What is the difference between a header switch and a footer switch?
From PDVerse Low-Power Physical Design Mentor Guide · pdVerse Mentor Guide
Definition
A header switch is placed on the VDD (power) side of a domain, between the always-on power rail and the domain's switched/virtual power rail; a footer switch is placed on the VSS (ground) side, between the domain's switched ground rail and the always-on ground rail.
Mentor Explanation
The pin names for each type tell the story. For the header switch cell, the pins are VDDG (the primary, always-on power), VDD (the switched/virtual power delivered to the domain), NSLEEPIN (control input), and NSLEEPOUT (optional delayed acknowledge output). For the footer switch cell, the true ground is VSSG (always-on) and the switched ground delivered to the domain is VSS, controlled by SLEEPIN/SLEEPOUT — when SLEEPIN is active, the switch opens and VSS is logically disconnected from VSSG. This split holds conceptually in implementation too: header switches connect the power rail to the power-supply pins in the power-down domain, and footer switches connect the ground rail to the ground-supply pins.
Example
A design using PMOS header switches keeps its footer (ground) always connected, while a design using NMOS footer switches keeps VDD always connected and gates the return path to ground instead.
Why It Matters
I want to see that a candidate knows headers and footers are functionally equivalent choices (power-side vs. ground-side gating) rather than one being 'better' by default — library and process considerations usually drive the choice.
Common Beginner Mistake
Mixing up which side is switched: some beginners assume both VDD and VSS get gated together, but coarse-grain power gating typically switches only one rail (header OR footer), not both simultaneously.
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