Polling or Interrupt based method

When deciding upon polling or interrupt you have to fully understand the nature of the event you are trying to follow and your response to it.

Interrupts require no processing when nothing is happening, but require all your attention when something is happening. If the event is external and has noisy edges or fast pulses then this can cause major headaches with interrupts, you have to be careful around the setup of interrupts.

In this example the interrupt routine is responding to a laser beam having become clear and is setting itself up for an event where it becomes blocked:

   BEAM_INTR_EN = TRUE;   /*re-enable the beam interrupts*/

   /*Set the beam interrupt for the next clear to blocked event*/
   BEAM_INTR_EDGE = CLEAR_TO_BLOCKED;
   BEAM_INTR_FLAG = FALSE; /*Clear the interrupt*/

There are 2 weak points of this code: 1) If the laser beam has become blocked again before the interrupt flag is cleared (BEAM_INTR_FLAG = FALSE;). The interrupt will have been missed and the code will be out of sync with the laser beam state.

2) When setting up interrupts in either in the background routine or for a higher priority than the priority this code is on, care must be taken when enabling the interrupt. If the interrupt flag was already set (incorrectly) prior to it being enabled, the interrupt routine would be called incorrectly as soon as it was enabled and maybe for the wrong edge.

The easiest way to fix 1) is to double check after you set up the interrupt, if it has occurred then force an interrupt. To fix 2) move the enabling of the interrupts to after the the double check:

   /*Set the beam interrupt for the next clear to blocked event*/
   BEAM_INTR_EDGE = CLEAR_TO_BLOCKED;
   BEAM_INTR_FLAG = FALSE; /*Clear the interrupt*/

   /*Double check beam state to see if it has already gone blocked*/
   if (BEAM_STATE == BEAM_BLOCKED)
   {
      BEAM_INTR_FLAG = TRUE; /*Force the in