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ARM9 No Task Switching

Posted by Kaushal on September 4, 2011
Hi all,

I am trying to port FreeRTOS on Samsung S3C2440 (ARM9) processor (FriendlyArm's Mini2440 board) using ADS 1.2 IDE

I am able to start the scheduler but no task switching takes place.

If I configure AppTickHook enabled from FreeRTOSConfig.h, then it is the only task being executed.

If AppTickHook is disabled then only the demo task is executed...it does not suspend (using vTaskSuspend() api) nor does it get delayed (using VTaskDelay() api).

I am using RVDS/ARM7_LPC21xx port files as reference

This is my assembly code for the SWI handler(in startup file), vPortYield and vPortYieldProcessor(both in portASM.s) which i think is the problem:

in startup.s:

$HandlerLabel HANDLER $HandleLabel

subsp,sp,#4 ;decrement sp(to store jump address)
stmfdsp!,{r0} ;PUSH the work register to stack(lr does''t push because it return to original address)
ldr r0,=$HandleLabel ;load the address of HandleXXX to r0
ldr r0,[r0] ;load the contents(service routine start address) of HandleXXX
str r0,[sp,#4] ;store the contents(ISR) of HandleXXX to stack
ldmfd sp!,{r0,pc} ;POP the work register and pc(jump to ISR)
; Setup SWI handler

In portASM.s:
; Within an IRQ ISR the link register has an offset from the true return
; address, but an SWI ISR does not. Add the offset manually so the same
; ISR return code can be used in both cases.
; Perform the context switch.
portSAVE_CONTEXT; Save current task context
LDR R0, =vTaskSwitchContext; Get the address of the context switch function
MOV LR, PC; Store the return address
BXR0; Call the contedxt switch function
portRESTORE_CONTEXT; restore the context of the selected task

Thanks in advance for any help!!!

RE: ARM9 No Task Switching

Posted by Dave on September 5, 2011
I don't know how the interrupt controller works on your chip, but for chips with a vectored controller I would expect the vector table to look something like the code below. Note this is taken from an NXP example, so the IRQ interrupt will be different on your chip. The thing I notice about your code is the SWI handler is that it is not branching directly to FreeRTOS but branching indirectly through R0. I don't know if this needs to work this way on your chip, but expect that will stop it working unless all interrupts do the exact same thing. See how it is different below.

Vectors         LDR     PC, Reset_Addr         
LDR PC, Undef_Addr
LDR PC, PAbt_Addr
LDR PC, DAbt_Addr
NOP ; Reserved Vector
LDR PC, [PC, #-0x0FF0] ; Vector from VicVectAddr

Reset_Addr DCD Reset_Handler
Undef_Addr DCD Undef_Handler
SWI_Addr DCD vPortYieldProcessor
PAbt_Addr DCD PAbt_Handler
DAbt_Addr DCD DAbt_Handler
DCD 0 ; Reserved Address
IRQ_Addr DCD IRQ_Handler
FIQ_Addr DCD FIQ_Handler

Maybe your chip has a completely different interrupt flow though. In any case, I suggest setting configUSE_PREEMPTION to 0 first, then have two tasks that yield to each other. Only when that is working add in the tick ISR. Then when that is working change configUSE_PREEMPTION to 1 and see if it still works.

RE: ARM9 No Task Switching

Posted by Kaushal on September 5, 2011
I have already tried this:

HandleSWI DCD vPortYieldProcessor

In this case, the task is executed once and the program is restarted.

I will try with configUSE_PREEMPTION 0/1 as you suggested and get back.....

RE: ARM9 No Task Switching

Posted by Kaushal on September 5, 2011

It's working....here are the changes:

In startup.s :
b HandlerSWI
LDR PC, HandlerSWI

SWI_Addr DCD vPortYieldProcessor

; Setup SWI handler
ldr r0,=HandleSWI
ldr r1,=vPortYieldProcessor
str r1,[r0]

In portASM.s:
//added "bx lr" to portYeild

bx lr

Thanks a lot davedoors!!!! Kudos!

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