> Originally, my (simplified) interrupt code looked like this:
>
> interrupt void isr()
>    {
>    if (TMR1IF)
>       { /* fairly long processing, but well under 500us */ }
>    if (TXIF)
>       { /* TXIF code, seems to work */ }
>    while (RCIF)
>       {
>       if (RCSTA & 0x06)  /* if OERR or FERR */
>          { /* error processing */ }
>       else
>          txbuf[txbufptr++] = RCREG;
>       }
>    }

> Bill Couture wrote :
>
> > It's actually about 300us out of the 500us between TMR1
> > interrupts.
> > And, yes, it does have to be done in the interrupt.
> > As noted elsewhere, I could use the TMR1IF flag to poll
> > when the PID loop needs to execute, but I found that the
> > resultant jitter could be seen (and heard!) in the
> > mechanical system.
>
> Note that it isn't a choice between doing *everything* in
> the ISR and doing *nothing* in the ISR (with regard to the
> TMR1 iterrupt).
>
> Let the TMR1IF generate an interrupt (as now). Then in the ISR
> just set your own "PID-flag" saying that your PID routine has
> to be run now (or rather "soon"). Then leave the ISR and
> let your main-loop detect your PID-flag and run the PID
> code. During that time, you are able to service other
> interrupts like the UART receive interrupt. Make sure that
> you make *that* ISR code as short as possible also (just
> saving the received byte) so you don't hold up your
> PID calculations unnecessarily.

> Since the PID is in the interrupt, it runs EXACTLY on
> schedule.  Although,
> there is some jitter, based on the values in the loop.  For
> example, with
> PID, Kp, and delta_position all long ints
>    PID += Kp * delta_position;
> is *MUCH* slower (about a factor of 2) if delta_position is negative
> intead of positive. In fact, if you do it right, you can save
> processing
> time with statements like
>    pos_delta_pos = (current - desired);
>    neg_delta_pos = (desired - current);
>    if (current > desired)
>       PID += Kp * pos_delta_pos;
>    else
>       PID -= Kp * neg_delta_pos;
> (you do have to be careful that you don't spend more time saving time,
> but if you're careful you can save considerable time.)