What is an always-on cell, and where is it physically placed?
From PDVerse Low-Power Physical Design Mentor Guide · pdVerse Mentor Guide
Definition
An always-on cell is a standard cell that is functionally identical to an ordinary buffer, inverter, or similar gate, but is powered from a backup/true supply (VDDG/VSSG or VDDG/VSS) that never shuts off, and it is physically placed inside a shutdown domain to carry signals that must remain live through that domain's power-down period.
Mentor Explanation
An always-on cell is powered from the active/true domain supply but is physically placed inside the shutdown domain, used for control signals such as enable pins, save/restore pins, and switch control pins that must stay live. A representative library cell for a header-type always-on buffer (AO_X1) carries the 'always_on:true' attribute and separate pg_pin declarations for VDD (primary_power), VDDG (backup_power), and VSS (primary_ground). Because tracing every always-on path by hand would be tedious, automatic inferencing handles it: for the enable pins of level shifters/isolation cells, the save/restore pins of retention cells, and the control pins of switch cells, the tool traces backward through cells and nets until it reaches an always-on region, marking everything on that path as always-on. This is reflected physically too, with an always-on logic cell showing separate primary-power and backup-power pins on the same buffer symbol — and the tool performs this optimization only if the target library actually contains always-on inverters/buffers marked with the 'always_on' attribute.
Example
A SAVE signal generated in an always-on power controller has to physically route through a shutdown domain's floorplan area to reach a retention register; an always-on buffer inserted along that path, powered from VDDG, keeps the signal valid even while everything else in that domain is dead.
Why It Matters
This tests the connection between a logical requirement ('this signal must never go dark') and its physical implementation consequence ('so this specific cell, though sitting inside a dead domain's footprint, needs its own live power connection').
Common Beginner Mistake
Assuming an always-on cell needs to be physically located outside the shutdown domain's boundary — it is placed inside the shutdown domain, just wired to a separate backup supply rather than the domain's switched rail.
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