Re: [avr-gcc-list] Problem with interrupt on a MEGA8535

[Paulo Marques]

dlc wrote:
> I have an odd problem and I'm hoping that by trying to explain it I can 
> solve it...
>
> What I have is an timer overflow ISR that tics off every 10us.  I've 
> coded it thusly:
>
> // variables at the top (global)
> //ISR variables, volatile by nature
> volatile uint16_t t_10us=0;
> volatile uint32_t t_1ms=0;
> volatile uint16_t repeat=0;
> volatile uint16_t  match=0;        // The single servo used
> uint8_t    servo = 150;
>
> // past initializations and such ...
>
> SIGNAL(SIG_OUTPUT_COMPARE0)
> /*
>  * 10 microsecond ISR
>  */
> {
>     t_10us++;
>     if (t_10us > 100)
>     {
>         t_1ms++;            //1ms background clock
>         t_10us = 0;
>         repeat++;
>         if (repeat == 19)
>         {
>             repeat = 0;
>             SPIN = 1;        //raise servo pin high
>             match = 0;        //start servo timer
>         }
>     }
>     match++;                //increment every 1
>     if (match == servo)
>     SPIN=0;                //drop servo bit
> }

You really should just use the output compares to do timed pulses, 
instead of wasting all your cpu power just doing this.

Anyway, you problem is that you're updating servo in the main loop while 
updating match in the interrupt, so chances are they are running paste 
each other...

The proper way to write that code (apart from using output compares) 
would be something like:


volatile uint16_t next_pulse;

SIGNAL(SIG_OUTPUT_COMPARE0)
{
    match++;                   // increment every 1
    if (match > next_pulse)
      SPIN = 0;                // drop servo bit        
    else
      SPIN = 1;                // raise servo pin high

    if (match >= 20000) {
      next_pulse = servo;
      match = 0;                //start servo timer
    }
}


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

"Prediction is hard. Especially of the future."
Niels Bohr