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

[Dave Hansen]

From: "Flemming Steffensen (sent by Nabble.com)" <address@hidden>
[...]
> I noticed the problem using this code:
> ------------
>
> unsigned short RotCount;
>
> SIGNAL(SIG_INTERRUPT0){
>         RotCount++;
> }
>
> void test(void){
>         while (1){
>                 RotCount = 0;
>                 while (RotCount < 1000) {}
>                 RotCount = 0;
>                 while (RotCount < 2000) {}
>         }
> }
> ------------
>
> My problem is that the two inner whiles sometimes exists prematurly.

You have two problems.

First, RotCount should be declared volatile, since it's shared between 
main-line code and an ISR.  This will prevent the compiler from optimizing 
access to RotCount away completely inside the loop.

Second, and the one whose symptoms you're seeing, RotCount is a 16-bit 
value.  Occasionally, you're getting an interrupt between the comparisons of 
the first and second bytes.

You need to protect access to RotCount from interruption.  For example, you 
could change the first loop to something like

  unsigned short temp;
  do
  {
     Disable();   // disable interrupts
     temp = RotCount;
     Enable();   // Re-enable interrupts

  } while (temp < 1000);

You of course need to do something similar for the second loop, and whenever 
you write RotCount as well.

HTH,
  -=Dave