What is the difference between a switched power supply and an alwayson power supply?
From PDVerse Low-Power Physical Design Mentor Guide · pdVerse Mentor Guide
Definition
A switched power supply (sometimes called a virtual or internal supply) is one that a power switch cell can turn fully on or off — it disappears electrically when the domain shuts down — while an always-on power supply is the true, unswitched rail that stays live at all times, typically feeding the power switch itself and any always-on cells inside the shutdown domain.
Mentor Explanation
A beginner often pictures 'the power' as one single thing per block, but power-gated designs deliberately split this into two: a real always-on rail (VDDG in this header-switch naming) and a downstream switched rail (VDD) that the switch cell derives from it. Think of it this way — the always-on rail is the source, and the switched rail is the tap that can be shut off.
Example
In a header switch cell example, pin VDDG is tagged pg_type: primary_power (the true always-on supply) while the internally generated VDD pin is tagged pg_type: internal_power with a switch_function of "!NSLEEPIN" — meaning it is derived from VDDG but controlled by the sleep signal.
Why It Matters
Always-on/switched separation is what makes control signals, retention, and isolation cells work at all — those special cells need to keep functioning precisely when the switched rail is off, so they must be powered from the always-on rail instead.
Common Beginner Mistake
A beginner mistake is assuming that because a block is 'in the shutdown domain,' every cell in it loses power — in practice, always-on cells are deliberately placed inside the shutdown domain's boundary but wired to the backup/always-on rail so their logic keeps functioning.
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