3.1. Blinky demo

The ‘blinky’ demo is in the directory app/blinky.

3.1.1. Building and running the demo

To build a new firmware and to flash the board with it, follow the instructions in Quick start.

To choose the ‘blinky’ profile, you will have to execute make with

$ make boards/32f407disco/configs/disco_blinky_ada_defconfig

3.1.2. Understanding the demo code LEDs configuration

To use a device, first, it must be declared and registered in the kernel. A device_t structure is associated to the LEDs. It describes its memory mapping, the GPIOs and the EXTIs of the device.

device_t    leds;
memset (&leds, 0, sizeof (leds));

strncpy (leds.name, "LEDs", sizeof (leds.name));
leds.gpio_num = 4; /* Number of configured GPIO */

leds.gpios[0].kref.port = GPIO_PD;
leds.gpios[0].kref.pin = 12;
leds.gpios[0].mask     = GPIO_MASK_SET_MODE | GPIO_MASK_SET_PUPD |
                         GPIO_MASK_SET_TYPE | GPIO_MASK_SET_SPEED;
leds.gpios[0].mode     = GPIO_PIN_OUTPUT_MODE;
leds.gpios[0].pupd     = GPIO_PULLDOWN;
leds.gpios[0].type     = GPIO_PIN_OTYPER_PP;
leds.gpios[0].speed    = GPIO_PIN_HIGH_SPEED;

leds.gpios[1].kref.port = GPIO_PD;
leds.gpios[1].kref.pin = 13;
leds.gpios[1].mask     = GPIO_MASK_SET_MODE | GPIO_MASK_SET_PUPD |
                         GPIO_MASK_SET_TYPE | GPIO_MASK_SET_SPEED;

Then, the sys_init() syscall is used to register the device with the kernel:

ret = sys_init(INIT_DEVACCESS, &leds, &desc_leds);

The desc_leds variable is a descriptor returned by the syscall. It’s ignored in that example (it’s used only by some few syscalls sys_cfg, configuring devices). Button configuration

The button must also be declared and registered with the kernel. Note the registration of the ISR handler with:

button.gpios[0].exti_trigger = GPIO_EXTI_TRIGGER_RISE;
button.gpios[0].exti_lock    = GPIO_EXTI_UNLOCKED;
button.gpios[0].exti_handler = (user_handler_t) exti_button_handler;

The GPIO_EXTI_TRIGGER_RISE configures the IRQ associated to the GPIO to be triggered only on a rising edge (corresponding to a button push in our case).

The GPIO_EXTI_UNLOCKED has a special meaning. The GPIO_EXTI_LOCKED means that the EXTI line is muted and that it must be voluntary “unlocked” using a specific syscall to receive further EXTIs.

The exti_handler field is initialized with the address of the ISR handler that will be executed for each EXTI interrupt. Leaving the initialization phase

When all devices are registered, they still can’t be used by the app. Before, the initialization phase must be leaved using the sys_init(INIT_DONE) syscall before using them:

/* Devices and resources registration is finished */
ret = sys_init(INIT_DONE);

Be aware that after that, no more further device or resource registration is possible. ISR handler

In our example, the ISR handler exti_button_handler() set the global variable button_pushed to notify the interrupt event:

void exti_button_handler ()
  uint64_t        clock;
  e_syscall_ret   ret;

  /* Syscall to get the elapsed cpu time since the board booted */
  ret = sys_get_systick(&clock, PREC_MILLI);

  if (ret == SYS_E_DONE) {
          /* Debounce time (in ms) */
          if (clock - last_isr < 20) {
              last_isr = clock;

  last_isr = clock;
  button_pushed = true;

The only subtlety here is the debouncing handling inside the ISR to avoid burst of interrupts. The debouncing time is arbitrary fixed to 20 milliseconds. The sys_get_systick() syscall is used to return elapsed CPU time since the board booted. Main loop

After the initialization phase, the main function executes a loop that waits for interrupt notifications by checking the value of button_pushed. When the Button is pushed, LEDs blinking pattern is switched.

while (1) {

    if (button_pushed == true) {
        printf ("button has been pressed\n");

        /* Change leds state */
        green_state   = (green_state == ON) ? OFF : ON;
        orange_state  = (orange_state == ON) ? OFF : ON;
        red_state     = (red_state == ON) ? OFF : ON;
        blue_state    = (blue_state == ON) ? OFF : ON;

        /* Show leds */
        display_leds  = ON;

        button_pushed = false;

To make the LEDs blinking, their related GPIO must be set to ON of OFF using the sys_cfg() syscall:

if (display_leds == ON) {
    ret = sys_cfg(CFG_GPIO_SET, (uint8_t) leds.gpios[0].kref.val, green_state);
    if (ret != SYS_E_DONE) {
        printf ("sys_cfg(): failed\n");
        return 1;
} else {
    ret = sys_cfg(CFG_GPIO_SET, (uint8_t) leds.gpios[0].kref.val, 0);
    if (ret != SYS_E_DONE) {
        printf ("sys_cfg(): failed\n");
        return 1;

Then, the task sleeps 500 milliseconds:

/* Sleeping for 500 ms */

If the button is pushed during that sleeping time, the task is awake due to the SLEEP_MODE_INTERRUPTIBLE option.