STM32 Mbed CodeIDE (4/8): serial debugging with printf

LEDs cannot tell you what values the code was using when something went wrong. A controller often needs an instrument panel for the programmer, and the serial console provides that panel so you can find a timing or logic mismatch instead of guessing from the camera alone.

When a controller does not behave as expected, guessing is slow. Serial output lets you print values and checkpoints:

printf("cycle=%d led=%d\n", cycle, led_state);

In these Mbed programs, printf() sends text from the running STM32 board to the serial console in CodeIDE. The %d markers are replaced by integer values, and \n starts a new line so each reading is easier to copy.

This course uses integer serial output. In this Mbed environment, floating-point printf("%f") output is not a good first debugging tool. Use integers such as milliseconds, millivolts, percentages, and degrees.

In the STM32 Mbed CodeIDE lab, serial output appears in the console panel near the bottom of the lab view. CodeIDE manages the USB serial connection and baud rate for you; just compile, upload, and read the printed lines.

Plan for two builds and about 22 minutes of active lab time: one required buggy run and one corrected run. If queued, prepare the table and a five-interval timing method. Do not diagnose by changing the source before you have serial and camera evidence from build 1.

#include "mbed.h"

DigitalOut status_led(PB_5, 0);

int main() {
    int cycle = 0;
    const int interval_ms = 500;
    const auto interval = 1000ms;

    printf("STM32 Mbed serial debug start\n");

    while (true) {
        status_led = !status_led.read();
        cycle = cycle + 1;

        printf("cycle=%d led=%d interval_ms=%d\n", cycle, status_led.read(), interval_ms);

        ThisThread::sleep_for(interval);
    }
}

The first lines should look similar to this:

STM32 Mbed serial debug start
cycle=1 led=1 interval_ms=500
cycle=2 led=0 interval_ms=500
cycle=3 led=1 interval_ms=500

Fill three table rows using different cycle=... lines, not the startup line. Paste each full line. For Camera behavior at that line, record the LED state or change visible at about the same time; do not assume that raw 0/1 universally means on/off. Write a short observation such as "cycle increased by 1" or "the printed LED value alternated." Judge timing only from the five-interval comparison below.

Use only the serial line, your camera timing, and the two timing constants in the code to diagnose build 1.

After answering, replace the two timing declarations with this correction template. It contains exactly two compile-blocking placeholders. Choose either a 500 ms or 1000 ms interval, but both TODO replacements must represent the same duration. Search for TODO before compiling.

// 1: integer milliseconds printed in the serial line.
const int interval_ms = TODO_REPORTED_MS;
// 2: the same duration as an Mbed chrono literal, including ms.
const auto interval = TODO_WAIT_MS;

Save, compile build 2, upload, and repeat the same five-interval camera measurement. Only after recording the corrected evidence, click Mark practice as done.