STM32 Mbed CodeIDE (7/8): servo position control

PWM for an LED changes average brightness. A servo also uses repeated pulses, but the important value is the pulse width:

- about 500 microseconds: one end of travel
- about 1500 microseconds: middle
- about 2500 microseconds: other end of travel

In Mbed, the servo signal is produced with PwmOut. servo.period_ms(20) sets the usual 20 ms repetition period, and servo.pulsewidth_us(...) changes the high-pulse width that the servo interprets as position.

The exact mechanical angle can vary. Your job is to show a controlled relationship: low potentiometer value gives one position, high value gives a different position, and the serial output matches your mapping.

The camera may show the servo only as a small arm on the training board. Treat angle as an estimate calculated by your code. The strongest evidence is the combination of serial pulse_us, serial angle estimate, and a visible change in broad servo position.

Pin and evidence checklist for this lesson:

- Servo signal output: PA_15
- Potentiometer used later: Pot. pot2 -> PC_1
- Required serial evidence: pulse_us
- Camera evidence: broad servo position change, not a precise measured angle

#include "mbed.h"

PwmOut servo(PA_15);

void set_servo_us(int pulse_us) {
    servo.pulsewidth_us(pulse_us);
}

int main() {
    servo.period_ms(20);
    printf("Servo three-position test start\n");

    while (true) {
        set_servo_us(500);
        printf("pulse_us=500 position=low\n");
        ThisThread::sleep_for(2000ms);

        set_servo_us(1500);
        printf("pulse_us=1500 position=middle\n");
        ThisThread::sleep_for(2000ms);

        set_servo_us(2500);
        printf("pulse_us=2500 position=high\n");
        ThisThread::sleep_for(2000ms);
    }
}

Fill three table rows, one for each fixed pulse width: 500 us, 1500 us, and 2500 us. Copy the printed pulse width and printed position. In the Confidence column, write clear, partial, or unclear based on whether the camera showed a visible position change. If motion is unclear, write unclear and describe what you could see instead of inventing a precise angle.

Now map AnalogIn PC_1 to an approximate angle and pulse width. Use the Pot. pot2 slider for PC_1.

#include "mbed.h"

AnalogIn pot(PC_1);
PwmOut servo(PA_15);

int main() {
    servo.period_ms(20);
    printf("Potentiometer servo control start\n");

    while (true) {
        float reading = pot.read();
        int percent = (int)(reading * 100.0f);
        int angle = (int)(reading * 180.0f);
        int pulse_us = 500 + (int)(reading * 2000.0f);

        servo.pulsewidth_us(pulse_us);

        printf("percent=%d angle=%d pulse_us=%d\n",
               percent,
               angle,
               pulse_us);
        ThisThread::sleep_for(500ms);
    }
}

Fill three table rows for low, middle, and high potentiometer positions. Copy the printed percent, angle estimate, and pulse width before judging the motion. The serial pulse_us value is required evidence; the camera observation is only a broad position check.