CODEWITHSUNDEEP
cloopsinterruptavrstm32
Anddo
Anddo

Reputation: 23

Loop "while" is not satisfying both conditions with interrupt

I'm waiting some time for a real-world event (e.g. push a button for 3 seconds) on an AVR or STM32 MCU, and I have trouble with code like:

#define PRESS_BUTTON
int waiting = 0;
int t_ms = 0; // time counter
//...

int main(void)
{           
    while(1)
    {           
        waiting = t_ms + 3000; // waiting button 3 sec

        while ((t_ms < waiting) && (!PRESS_BUTTON)) // infinite loop
        {}                  
        printf("out"); // not printed
        waiting = t_ms = 0;
    }    
}

ISR( TIMER0_OVF_vect ) // timer interrupt
{
    t_ms++;
}

But if I add a printf() inside the while loop, it works!

The same thing happens if I use a do...while loop either. What is causing this?

Upvotes: 2

Views: 621

Answers (1)

Dellowar
Dellowar

Reputation: 3352

You need to declare t_ms with volatile 

volatile int t_ms =0;

Volatile tells the compiler that the variable may be changed due to external factors, and because of this the compiler will never assume it will stay the same.

In other words, it will force the compiler to check every loop to see if t_ms has changed instead of assuming it will never change.

Upvotes: 3

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