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

[From: Amit Kucheria]

On Wed, 2007-01-31 at 20:48 -0500, ext Larry Battraw wrote:
>   First off, kudos to the maemo team for making such a nifty and
> easy-to-use interface for the keypad LED.  I spent a inordinate amount
> of time just tinkering with the brightness, blinking period, and so
> forth :-)  For those interested, the relevant sys entries are here:
> /sys/class/leds/keypad

Don't thank us too soon. :)

_Do Not_ use brightness, on_period and off_period files in that sysfs
directory. They are linked to the hardware PWM but have some problems
that I will describe later.

Only use what is shown in the following example for a SW controlled
blinking:
/ # cd /sys/class/leds/keypad
/sys/class/leds/keypad # echo timer > trigger
/sys/class/leds/keypad # echo 128 > blink_brightness 
/sys/class/leds/keypad # echo 1000 > delay_on
/sys/class/leds/keypad # echo 500 > delay_off

> Should be self explanatory, particularly if you turn on the blinking
> from the control panel so you can cat out the values it uses for
> delay_off and delay_on, etc.
>   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