[chbot] Synco's reentrant chocolate fish code with critical bug fixes

[Peter Harris]

Hi Col et Al
The size of number_of_times_the_timer_has_overflowed_during_pulse is
irrelevant as the multi-read trick will catch overflows.

There are a couple of critical bugs in my original code which are fixed
below. There are also a couple of optimisations.

  Peter

#define TIMERWRAP     1024
static uint number_of_times_the_timer_has_overflowed_during_pulse = 0;

ISR(ExternalInterrupt)  // highest priority
{
  if (triggered on +ve edge) {
     // Start of pulse detected
     do
     {
        number_of_times_the_timer_has_overflowed_during_pulse = 0;
        count = Timer;
     }while (number_of_times_the_timer_has_overflowed_during_pulse > 0);
     //The while loop will catch an overflow occurring at the start of the
pulse
     // FIX Changed this to a do while so that count will always be set.
Previously it would only work after an overflow

     count = TIMERWRAP - count;  // This only needs to be done once.
     set to trigger on -ve edge;       // This only needs to be done once.

  }
  else {
      // end of pulse
      do
      {
         temp_overflow_count =
number_of_times_the_timer_has_overflowed_during_pulse;
         temp_timer = Timer;
      } while (temp_overflow_count !=
number_of_times_the_timer_has_overflowed_during_pulse);
      // The while loop will catch an overflow occurring at the end of the
pulse

      count += temp_timer +(temp_overflow_count * TIMEWRAP);
      // FIX moved count calculation out of the while loop so it will only
be updated once.

      set to trigger on +ve edge; // This only needs to be done once too
  }
}

ISR(TimerOverflow)   // overflows at TIMERWRAP ticks
{
  number_of_times_the_timer_has_overflowed ++;
  ....
}

ISR(other)
{
 ...
}
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