Save Power 4: Dynamic Voltage Scaling

CMOS delay is inversely proportional to supply voltage.

Voltage to a region may be varied dynamically. A higher supply voltage (at fixed f) uses more power (square law) but allows a higher f. Operating region of the frequency/voltage curve is roughly linear. But, logic with higher-speed capabilities is smaller which means it (generally) consumes greater leakage current which is being wasted while we are halted. Let's only raise VCC when we ramp up f.

Method:

1. Adjust f for just-in-time completion (e.g. in time to decode the next frame of a real-time video),
2. then adjust VCC so logic just works.

But Zeno applies still: always aim for 'a' as high as possible and minimal halt cycles.

Overall: power will then have cubic dependence on f. But this way we achieve peak performance under heavy loads and avoid cubic overhead when idle.

So a typical SoC uses not only many dynamic clock gated islands, but also some sub-continents with automatic frequency and voltage variation.

Power isolation originally used on a longer and larger scale (complete continents) but now a lot of power islands are being used.

It is possible to locally and quickly adjust supply voltage with a series transistor - but wasteful.

An off-chip power supply can be efficiently adjusted, but limited to only a few voltage islands and tens of milliseconds inertia.

For sub 45nm, voltage scaling may not be viable and static leakage may be very high: so clock within the frequency band that works and then power off until next deadline.