🛠 Piloter un micro-servo avec le PWM

Objectives

Configure a PWM slice on the RP2350 (16 PWM channels, 8 double slices) at 50 Hz with sufficient resolution to generate 1 to 2 ms pulses, and drive a micro-servo in angle sweeping. Difference with the RP2040: same PWM principles, but the RP2350 allows for finer clock divisions at 150 MHz.

Hardware

• 1 × Raspberry Pi Pico 2
• 1 × micro-servo (SG90/MG90S)
• External power supply recommended for more than 2 servos; 1 servo is supported from USB 5 V with common grounds
• Pin: GP2 (PWM slice 1A output, confirmed by official Pico 2 examples)

Complete Code

/* servo.c — balayage 0..180 deg sur GP2, PWM 50 Hz */
#include "pico/stdlib.h"
#include "hardware/pwm.h"

#define SERVO_PIN 2
#define WRAP 19999   /* 125 MHz / (125 * 20000) = 50 Hz */

static void servo_angle(float deg) {
    /* 0 deg -> 1 ms, 180 deg -> 2 ms ; 1 pas = 10 ns avec clkdiv 125 */
    uint32_t level = 100 + (uint32_t)(deg * 5.0f);  /* 100..1000 */
    pwm_set_chan_level(pwm_gpio_to_slice_num(SERVO_PIN), PWM_CHAN_A, level);
}

int main(void) {
    gpio_set_function(SERVO_PIN, GPIO_FUNC_PWM);
    uint slice = pwm_gpio_to_slice_num(SERVO_PIN);
    pwm_set_clkdiv(slice, 125.0f);      /* 1 pas = 1 us / 100 ? voir texte */
    pwm_set_wrap(slice, WRAP);
    pwm_set_enabled(slice, true);

    for (float a = 0; ; a += 5) {          /* triangle 0->180->0 */
        float ang = (a <= 180.0f) ? a : 360.0f - a;
        servo_angle(ang);
        if (a >= 355.0f) a = 0;
        sleep_ms(20);
    }
}

Expected Verification

The servo regularly sweeps from 0° to 180° and returns, in a loop. On an oscilloscope: 20 ms period (50 Hz), pulse width varying from approximately 1 ms to 2 ms. Adjust the level limits according to your servo.

Common Errors

• Incorrect clock divider: with a wrap of 19999, a clkdiv of 125 yields 50 Hz; a forgotten clkdiv produces a kHz signal that causes the servo to buzz and overheat.
• Powering multiple servos from the board's 3V3 pin: the limit is ~300 mA on the 3V3 pin; use an external 5 V supply and connect the grounds.
• Exceeding mechanical limits: limit to 0–180° (some standard servos are 170°) to prevent stalling.
• Forgetting pwm_set_enabled: the counter runs but the output remains inactive.