What is a power switch and what does it do?
From PDVerse Low-Power Physical Design Mentor Guide · pdVerse Mentor Guide
Definition
A power switch is a special cell (or bank of cells) placed on a domain's power or ground connection that can electrically connect or disconnect that domain from its supply rail, based on a control signal.
Mentor Explanation
In short, a power switch cell shuts off power to a domain. Structurally, a power switch has an input supply net, an output (switched) supply net, at least one control input, and optionally acknowledge outputs. When the control signal indicates 'off,' the output supply net is disconnected from the input supply, and everything downstream of that switch effectively loses power — which is exactly how coarse-grain power gating shuts down a block.
Example
A CPU core scheduled to sleep has its VDD routed through a header switch; when the power controller asserts the switch's sleep control, the core's internal VDD net drops and the core stops consuming both dynamic and leakage power.
Why It Matters
Power switches are the physical mechanism behind every 'shutdown domain' story in a low-power interview — I check that candidates can connect the switch cell itself to the higher-level concept of power gating.
Command
create_power_switch SW1 -domain PD_TOP -output_supply_port {SWOUT VDD1g}
-input_supply_port {SWIN1 VDD1} -control_port {CTRL swctl} -on_state {ON
VDD1 {!swctl}}Common Beginner Mistake
Describing a power switch as simply 'a big transistor' without mentioning that it needs control (and often acknowledge) signals that must themselves stay always-on and properly sequenced.
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