These specify: which cells belong to which power domain, supply nets and switches, isolation and level-shifting requirements, retention strategies, and power state transitions. The UPF drives all downstream tools — synthesis, place-and-route, and verification.
**Floorplan Considerations**: **Domain contiguity** — cells in the same power domain should be physically grouped to minimize power switch overhead and simplify power grid routing; **boundary cells** — isolation cells (clamp to 0/1 or hold last value) and level shifters must be placed at every signal crossing between domains; **power switch placement** — header/footer MTCMOS switches sized for rush current and inserted in dedicated rows; **ring isolation** — guard rings or spacing between domains at different voltages to prevent latch-up.
**Power Grid Design**: Each domain needs its own power/ground network. Domains sharing the same voltage can share power grids. Power switches create a virtual VDD (VVDD) rail that can be disconnected from actual VDD. The power grid must handle: **rush current** (inrush when a gated domain powers on — can cause IR drop spikes), **static IR drop** (voltage loss across power grid resistance), and **dynamic IR drop** (voltage fluctuation during switching activity).
**Cross-Domain Verification**: Every signal crossing a power domain boundary must have proper isolation and/or level shifting. Missing isolation cells cause floating outputs that draw crowbar current and potentially damage downstream logic. Verification tools (UPF-aware) flag: missing isolation, incorrect level shifter type (high-to-low vs. low-to-high), signals crossing from off domain to on domain, and retention register connectivity.
**Floorplan power domain partitioning is the architectural foundation of modern low-power chip design — it translates power management intent into physical reality, and errors in domain partitioning propagate through every subsequent design step, making early floorplan decisions among the most consequential in the entire design flow.**