Kindle 3 has a two-color LED in the power button. With original software, you can see it glow briefly when you slide the button or when the battery is charging. A Kindle running Debian of course allows many more interesting possibilities.
The two-color LED is actually composed of two separate LEDs that are controlled separately: an amber and a green one.
The amber LED is lit when the battery is charging. Currently it is not clear to me how exactly it is controlled, but it seems to involve both the hardware-controlled charger LED pin of the MC13892 power management IC and a software-controlled GPIO. The relevant code starts at line 929 in arcotg_udc.c. Note that pmic_green_led_disable() function forces the MC13892 pin state (which as I understand actually affects the amber LED, not the green one).
In any case, there doesn't seem to be any user-space interface available for controlling this LED. It's turned off by default.
The green LED is much easier to control. It's connected directly to one of the signaling LED drivers on the MC13892 and there's a convenient, if somewhat weird interface for those in sysfs. Relevant code is in pmic_light.c.
All operations are made through the /sys/devices/platform/pmic_light.1/lit file. You can write statements in the form of "command value" to this file to control the LED drivers. Supported commands are:
- ch - select channel on which subsequent commands will operate. Kindle's LED uses channel 4, so before issuing any other commands, always write ch 4. Better not touch other channels.
- cur - set LED current. Can be set from 0 to 7 to control LED brightness. Kindle's built-in software uses 7.
- dc - set PWM duty-cycle. Can be set from 0 to 32. 0 means constantly off, 32 means constantly on.
- bp - set PWM frequency. Can be set from 0 to 3. At 0, frequency is high enough for blinking to be invisible to human eye and PWM setting can be used as an additional brightness control. On higher settings, the PWM output is seen as blinking (see MC13892 datasheet for exact frequencies).
- ra - Either 0 or 1. If set to 0, PWM duty-cycle changes are immediate. If set to 1, changes happen via a hardware generated ramp, making smooth visual transitions.
As an example, here's what I have in /etc/rc.local to turn off the green LED at boot:
echo "ch 4" > /sys/devices/platform/pmic_light.1/lit echo "cur 0" > /sys/devices/platform/pmic_light.1/lit echo "dc 0" > /sys/devices/platform/pmic_light.1/lit