9.1 Proposal: Power.

Nate Ridderman nate.ridderman at gmail.com
Thu Oct 23 19:44:26 EDT 2008


Do we have the ability to pulse width modulate the backlight LEDs? What is
the resolution on the PWM? It's hard to know if this is feasible without a
hardware schematic and specs on the backlight driver. The CL1 spec mentions
a PWM signal, but maybe it only has four bits of resolution?

In the cell phone world, PWM is used pretty regularly as an approach to dim
backlights. It's much cheaper than having a bunch of analog current sensing
circuitry. I believe 100 Hz or greater is required to reduce flickering, but
this might be higher on a larger screen. Sometimes using PWM on LEDs can
create spectral noise, especially if there is no soft start mechanism in the
LED driver, so the antenna desensitivity would probably need to be tested
against this approach.

Thanks,
Nate

On Thu, Oct 23, 2008 at 4:45 PM, Jim Gettys <jg at laptop.org> wrote:

> How well we can do that isn't clear.
>
> We have 16 brightness levels, but we didn't think about making them
> logarithmic in response to correspond to the eye's behavior, so there
> are really fewer than that that are useful.
>
> Please experiment and see if it is helpful, of course...
>                      - Jim
>
> On Thu, 2008-10-23 at 10:14 -0400, Eben Eliason wrote:
> > I could be talking nonsense, and perhaps this would consume more power
> > than it saves, but if you were able to slowly dim the backlight over
> > the course of a minute or so, instead of waiting a minute and then
> > dropping it suddenly, we could prevent the sudden change which causes
> > a break in concentration.  (As long as the screen is bright enough to
> > be usable when dim, of course.)
> >
> > - Eben
> >
> >
> > On Wed, Oct 22, 2008 at 4:21 PM, Chris Ball <cjb at laptop.org> wrote:
> > > Hi,
> > >
> > >   > When running on battery with Energy Saver set to "Better Battery
> > >   > Life" (which sets "Automatically reduce the brightness of the
> > >   > display before display sleep") the backlight dims after 30 seconds.
> > >   > On AC with the equivalent setting it's 2 minutes, 30 seconds.  In
> > >   > each case the dimming time seems to be 50% of the time until the
> > >   > screen is turned off.  1 minute is the minimum time before display
> > >   > sleep.
> > >
> > > Thanks!  I was hoping someone would have numbers.  Our backlight dim
> > > currently happens fifty seconds after idleness starts, so we're
> > > definitely less aggressive than OS X already..
> > >
> > > - Chris.
> > > --
> > > Chris Ball   <cjb at laptop.org>
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> --
> Jim Gettys <jg at laptop.org>
> One Laptop Per Child
>
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