Doug S (foo.delete@this.bar.bar) on September 13, 2021 11:57 am wrote:
> --- (---.delete@this.redheron.com) on September 13, 2021 11:19 am wrote:
> > - one thing Apple do (I don't know about other designs)
> > is slide voltage up and down while keeping frequency
> > unchanged. Obviously changing frequency is somewhat disruptive in a way that changing voltage is not.
> > Apple have some flexibility to do this given the digital
> > power estimators, a knowledge of the capacitance in
> > the system, and an ability to prevent catastrophe if the
> > system is oversubscribed by having instruction issue
> > paused by the DPE for a cycle or two. Maybe also the fact
> > that every SRAM is decoupled from logic with a voltage
> > shifter between the two, so you have flexibility to down-voltage logic while not losing SRAM retention.
> > (At least they have a long sequence of patents about this, so you'd hope it's implemented!)
>
>
> I'm a little confused on this point. Why would you want to alter voltage while keeping frequency
> constant? Shouldn't you always want to run at the lowest voltage capable of handling a given frequency
> to minimize power? In what circumstance would you for example want to increase voltage while keeping
> frequency constant? That makes no sense to me, but maybe I'm missing something.

Yes. But the minimum voltage to sustain a given frequency will change over time due to all sorts of factors!

David