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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
consumes greater leakage current which is being wasted while we are halted.

Let's only raise VCC when we ramp up f.


  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. So 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 is used on a longer and larger scale (complete continents).

49: (C) 2008-11, DJ Greaves, University of Cambridge, Computer Laboratory.