[maemo-developers] [maemo-developers] n800 LED interface

[From: Larry Battraw]

On 2/1/07, Amit Kucheria <amit.kucheria at nokia.com> wrote:
> >   My question is this: for the default trigger source (timer), is the
> > blinking just a hardware PWM built into the CPU or is the CPU involved
> > in counting timer interrupts to handle on/off periods and toggling the
> > LED?  I ask because there was some discussion on ITT about how someone
>
> Unfortunately, the CPU is involved because the built in PWM prevents the
> CPU from sleeping and there is not a lot we can do about it. So the SW
> solution wakes up device with a period "delay_on".
>
> > had their n800 run down overnight with the LED left on.  It seemed
> > unlikely but I wanted to make sure a bunch of CPU cycles weren't being
> > burned to run it.
>
> That is why they are disabled by default.
>
> Regards,
> Amit

  So would it be accurate to say using the blinking functionality goes
through the following steps?

CPU sets timer to interrupt after delay_off period
timer interrupt occurs
CPU sets hardware PWM/LED to specified brightness
CPU sets timer to interrupt after delay_on period
timer interrupt occurs
CPU turns off hardware PWM/LED
repeat ->

  I'm trying to determine what's consuming the power.  If what I've
described above is accurate, then it sounds like the CPU can't sleep
during the ON portion of the cycle due to hardware PWM for brightness
control.

Thanks-
Larry