Power Domains & Voltage Islands
What a power domain is, and how voltage islands let blocks run at different voltages or switch off independently.
A power domain is a group of logic that shares one supply and is controlled as a unit: it is on, off, or at a given voltage together. Splitting a chip into domains is an architecture decision made early, because it decides which low-power techniques are possible later. A modern phone SoC has tens of domains.
Why split a chip into domains
- To switch blocks off. A GPU, modem or camera pipeline sits idle most of the time. Its own switchable domain lets it be power gated, removing its leakage.
- To run blocks at different voltages. The CPU needs a high voltage for its top frequency, while a slow always-on controller can run at a much lower one. Separate supplies let each block run at the lowest voltage that meets its own timing.
- To scale voltage at runtime. A domain with its own regulator can use DVFS without affecting the rest of the chip.
Key terms
| Term | Meaning | Example |
|---|---|---|
| Power domain | Logic sharing one primary supply, controlled together | PD_CPU, PD_GPU |
| Voltage island | A domain whose voltage differs from its neighbours | CPU at 0.8 V next to fabric at 0.9 V |
| Always-on (AON) domain | Never powered down; holds wake-up logic | Power controller, RTC, interrupt logic |
| Switchable domain | Can be fully powered off through switch cells | GPU, modem, accelerators |
| Power state | One legal combination of supply states | CPU on at 0.8 V, GPU off |
What every boundary needs
Once the domains exist, every signal that crosses from one domain to another needs a decision. Two questions decide it: are the voltages different, and can the source be off while the destination is on?
| Source vs destination | Needs |
|---|---|
| Same voltage, both always on together | Nothing special |
| Different voltages, both on | Level shifter |
| Source can be off while destination is on | Isolation cell |
| Different voltages and source can be off | Enable level shifter (level shifter with isolation built in) |
| Destination can be off, source on | Usually nothing; an off input is harmless |
Worked example: four domains
An SoC has an always-on domain at 0.9 V (power controller and wake logic), a CPU domain at 0.8 V that can be switched off, a GPU domain at 0.9 V that can be switched off, and an I/O domain at 1.8 V. Here is what three of the crossings need.
| Crossing | Voltage | Source can be off? | Cells needed |
|---|---|---|---|
| CPU → AON (interrupt request) | 0.8 → 0.9 V | Yes | Enable level shifter, clamped to 0 so no false interrupt |
| GPU → AON (status) | 0.9 → 0.9 V | Yes | Isolation cell only |
| AON → CPU (wake, reset) | 0.9 → 0.8 V | No, AON is always on | Level shifter only (high to low) |
What the floorplan has to do
Physical design turns each domain into a region of the floorplan with its own power rails. Switchable domains get arrays of power switch cells between the always-on supply and the domain's virtual supply. Isolation and level shifter cells go at the boundary, powered from the right supply. Any always-on signal that must pass through a switchable region is routed with always-on buffers that take their power from a secondary supply pin, so the signal survives when the region is off.
In interviews, domain questions usually end with "what cells do you need at this boundary?" Answer with the two questions: different voltage means a level shifter, a source that can turn off means isolation, both means an enable level shifter.
Every extra domain adds switches, boundary cells, power grid complexity, verification states and sequencing logic. Create a domain only when the power it saves is worth that cost. The power state table, covered later in this path, keeps the legal combinations under control.
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