NVIC Priority Groups

Question

Posted on April 07, 2014 at 14:48I have read through Yiu's Cortex M3 book on the NVIC, and read this section of the COrtex M3 manual (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0337e/ch05s03s02.html), and am still missing the answer to one vital, basic question: does the Priority Group have any bearing on the actual priority assigned to an IRQ, or do the groups just allow you to have available different numbers of preemptive priorities/subpriorities? So for example, if I want 16 subpriorities, I have to use Group 0, as its the only group with 4 bits available for subpriorities.  But if I've assigned an IRQ to Group 0, subpriority 0, is this necessarily higher than one assgined to Group 1, pre-emption priority 0, subpriority 0? If not, how are these arbitrated?  EDIT: Or, do I assign a Preemption Priority to a particular group (which could be any of the four), and the IRQs that are then included in that group would all have the same preemption priority (but possibly different subpriorities)?

Tesla DeLorean · Answer

Posted on April 07, 2014 at 16:45Priority is controlled by the sub-priority, and the NVIC# when that is ambigous. If you want specific interrupts to break into others, they must have a lower pre-emption level. Otherwise the ordering is managed as the interrupt exits, and the processor makes a tail-chaining decision. Ideally your interrupts should be brief and simple, if things will take a long time queue the work so it can be handled in a deferred manner. This can avoid a lot of grief with priorities, and priority inversions when resources are locked.

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