Re: [avr-gcc-list] BUG? Comparing of words error.

[Dimitar Dimitrov]

On 18.1.2006 16:47, Flemming Steffensen (sent by Nabble.com) wrote:
> unsigned short RotCount;
>
> SIGNAL(SIG_INTERRUPT0){
>         RotCount++;
> }
>
> void test(void){
>         while (1){
>                 RotCount = 0;
>                 while (RotCount < 1000) {}
>                 RotCount = 0;
>                 while (RotCount < 2000) {}
>         }       
> }

I noticed two problems with this code:
 1) RotCount should be declared volatile. See AVR-Libc FAQ #1 for details.
 2) Operations with two byte variables (RotCount in this example) are NOT 
atomic. Think what will happen if the interrupt triggers between comparing 
the two RotCount bytes. The first compare will be using the old value of 
RotCount, whereas the second compare will use the new one. 

I suggest a program like this:
-------
#define enable_int0()   (GICR |= (1<<INT0))
#define disable_int0()  (GICR &= ~(1<<INT0))

volatile unsigned short RotCount;

SIGNAL(SIG_INTERRUPT0){
        RotCount++;
}

void test(void){
        unsigned short temp;
        while (1){
                enable_int0();
                RotCount = 0;
                disable_int0();

                do {
                        enable_int0();
                        temp = RotCount;
                        disable_int0();
                } while (temp < 1000);

                enable_int0();
                RotCount = 0;
                disable_int0();
                do {
                        enable_int0();
                        temp = RotCount;
                        disable_int0();
                } while (temp < 2000);
        }       
}
---------

Hope that helps.