[libre-riscv-dev] effects of powered-off chip sections on current leakage

[Jacob Lifshay]

On Fri, Jun 12, 2020, 17:21 <bugzilla-daemon at libre-soc.org> wrote:

> https://bugs.libre-soc.org/show_bug.cgi?id=376
>
> --- Comment #24 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
> (In reply to Jacob Lifshay from comment #23)
> > (In reply to Luke Kenneth Casson Leighton from comment #21)
> > > (In reply to Jacob Lifshay from comment #19)
> > > > > > Also, it might be worthwhile to add one more
> > > > > > core and disable one at manufacture time as a way to increase
> yield.
> > > > >
> > > > > that's a good idea.  have to bear in mind that current leakage
> occurs
> > > > > regardless of whether the silicon is in use or not.
> > > >
> > > > If we set up each core as a different power domain (which will also
> help
> > > > with idle power), the disabled core could be powered-down.
> > >
> > > (you still get current leakage even when powered down, is my point)
> >
> > I meant something like having a mosfet in the power line to the whole
> core,
> > so the entire power supply could be turned off. Assuming that mosfet
> wasn't
> > garbage, the power usage for the whole core could be reduced to the
> > microwatt level.
>
> apologies: you're still not getting it.  even the *existence* of the gates,
> even when fully powered down, with zero oower connected in any way, shape
> or
> form, *still* causes not insignificant current leakage.
>

Please explain in more detail, perhaps with some supporting references --
it still makes close to zero sense to me how additional unused silicon area
could affect the rest of the chip, since that area itself couldn't be
drawing any power since all wires into it that are at 0V due to the power
rails and I/O signals being switched off.


> this shows up particularly badly in the design of GSM ASICs.  thousands of
> correlators are required to get an initial lock within "seconds" expected
> by
> users... or... and then they are "powered down" exactly as you suggest.
>
> unfortunately even their very existence, even powered down, causes current
> leakage, adversely affecting power consumption and draining battery life.
>
> the solution had to involve "A-GPS" where the LAT/LONG (or even just one of
> those) was obtained from the bearest celltower.  this reduced the
> correlator
> search soace to the point where a decent DSP could handle the job instead.
>

neat! didn't know about A-GPS before, though I assumed it was related to
GPS somehow.

>
Jacob

>