Cortex-M3 Processor (Reference Material)
UG0331 User Guide Revision 15.0
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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.
3.5.3.6
Interrupt Priority Grouping
To increase priority control in systems with interrupts, the NVIC supports priority grouping. This divides
each interrupt priority register entry into two fields:
•
An upper field that defines the
group priority
•
A lower field that defines a
subpriority
within the group.
Only the group priority determines preemption of interrupt exceptions. When the processor is executing
an interrupt exception handler, another interrupt with the same group priority as the interrupt being
handled does not preempt the handler,
If multiple pending interrupts have the same group priority, the subpriority field determines the order in
which they are processed. If multiple pending interrupts have the same group priority and subpriority, the
interrupt with the lowest IRQ number is processed first.
For information about splitting the interrupt priority fields into group priority and subpriority, see
Application Interrupt and Reset Control Register,
3.5.3.7
Exception Entry and Return
Descriptions of exception handling use the following terms:
Preemption: When the processor is executing an exception handler, an exception can preempt the
exception handler if its priority is higher than the priority of the exception being handled. See
page 41 for more information about preemption by an interrupt.
When one exception preempts another, the exceptions are called nested exceptions. See
page 41 more information.
Return: This occurs when the exception handler is completed, and:
•
there is no pending exception with sufficient priority to be serviced
•
the completed exception handler was not handling a late-arriving exception.
The processor pops the stack and restores the processor state to the state it had before the interrupt
occurred. See
page 42 for more information.
Tail-chaining: This mechanism speeds up exception servicing. On completion of an exception handler, if
there is a pending exception that meets the requirements for exception entry, the stack pop is skipped
and control transfers to the new exception handler.
Late-arriving:
This mechanism speeds up preemption. If a higher priority exception occurs during state
saving for a previous exception, the processor switches to handle the higher priority exception and
initiates the vector fetch for that exception. State saving is not affected by late arrival because the state
saved is the same for both exceptions. Therefore the state saving continues uninterrupted. The
processor can accept a late arriving exception until the first instruction of the exception handler of the
original exception enters the execute stage of the processor. On return from the exception handler of the
late-arriving exception, the normal tail-chaining rules apply.
3.5.3.7.1
Exception Entry
Exception entry occurs when there is a pending exception with sufficient priority and either:
•
The processor is in Thread mode
•
The new exception is of higher priority than the exception being handled, in which case the new
exception preempts the exception being handled.
When one exception preempts another, the exceptions are nested.