SysTick interrupt priority

[ivan239955_stm1_stmicro_com]

Posted on December 17, 2010 at 13:56

SysTick interrupt priority

[mikeshanahan9]

Posted on May 17, 2011 at 14:18

Hi,

I’ve been having fun with SysTick’s priority too!

It appears that, because it’s a ‘core’ function, it is described within PM0056 (15491 Rev 3), rather than RM0008.

Its priority is set by the PRI_15 field within SCB_SHPR3 (System Control Block, System Handler Priority Register) – see PM0056 p139.

It would seem that altering the Group setting would affect it (and all other ‘system’ priorities), although I can’t find an explicit reference to this.

It defaults to 0x00 (highest possible priority), although a catch is that core_cm3.h: SysTick_Config() overrides it to 0x0F (read back as 0xF0), but without explaining why!

You can easily override it back by calling NVIC_SetPriority(), but I’m nervous about this – why did ARM think that it needed to be overriden to the lowest priority (or is this a bug…)?

Perhaps others know more?

Best Regards,

MikeShan.

[ivan239955_stm1_stmicro_com]

Posted on May 17, 2011 at 14:18

Thanks for a valuable information.

So I understand that I should set the SysTick priority according to my intention after SysTick_Config(), NVIC_PriorityGroupConfig() and NVIC_Init() for HW interrupt sources have been called.

As I use several HW interrupts with different interrupt periods and different time consumption of handlers I must draft a time budget initially. 

[Tesla DeLorean]

Posted on May 17, 2011 at 14:18

Internally SysTick is number 15 within the core, and that is higher than all the IRQ's generated by the peripherals.

You need to tell NVIC how to order them (priority/preemption) to fit  *your* application and usage. That's why they are programmable.

 

Seeing as a lot of people slap a ridiculous amount of processing in the SysTick interrupts, it usually make sense to give it a lower priority, so that quick routines servicing hardware efficiently can get their job done and leave.

If you are simply incrementing a counter with the SysTick interrupt, and leaving immediately, and want low latency, give it a high priority. If you have a 1 ms SysTick, and your ''tasks'' are sucking 500 us then give it a low priority.