power consumption depending on screen content

Joerg Reisenweber joerg at openmoko.org
Fri Oct 9 23:23:27 CEST 2009

[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    

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 

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 
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.

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