STM32F2xx_StdPeriph_Lib NVIC_Init(): preemption priority, issue with PRIGROUP

[reich]

Posted on June 07, 2013 at 11:56

STM32F2xx_StdPeriph_Lib_V1.1.0

NVIC_Init()

 inmisc.c has problems with AIRCR.PRIGROUP< 0b011.

After reset, SCB->AIRCR.PRIGROUP ==0b000

Calculation is then asfollows:tmppriority

= 

 (

7 - PRIGROUP)  ==> 7- 0b00  ==> 7

 tmppre = 4- tmppriority

 ==> (uint_8)(-3)  ==> 0xFD

 tmpsub = 0x0F >> tmppriority  ==> 0x0F >> 7  ==>0x00

 tmppriority =

PreemptionPriority

<<tmppre  ==> (0..F) << 0xFD  ==> 0x00  // always zero, independentlyof

PreemptionPriority

tmppriority |=

SubPriority

& tmpsub

  ==> (0..F) & 0x00  ==> 0x00  // alwayszero, independently of

SubPriority

 tmppriority <<= 4

  ==> 0x00  // always zero, independently of

PreemptionPriority

or

SubPriority

 

NVIC->

IP

[

IRQChannel

]

= tmppriority

That is: given Preemption- orSubPriority has no effect, all interrupts get programmed with IP==0x00,thus having the same group priority(0).

Result: nopreemption.

WithPRIGROUP == 0b011:

tmppriority

= 

 (

7 - PRIGROUP)  ==> 7 - 0b011 ==>4

 tmppre = 4- tmppriority

 ==> 0

 tmpsub = 0x0F >> tmppriority  ==> 0x0F >> 0  ==>0x0F

 tmppriority =

PreemptionPriority

<< tmppre  ==> (0..F)<< 0  ==>

PreemptionPriority

(0..F)tmppriority |=

SubPriority

&tmpsub

  ==> (0..F) & 0x0F  ==> 

PreemptionPriority | SubPriority

(0..F) // this makes no sense, IMHO

 tmppriority <<= 4  ==>0x00..0xF0

 NVIC->IP[IRQChannel]

= tmppriority

Is this behaviour by design?What is the recommended procedure to set IRQ preemption priority using NVIC_Init()? #stm32-nvic-peripheral-library

[waclawek.jan]

Posted on June 07, 2013 at 12:11

misc.c:

 How to configure Interrupts using driver 1. Configure the NVIC Priority Grouping using NVIC_PriorityGroupConfig() JW

[reich]

Posted on June 07, 2013 at 12:18

This sets only SCR->AIRCR.PRIGROUP, but does not solve the issue mentioned.

[zzdz2]

Posted on June 07, 2013 at 12:47

Is this behaviour by design?

What is the recommended procedure to set IRQ preemption priority using NVIC_Init()?I think you are right, NVIC_Init() is likely to be buggy. Note that you don't really need the stm32 library, you are free to set priority registers as you like.You can include core_cm3.h alone, it contains everything you need to access those registers and it also has some nice functions to set priorities and groups.

[waclawek.jan]

Posted on June 07, 2013 at 14:04

> This sets only SCR->AIRCR.PRIGROUP, but does not solve the issue mentioned.

Yes, it does, if you follow the instructions, i.e. use first NVIC_PriorityGroupConfig() with one of the NVIC_PriorityGroup_x - note how they are defined in misc.h. I don't say it can't be improved, but as Knik said above, you might be better off not using it, if it does not fit your style. JW

[reich]

Posted on June 07, 2013 at 18:29

Sure, actually I'd even prefer to not use the STM32 peripheral library at all, but there are forces beyond my control that drive me to use it 😉

Anyway, programming SCR->AIRCR.PRIGROUP either directly or via NVIC_PriorityGroupConfig() still does not solve the problem that NVIC_Init() a) fails for PRIGROUP < 0b011, andb) with PRIGROUP == 0b011 ORs PreemptionPriority and SubPriority together, leading to unexpected results.Because of b), NVIC_Init() only works as expected if PreemptionPriority == SubPriority, or SubPriority == 0.

[zzdz2]

Posted on June 07, 2013 at 18:47

I think I know what's the actual problem, stm32 only has 16 priority levels so setting grouping below 0b011 doesn't make sense.

Edit:If you set grouping to 0b011 subpriority has to be zero.