🛠 Servo moteur sur PWM0 (GPIO18) : 50 Hz et 2,5 à 12,5 %

Objectives

Control a standard angle servo using the CM1's hardware PWM. On the BCM2835, the PWM0 channel is routed to GPIO18 via alternate function. A standard servo expects a 20 ms period (50 Hz) and reads position based on pulse width: approximately 1 ms at center, with the useful range extending from 0.5 ms (2.5% duty cycle) to 2.5 ms (12.5%). Configure the PWM via the pwm device tree overlay, then use sysfs/pigpio, avoiding software PWM which suffers from jitter.

Hardware

• A Compute Module 1 on an IO Board
• 1 angle servo (SG90/MG90S) powered separately at 5 V if high torque is required
• GPIO18 (pin 12 on J6) connected to the signal wire, with common grounds

Complete Code

# Activer le PWM matériel
echo 'dtoverlay=pwm-28' | sudo tee -a /boot/config.txt
sudo reboot

# Pilotage pigpio (daemon PWM matériel)
sudo pigpiod
pigs p 18 0            # arrêt
pigs p 18 64           # ~1,56 % -> butée gauche (50 Hz)
pigs p 18 128          # ~3,1 % -> centre
pigs p 18 256          # ~6,25 %
pigs p 18 512          # ~12,5 % -> butée droite

# Balayage doux
for d in 64 96 128 160 192 224 256; do pigs p 18 $d; sleep 0.3; done

Expected Verification

The servo moves decisively to each setpoint and holds its position without excessive buzzing. The useful range extends between pigs p 18 64 (0.5 ms/2.5%) and pigs p 18 512 (2.5 ms/12.5%) for an 8-bit output at 50 Hz; beyond 12.5%, the servo hits its mechanical stop and draws excessive current. Hardware PWM does not exhibit the dropouts associated with software methods during SD card access.

Common Errors

• Using software PWM (pwm mode without hardware): micro-interruptions cause the servo to jitter; verify that the overlay is active (ls /sys/class/pwm).
• Powering the servo from the CM1's 3.3 V rail: insufficient current causes the SoC to reset when the motor starts; use an external 5 V supply with a common ground.
• Duty cycle out of range: above 12.5% or below 2.5%, the servo stalls at its limits and overheats.
• Confusing PWM0 (GPIO18) and PWM1 (GPIO13): the alternate function pinout differs; consult the BCM2835 function table.
• Forgetting the common ground between the servo power supply and the CM1: the servo vibrates randomly.