Power switch (header/footer)
Beginner
A big switch on the chip that can cut the power to one part, like a light switch for a whole room.
Novice
A transistor used as an on/off switch between the chip’s main supply and a block’s own local supply wire, so the block can be powered off. Many small switch cells are spread across the block.
Expert
Either a PMOS “header” on the supply side or an NMOS “footer” on the ground side. Sized for its on-resistance, which adds voltage drop whenever it is on, against its leakage when off and its area. The on/off signal can reach every switch at once (star) or pass from switch to switch (daisy chain), which staggers turn-on to limit rush current.
Explained in Power planning (Design Flow).
See also: power gating, Rush current (in-rush current).