How to handle global variables in interrupts

Question

I am a newbie in this of embedded systems. For my final degree project I am developing a system working with a STM NUCLEO Board with a STM32F411RE microcontroller.

In addition, I am working with stmcube32 for loading initialization code.

At this point I am stuck, but first let me explain you a litle background:

• I have three interrupt routines handling the same flag, as you can see in the source file stm32f4xx_it.c:

  void EXTI15_10_IRQHandler(void) {
  
      /* USER CODE BEGIN EXTI15_10_IRQn 0 */
  
      if (__HAL_GPIO_EXTI_GET_IT(BOTON1_Pin) != RESET) {
          __HAL_GPIO_EXTI_CLEAR_IT(BOTON1_Pin);
          boton_Flag = 1;
      }
      if (__HAL_GPIO_EXTI_GET_IT(BOTON2_Pin) != RESET) {
          __HAL_GPIO_EXTI_CLEAR_IT(BOTON2_Pin);
          boton_Flag = 2;
      }
      if (__HAL_GPIO_EXTI_GET_IT(BOTON3_Pin) != RESET) {
          __HAL_GPIO_EXTI_CLEAR_IT(BOTON3_Pin);
          boton_Flag = 3;
      }
      /* USER CODE END EXTI15_10_IRQn 0 */
      //HAL_GPIO_EXTI_IRQHandler(GPIO_PIN_10);
      //HAL_GPIO_EXTI_IRQHandler(GPIO_PIN_11);
      //HAL_GPIO_EXTI_IRQHandler(GPIO_PIN_12);
      /* USER CODE BEGIN EXTI15_10_IRQn 1 */
  
      /* USER CODE END EXTI15_10_IRQn 1 */
  }
  

• In the header file stm32f4xx_it.h it is declared my global variable this way:

   extern volatile uint8_t boton_Flag;
  

• BOTON_1, BOTON_2, BOTON_3 are the names assigned to the pins, which are configured as interrupts by raising trigger edge detection.

• While debugging, the three interrupts work perfectly when pressing the buttons, each one at its right time.

• The variable boton_Flag is defined in main.c this way:

  volatile uint8_t boton_Flag = 0;
  

• Inside the main function I pass my global variable boton_Flag to a function which is declared in controlpanel.h (I have #include "controlpanel.h" at the beginning of the file):

  int main(void) {
  
      /* USER CODE BEGIN 1 */
  
      /* USER CODE END 1 */
  
      /* MCU Configuration----------------------------------------------------------*/
  
      /* Reset of all peripherals, Initializes the Flash interface and the Systick.*/
      HAL_Init();
  
      /* Configure the system clock */
      SystemClock_Config();
  
      /* Initialize all configured peripherals */
      MX_GPIO_Init();
      MX_TIM3_Init();
      MX_TIM4_Init();
  
      /* Initialize interrupts */
      MX_NVIC_Init();
  
      /* USER CODE BEGIN 2 */
      init_PanelConfig();
  
      numeroJugadores(boton_Flag);
  
      /* USER CODE END 2 */
  
      /* Infinite loop */
      /* USER CODE BEGIN WHILE */
      while (1) {
          /* USER CODE END WHILE */
  
          /* USER CODE BEGIN 3 */
  
      }
      /* USER CODE END 3 */
  }
  

• The function numeroJugadores(boton_Flag); does the following:

  uint8_t numeroJugadores(uint8_t boton_Flag) {
      uint8_t players = 4;
  
      while (boton_Flag != 2);
      boton_Flag = 0;
  
      while (boton_Flag != 2) {
          if (boton_Flag == 1)
              boton_Flag = 0;
          {
              players++;
              if ((players > 4) || (players < 1) ) {
                   players = 0;
              }
          }
          while (boton_Flag != 1);
  
          switch(players) {
            case 1:
              lcd_clear();
              lcd_goto(0, 0);
              lcd_on();
              lcd_puts("Numero de");
              lcd_goto(0, 1);
              lcd_puts("jugadores: 1");
              break;
            case 2:
              lcd_clear();
              lcd_goto(0, 0);
              lcd_on();
              lcd_puts("Numero de");
              lcd_goto(0, 1);
              lcd_puts("jugadores: 2");
              break;
            case 3:
              lcd_clear();
              lcd_goto(0, 0);
              lcd_on();
              lcd_puts("Numero de");
              lcd_goto(0, 1);
              lcd_puts("jugadores: 3");
              break;
            case 4:
              lcd_clear();
              lcd_goto(0, 0);
              lcd_on();
              lcd_puts("Numero de");
              lcd_goto(0, 1);
              lcd_puts("jugadores: 4");
              break;
          }
      }
      return players;
  }
  

So, what is my problem? When debugging, I realise that the program remains stuck in the first statement while (boton_Flag != 2); inside the function above.

It should stay there until I press the BOTON2 to change the flag to value 2 and this way the program keep running. However, when I press the button, in spite of the program JUMPS TO THE INTERRUPT AND CHANGES THE VALUE, when it returns to the function, the value is again 0 (its initialized value).

My conclusion, which may be wrong, is that I am not passing the variable to the function correctly, and the program interpret it as a local variable inside the function, but I am not sure at all.

I hope to have been explained as well as possible, if not, please let me know.

Answer 1

You're right, you aren't passing the variable correctly. The way you have it, when the function is called you create a local variable (local to the function) and assign it the value of your global boton_flag.

There are two ways that you could fix this:
1. Just use the global variable in the function. If it's already global there is no need to pass it to your function.
2. Pass boton_flag by reference. i.e. numeroJugadores(&boton_Flag);. If you do it this way, you'll have a pointer to boton_flag which slightly changes the code in numeroJugadores (you have to dereference boton_flag every time you use it).