Conditions in Timers (STM32F103C6)

Question

I am developing a project on STM32F103C6 and am new to it. I've implemented a timer with interrupt, which checks the temperature value measured by a sensor(LM35) every second, and if it is more than 38 degrees, the system triggers a buzzer.

The system triggers the buzzer as expected but the system cannot read temperature value from the sensor anymore. That is to say, it stucks in the timer.

Here is the piece of code which performs this part:

void HAL_TIM_PeriodElapsedCallback(TIM_HandleTypeDef *htim)
{
     if(htim == &htim2){
         
         char temp_string[2];
         
        sprintf(temp_string,"%d",temperature);
        
        if(strcmp(temp_string,"38")>0){
            HAL_GPIO_WritePin(GPIOB,GPIO_PIN_3,GPIO_PIN_SET);
            
            
        }
        else{
            HAL_GPIO_WritePin(GPIOB,GPIO_PIN_3,GPIO_PIN_RESET);
        }
        
        
         
         
     }
}

What can be the reason for the problem?

Any help is appreciated.

Answer 1

The first error is not having enough room for the string. You need at least 3 characters to hold two characters and the terminating null character. Basically, if the temperature is larger than 9 degrees, it won't fit and trashes your stack.

The next error is trying to convert the number into string in interrupt context. Depending on what other things you are doing, it may not work. If you are sure that your sprintf() is re-entrant then go ahead, but in general, you just don't do conversions to stringa and comparing strings in interrupts.

The string conversion step is useless extra step anyway, as you can just compare the temperature with an integer and achieve the same result faster.