2.5.3.5 Exception Priorities

As Table 2-16 shows, all exceptions have an associated priority, with:

A lower priority value indicating a higher priority
Configurable priorities for all exceptions except Reset, HardFault, and NMI

If software does not configure any priorities, then all exceptions with a configurable priority have a priority of 0. For information about configuring exception priorities see 2.7.2.8 System Handler Priority Registers and 2.7.1.8 Interrupt Priority Registers.

Important: Configurable priority values are in the range 0–255. This means that the Reset, HardFault, and NMI exceptions, with fixed negative priority values, always have higher priority than any other exception.

For example, assigning a higher priority value to IRQ[0] and a lower priority value to IRQ[1] means that IRQ[1] has higher priority than IRQ[0]. If both IRQ[1] and IRQ[0] are asserted, IRQ[1] is processed before IRQ[0].

If multiple pending exceptions have the same priority, the pending exception with the lowest exception number takes precedence. For example, if both IRQ[0] and IRQ[1] are pending and have the same priority, then IRQ[0] is processed before IRQ[1].

When the processor is executing an exception handler, the exception handler is preempted if a higher priority exception occurs. If an exception occurs with the same priority as the exception being handled, the handler is not preempted, irrespective of the exception number. However, the status of the new interrupt changes to pending.