🛠 Comprendre les triggers en pilotant les 4 LED utilisateur

Objectives

Discover the Linux LED subsystem without soldering anything: the BeagleBone Black has 4 user LEDs defined in the mainline device tree (am335x-bone-common.dtsi) under the names beaglebone:green:heartbeat, mmc0, usr2, and usr3. Each LED exposes a "trigger": an activation source managed by the kernel (heartbeat, SD card activity, timer...). This is the gateway to hardware control under Debian.

Hardware

• 1 powered BeagleBone Black (a simple mini-USB cable is sufficient)
• SSH access or serial console (the 6-pin 3.3 V TTL debug connector is available if no screen is present)

Complete code

# Lister les LED exposées par le device tree
ls /sys/class/leds/
# beaglebone:green:heartbeat  beaglebone:green:mmc0
# beaglebone:green:usr2       beaglebone:green:usr3

# Voir les triggers disponibles ; l'actif est entre crochets [ ]
cat /sys/class/leds/beaglebone:green:usr2/trigger

# Chaque LED prend son rôle métier :
echo heartbeat > /sys/class/leds/beaglebone:green:heartbeat/trigger
echo mmc0      > /sys/class/leds/beaglebone:green:mmc0/trigger

# usr2 en minuteur 250 ms allumée / 500 ms éteinte
echo timer > /sys/class/leds/beaglebone:green:usr2/trigger
echo 250 > /sys/class/leds/beaglebone:green:usr2/delay_on
echo 500 > /sys/class/leds/beaglebone:green:usr2/delay_off

# Reprise la main : retirer le trigger puis piloter la luminosité
echo none > /sys/class/leds/beaglebone:green:usr3/trigger
echo 255 > /sys/class/leds/beaglebone:green:usr3/brightness
echo 0   > /sys/class/leds/beaglebone:green:usr3/brightness

Expected verification

The heartbeat LED pulses at the kernel's rhythm, mmc0 blinks during writes to the SD card (test with dd if a microSD is inserted), usr2 blinks at the configured rate. With the none trigger, usr3 instantly obeys echo commands.

Common errors

• Forgetting the colons in path names (beaglebone:green:usr2): tab completion is your friend.
• Writing to brightness without setting the trigger to none: the active trigger constantly overwrites your value.
• Confusing the LEDs: heartbeat is a programmable user LED, it is not the Ethernet port LED nor the power LED.
• On very old A5B revisions, the LEDs were intentionally made dimmer (resistance changed from 820 Ω to 4.75 kΩ): a dim LED is not a dead LED.
• The heartbeat trigger slows down when the system is heavily loaded: this is intended (it reflects kernel load).
• Expecting these LEDs on the P8/P9 pins: they are soldered onto the board, the cape pins are a different exercise.