User (Software) Priority

User-assigned interrupt priority is enabled by setting IPEN. Each peripheral interrupt source can be assigned a high- or low-priority level by the user. The user-assignable interrupt priority control bits for each interrupt are located in the IPRx registers, which are device-specific and can be found in the respective data sheet for each device.

The interrupts are serviced based on a predefined interrupt priority scheme detailed below.

  1. 1.Interrupts set by the user as a high-priority interrupt have higher precedence of execution. High-priority interrupts will override a low-priority request when:
    1. a.A low-priority interrupt has been requested or its request is already pending.
    2. b.A low and high-priority interrupt are triggered concurrently (i.e., on the same instruction cycle).(1)
    3. c.A low-priority interrupt was requested and the corresponding Interrupt Service Routine is currently executing. In this case, the lower priority interrupt routine will be interrupted then complete executing after the high-priority interrupt has been serviced.(2)
  2. 2.Interrupts set by the user as low priority have a lower priority of execution and are preempted by any high-priority interrupt.
  3. 3.Interrupts defined with the same software priority cannot preempt or interrupt each other. Concurrent pending interrupts with the same user priority are resolved using the natural order priority (when vectored interrupts are enabled) or in the order the interrupt flag bits are polled in the ISR (when vectored interrupts are disabled).
Important:
  1. 1.When a high-priority interrupt preempts a concurrent low-priority interrupt, GIEL may be cleared in the high-priority Interrupt Service Routine. If GIEL is cleared, the low-priority interrupt will NOT be serviced, even if it was originally requested. The corresponding interrupt flag needs to be cleared in user code.
  2. 2.When a high-priority interrupt is requested while a low-priority Interrupt Service Routine is executing, GIEL may be cleared in the high-priority Interrupt Service Routine. The pending low-priority interrupt will resume, even if GIEL is cleared.