What are clock gating and power gating?

Gating saves power in idle parts: stop a part's clock to pause it, or cut its power to switch it off.

1. Clock gating: pause
Each clock tick moves a part's work one step. Clock gating stops the ticks to an idle part, like pausing a video game: it keeps its data and restarts at once.
2. Power gating: switch off
Power gating cuts a part's power with a switch, like turning off a game console. Almost nothing leaks, but the part loses its data unless it saves it first.
3. Short break or long break
Waking from power gating takes longer, and saving data and switching back on cost energy. So firmware compares the power saved with these costs, and switches a part off only for long breaks.
4. Idle parts pause or switch off
The chip turns off or pauses idle graphics, AI engines, video engines and CPU cores, with no help from the user. Their clocks also slow or stop when nothing needs them.

Words to know

Clock: A steady, very fast beat inside a chip. Each tick moves the work one step.
Clock gating: Stopping the clock to an idle part. It keeps its data and wakes at once.
Power gating: Cutting an idle part's power with a switch. It saves more, but wakes more slowly.
Leakage: Current that leaks through switches that are off, whenever the part has power.
Firmware: Software built into a chip or board that runs the hardware from inside.

A simple test reads an idle part's clock or power state, to see whether it really switched off.

Why not always use power gating, since it saves the most?Think first, then tap to see the answer.

Waking takes longer and costs energy, and the part loses its data unless it saves it. For short breaks, clock gating is better.