9.1 Proposal: Power.

Gary C Martin gary at garycmartin.com
Fri Oct 24 22:22:57 EDT 2008

On 24 Oct 2008, at 23:40, Richard A. Smith wrote:

> 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.

I just tried some simple shell scripts stepping through the LED  
brightness to simulate a 15 to 7 drop, a pause and back again, also 15  
to 1 cycle (think .1sec step seemed fair place to start). The effect  
seems to add a good touch of polish to a potential screen dimming  
cycle, if it's not too complicated to implement reliably for real.

The steps between brightness values are certainly not linear to my eye  
(1-2 is a much larger step than 14-15), but used as a fade transition  
effect it didn't seem an issue. I also tried some rough PWM between 7  
& 8, via the shell, but couldn't get anything that didn't make me feel  
nauseous after a few moments use.

Not sure if this helps the original feedback (that the sudden screen  
dimming is annoying when trying to read), but the effect is at least  
more pleasing than a sudden sharp drop in brightness.


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