STM32 Mbed CodeIDE (3/8): buttons, pull-ups, and debounce

The CodeIDE guide lists PC_5, PC_6, and PC_10 among the button/sensor pins available in the STM32 WB55RG lab. This lesson uses PC_5 first and teaches you to verify the pressed value before building logic around it.

Many button circuits use a pull-up resistor:

- released: reads 1
- pressed: reads 0

That is called active-low. Do not assume it blindly; test it.

DigitalIn button(PC_5, PullUp);
int raw = button.read();

DigitalIn is the Mbed input object for reading a pin. PullUp asks Mbed to use the microcontroller's internal pull-up resistor, so the pin has a defined value when the button is released. button.read() returns the current 0/1 value at the moment your code calls it.

The remote lab UI has round button controls. In this lesson, use B1 for PC_5.

#include "mbed.h"

DigitalIn button(PC_5, PullUp);
DigitalOut led(PB_13, 1);

void led_on() {
    led = 0;
}

void led_off() {
    led = 1;
}

int main() {
    printf("Button probe start\n");

    while (true) {
        int raw = button.read();

        if (raw == 0) {
            led_on();
        } else {
            led_off();
        }

        printf("button_raw=%d\n", raw);
        ThisThread::sleep_for(500ms);
    }
}

Record the released and pressed readings separately. Copy the exact button_raw number from the console.

Example serial lines should look like this:

button_raw=1
button_raw=0

Now use B1 to start a three-LED sequence. The program treats button_raw=0 as pressed, matching the released/pressed evidence you just recorded.

Mechanical buttons can bounce: during one press, the electrical signal may rapidly switch between 0 and 1 for a few milliseconds. Debouncing means waiting briefly and checking again so one physical press is not treated as several presses.

The running variable marks that the sequence is active. The program waits 50 ms after the first pressed reading and checks the button again before accepting the press. While run_sequence() is executing, the program is busy running the LED sequence and does not accept new presses. After the LEDs turn off, it waits until the button is released before it accepts another press.

#include "mbed.h"

DigitalIn button(PC_5, PullUp);
DigitalOut led1(PB_13, 1);
DigitalOut led2(PB_14, 1);
DigitalOut led3(PB_15, 1);

void led_on(DigitalOut &led) {
    led = 0;
}

void led_off(DigitalOut &led) {
    led = 1;
}

void all_off() {
    led_off(led1);
    led_off(led2);
    led_off(led3);
}

void run_sequence() {
    led_on(led1);
    ThisThread::sleep_for(500ms);
    led_on(led2);
    ThisThread::sleep_for(500ms);
    led_on(led3);
    ThisThread::sleep_for(2000ms);
    all_off();
}

int main() {
    const int pressed_value = 0;
    bool running = false;
    all_off();
    printf("Button sequence start\n");

    while (true) {
        bool pressed = (button.read() == pressed_value);

        if (pressed && !running) {
            ThisThread::sleep_for(50ms);
            if (button.read() != pressed_value) {
                continue;
            }

            running = true;
            printf("press detected: running sequence\n");
            run_sequence();
            printf("sequence complete; release button\n");
            while (button.read() == pressed_value) {
                ThisThread::sleep_for(20ms);
            }
            running = false;
            printf("ready for next press\n");
        }

        ThisThread::sleep_for(20ms);
    }
}

Record three sequence tests. For each test, write the expected behavior before you test, then record the observed LED behavior and serial evidence after you test.