STM32 Mbed CodeIDE (6/8): PWM brightness and RGB control

Mbed uses PwmOut for PWM pins:

PwmOut red(PA_8);
red.period_ms(2);
red.write(0.25f);

PwmOut red(PA_8) creates a Mbed PWM output object. period_ms(2) sets how often the pulse pattern repeats, and write(0.25f) sets how much of each repeat is on. For an RGB LED, using the same period on red, green, and blue makes the channels behave as one coordinated color output.

write() takes a duty cycle from 0.0 to 1.0:

- 0.0f: always off
- 0.25f: on for 25 percent of each cycle
- 1.0f: always on

This lesson uses the RGB PWM pins PA_8, PA_9, and PA_10.

It also uses PC_0 as the analog control pin in the second half of the lesson. In the lab UI, PC_0 is the Pot. pot1 slider. This is different from lesson 5, which used PC_1 / Pot. pot2.

#include "mbed.h"

PwmOut red(PA_8);
PwmOut green(PA_9);
PwmOut blue(PA_10);

void set_rgb(float r, float g, float b) {
    red.write(r);
    green.write(g);
    blue.write(b);
}

int main() {
    red.period_ms(2);
    green.period_ms(2);
    blue.period_ms(2);

    printf("PWM RGB fixed levels start\n");

    while (true) {
        set_rgb(0.20f, 0.00f, 0.00f);
        printf("state=low_red r=20 g=0 b=0\n");
        ThisThread::sleep_for(2000ms);

        set_rgb(0.20f, 0.50f, 0.10f);
        printf("state=mixed r=20 g=50 b=10\n");
        ThisThread::sleep_for(2000ms);

        set_rgb(0.00f, 0.00f, 0.80f);
        printf("state=high_blue r=0 g=0 b=80\n");
        ThisThread::sleep_for(2000ms);
    }
}

Fill three table rows, one for each printed RGB state. Copy the printed state label and percentages, then describe the visible color or brightness you observed. If the camera makes a color subtle, write that in Matched expectation? instead of guessing.

Now combine analog input and PWM output. The potentiometer controls the red channel duty cycle. The serial percentage is your strongest evidence if camera exposure makes brightness subtle.

#include "mbed.h"

AnalogIn pot(PC_0);
PwmOut red(PA_8);
PwmOut green(PA_9);
PwmOut blue(PA_10);

int main() {
    red.period_ms(2);
    green.period_ms(2);
    blue.period_ms(2);

    green.write(0.0f);
    blue.write(0.0f);

    printf("Potentiometer controlled PWM start\n");

    while (true) {
        float duty = pot.read();
        int percent = (int)(duty * 100.0f);

        red.write(duty);
        printf("red_percent=%d\n", percent);

        ThisThread::sleep_for(500ms);
    }
}

Fill three table rows for low, middle, and high potentiometer positions. Copy the printed percentage and describe how the red brightness changed. If the camera exposure makes the brightness hard to judge, still copy the percentage and write "camera unclear" in Note.