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

[Paulo Marques]

Flemming Steffensen (sent by Nabble.com) wrote:
> Hi,
>
> This is my first post to this forum. I'm not even sure it's the right 
> place to post, but I think so.
>
> I'm trying this with an ATmega8, with a 500Hz oscillator connected at 
> INT0, using WinAVR (avr-gcc 3.4.3).
>
> I noticed the problem using this code:
> ------------
>
> unsigned short RotCount;

This should be "volatile unsigned short RotCount;"

> SIGNAL(SIG_INTERRUPT0){
>         RotCount++;
> }
>
> void test(void){
>         while (1){
>                 RotCount = 0;
>                 while (RotCount < 1000) {}
>                 RotCount = 0;
>                 while (RotCount < 2000) {}
>         }
> }

Since RotCount is a multi-byte variable you need to disable interrupts 
while accessing it.

My personal preference is doing something like:

unsigned short atomic_read_short(unsigned short *addr)
{
        unsigned short ret;
        cli();
        ret = (volatile)(*addr);
        sei();
        return ret;
}

void atomic_write_short(unsigned short *addr, unsigned short value)
{
        cli();
        (volatile)(*addr) = value;
        sei();
}


Then in your main loop, just do:

void test(void){
        while (1){
                atomic_write_short(&RotCount, 0);
                while (atomic_read_short(&RotCount) < 1000) {}
                atomic_write_short(&RotCount, 0);
                while (atomic_read_short(&RotCount) < 2000) {}
        }
}

I hope this helps,

--
Paulo Marques
Software Development Department - Grupo PIE, S.A.
Phone: +351 252 290600, Fax: +351 252 290601
Web: www.grupopie.com

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