🛠 Fondu LED progressif avec PWM sous sysfs (meson)

Objectives

Implement an LED fade (intensity variation) on the Le Potato by exporting a PWM output from the meson controller via /sys/class/pwm, after enabling the corresponding device tree overlay.

Hardware

• Le Potato AML-S905X-CC running Armbian mainline
• LED + 330 Ω resistor on a PWM-compatible pin of the 40-pin connector

• Stable 5 V/2.5 A power supply (micro-USB)

Complete Code

#!/bin/bash
# fondu.sh — fondu LED via sysfs PWM (adapter pwmchipN selon votre overlay)
CHIP=0                       # /sys/class/pwm/pwmchip$CHIP
OUT=/sys/class/pwm/pwmchip$CHIP
export $OUT/pwm0 2>/dev/null || echo 0 | sudo tee $OUT/export
duty_max=1000000             # période 1 ms en nanosecondes
echo $duty_max | sudo tee $OUT/pwm0/period
echo 1 | sudo tee $OUT/pwm0/enable
while true; do
  for ((d=0; d<=duty_max; d+=25000)); do
    echo $d | sudo tee $OUT/pwm0/duty_cycle >/dev/null; sleep 0.02; done
  for ((d=duty_max; d>=0; d-=25000)); do
    echo $d | sudo tee $OUT/pwm0/duty_cycle >/dev/null; sleep 0.02; done
done

Expected Verification

After enabling the overlay and rebooting, /sys/class/pwm/pwmchipN exists; after echo 0 > export, the pwm0 node appears and the LED fades in and out in a loop.

Common Errors

• PWM overlay not enabled: no /sys/class/pwm/pwmchipN linked to the header — enable the interface and reboot.
• Confusing period and duty_cycle: both are in nanoseconds; duty_cycle > period is ignored or capped.
• Writing to export without permissions: use sudo/tee.
• LED on a non-PWM pin: no effect even if sysfs responds — cross-reference with the schematic and gpioinfo.