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Dynamic Frequency Scaling

Let's adjust the clock frquency (while keeping VCC constant for now). Does this help ?

Frequency scaling is software controlled: update PLL division ratio. PLL has inertia: e.g. 1~millisecond.

Let's compare with dynamic clock gating:

Clock Gating. Frequency Adjustment.
Control: automatic, manual.
Granularity: register / FSM, macroscopic.
Clock Tree: mostly free runs, slows down.
Response time: instant, acceptable.
Can vary voltage: no, yes.

To compute quickly and halt we need a higher frequency clock but consume the same number of active cycles.

So the work-rate product, af, unchanged, so no power difference ?

Actually un-stopped regions consume power proportional to f.

Zeno: Tortoise and Achilles ?

Tortoise is best: keep going steadily and end just in time. (He appeals even more when we vary the voltage.)

But, dynamic clock gating still good for: bursty, localised activity.

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