servo-pwm-sysfs.sh
The Linux kernel PWM interface (sysfs) is identical on every board: 20 ms period, 0.5-2.5 ms pulse. This bash script sweeps a servo over 3 cycles then cuts power to stop the buzzing.
Hardware required
A BeagleBone AI-64 (TI J721E, 2 GHz) on the official Debian image, powered by 5 V/4 A USB-C, with its two 3.3 V P8/P9 cape headers. 1 hobby servo + external 5 V supply (never the board 5 V beyond a micro servo).
Wiring
Servo signal on the connector PWM pin (channel exposed in /sys/class/pwm once enabled), external supply, common grounds.
How it works
export creates the channel folder; period/duty_cycle are in nanoseconds. servo_angle converts the angle into a pulse; enable cleanly stops the signal. On Debian: sudo apt install gpiod python3-gpiod; gpiodetect lists the chips (main one is 600000.gpio). The P8/P9 pins are named in gpioinfo.
Going further
Wrap the function in a small Python daemon (python3-pwmio or sysfs writes) for remote control.
servo-pwm-sysfs.sh
#!/bin/bash
# Servo d'angle via /sys/class/pwm (interface noyau standard).
# Exemple pour le canal PWM 0 du connecteur (a adapter a la carte :
# ls /sys/class/pwm/ apres activation du PWM dans le firmware/overlay).
PWM="/sys/class/pwm/pwmchip0"
CANAL=0
# Exporter le canal s'il n'existe pas.
if [ ! -d "$PWM/pwm$CANAL" ]; then
echo $CANAL > "$PWM/export" 2>/dev/null || {
echo "PWM $CANAL indisponible : activer le canal (overlay/device tree)"; exit 1; }
sleep 1
fi
# Configuration : frequence 50 Hz = periode 20 000 000 ns.
echo 20000000 > "$PWM/pwm$CANAL/period"
echo 1500000 > "$PWM/pwm$CANAL/duty_cycle" # 1,5 ms = 90 degres
echo 1 > "$PWM/pwm$CANAL/enable"
servo_angle() { # $1 = angle 0..180
local ns=$((500000 + $1 * 2000000 / 180)) # 0,5..2,5 ms en ns
echo $ns > "$PWM/pwm$CANAL/duty_cycle"
}
for cycle in 1 2 3; do
for a in $(seq 0 10 180); do servo_angle $a; sleep 0.1; done
for a in $(seq 180 -10 0); do servo_angle $a; sleep 0.1; done
done
echo 0 > "$PWM/pwm$CANAL/enable" # coupe l'alimentation du servo
# Penser a « echo $CANAL > $PWM/unexport » au propre.