What is a switched ground net, and how does it relate to a footer switch?
From PDVerse Low-Power Physical Design Mentor Guide · pdVerse Mentor Guide
Definition
A switched ground net is the ground rail delivered to a domain through a footer switch — it is electrically disconnected from the always-on ground (VSSG) whenever the footer switch is turned off.
Mentor Explanation
The footer switch pin naming is VSS for the switched ground delivered into the domain and VSSG for the true, always-on ground feeding the switch cell; the control pin SLEEPIN determines whether the switch opens (SLEEPIN active means VSS is logically disconnected from VSSG). This mirrors the header-switch case exactly but on the ground side: instead of gating VDD, the footer switch gates the return path to ground, so cells inside the domain see a floating or rising 'ground' reference once the footer is off — which is precisely the internal power/ground pin classification the library attribute system captures.
Example
In a footer-switched domain, an NMOS transistor's normally-grounded source pin actually connects to the switched VSS net; when the footer switch opens, that net is no longer tied to true ground and the transistor stack effectively floats.
Why It Matters
I want to confirm candidates realize ground gating is symmetric to power gating — it's not just a VDD story, and the same rush-current and isolation concerns apply on the ground side.
Common Beginner Mistake
Assuming only VDD can be gated; footer (ground-side) switching is an equally valid and common power-gating architecture.
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