9.1 Proposal: Power.

Richard A. Smith richard at laptop.org
Fri Oct 24 18:40:10 EDT 2008

Nate Ridderman wrote:

> 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?

The DCON drives a 200Hz PWM to the feeback loop of the DC/DC converter 
that produces the LED's supply voltage.  The lo-side driver transistors 
for each of the 3 LED chains (12 LEDS, 3 chains of 4) are not connected 
to anything you can PWM.

>      > 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.)

It not possible to save power by doing this unless you ramped the 
backlight down during the period when the CPU whould have normally been 
awake.  The current scheme is already at its lowest it can be.  Jump to 
the lowest setting and then put the cpu to sleep.  Any deviation from 
that will use more juice.  If you wake up the CPU to do something you 
have taken a large step backwards.

Richard Smith  <richard at laptop.org>
One Laptop Per Child

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