What are the different states a power domain can be in (fixed voltage, switchable, DVS, DFS, DVFS)?
From PDVerse Low-Power Physical Design Mentor Guide · pdVerse Mentor Guide
Definition
A power domain can be operated at a static fixed voltage (simply set once for its function), can be switchable (fully powered on or off via power gating), can use variable frequency / DFS (dropping clock frequency, or shutting clocks off entirely, in idle mode), can use DVS (dynamically adjusting only voltage based on measured speed/performance needs), or can use full DVFS (dynamically adapting both voltage and frequency together via look-up tables or on-chip monitors).
Mentor Explanation
A beginner often treats these as one technique, but these form a spectrum of increasing complexity and increasing power benefit. Think of it this way: fixed-voltage domains are the simplest (different blocks just get different static voltages); variable frequency only touches the clock; DVS only touches voltage using on-chip monitors like a ring oscillator; and DVFS combines both, which is the most powerful but also the most expensive to architect, verify, and implement.
Example
In a variable-frequency example, some clocks (200/300 MHz) shut off entirely in idle mode via clock gaters, while others (10/100/500 MHz) are merely reduced to kHz-range frequencies rather than fully stopped.
Why It Matters
Choosing the right state for a given domain is a real architectural trade-off — DVFS gives the largest power savings but carries a high architecture, design, and verification impact, versus a fixed multi-voltage domain which is comparatively low-effort.
Common Beginner Mistake
A beginner mistake is reaching for full DVFS by default; plain multi-supply-voltage domains or clock gating can capture a lot of the benefit at a fraction of the implementation cost.
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