power consumption depending on screen content

[Flemming Richter Mikkelsen]

On 2009-10-09, Joerg Reisenweber <joerg at openmoko.org> wrote:
> [Timo Juhani Lindfors Fr  9. Oktober 2009]:
> > Joerg Reisenweber <joerg at openmoko.org> writes:
> > > A completely white screen with ~40% backlight consumes same amount of
> power as
> > > a completely black screen (-content) with backlight "dim" (=off)
> > >
> > > (=> a black screen eats as much power as a backlight set to 40%)
> > >
> > > (DUT: FR A7 SHR-U0808)
> >
> > I wrote
> >
> > http://iki.fi/lindi/openmoko/display-current-measurement1.pl
> >
> > to test this. However, here white screen with 40% brightness consumes
> > significantly more than black screen with 0% brightness:
> >
> > >dbg running echo 102 > /sys/class/backlight/gta02-bl/brightness
> > >dbg sleeping for 60 seconds for current to stabilize
> > >dbg taking 10 samples with 10-second delays
> > >dbg current now   102750, mean 102750.0000, deviation   -1.0000, range
> [...]
> > >dbg running xset dpms force off
> > >dbg sleeping for 60 seconds for current to stabilize
> > >dbg taking 10 samples with 10-second delays
> > >dbg current now    48000, mean 48000.0000, deviation   -1.0000, range
> 0.0000
> [...]
>
> 1: I didn't use xset dpms force off, I rather let SHR (or probably
> FSO-framework) do it's dim-job, like it always does by default when hooked up
> to wallcharger. No idea what is actually done on dim, or if it is
> exactly "xset dpms force off"
> (btw if dpms force off actually switches off LCM and glamo video clock, I'd be
> rather surprised to see any difference from a light or dark video buffer
> content)
>
> 2: I used current indicator LED on that wallcharger to positively find the
> sketch program when dimmed with a virgin white screen uses significantly less
> current than when dimmed with a painted black screen (and erasing all the
> black brought it back to "power-saving mode", so it's not been any difference
> in state of that program - like filled history buffer etc)
>
> 3: the probing during suspend was mainly due to the wallwart LED indicator
> has its optimum resolution here, whereas it hardly differs for 190mA vs 220mA
> e.g
> Nevertheless I suspect the power consumption difference will be similar in
> normal backlit state, and probably isn't related to dimming at all (except
> for the rationale it's rather senseless to keep a black screen during
> backlight LED off)
>
>
> The compare to ~40% backlight power consumption was a rough guess just to
> illustrate the magnitude of the effect, based on same LED readout on the
> wallwart charger
>
> I can't tell why your results are like that, but obviously your setup differs
> somewhat to mine.

What happens if you do the same test with the screens flex cable
disconnected?