ARM9 No Task Switching

imkaush wrote on Sunday, September 04, 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 $HandleLabel
	sub	sp,sp,#4	                            ;decrement sp(to store jump address)
	stmfd	sp!,{r0}	                            ;PUSH the work register to stack(lr does''t push because it return to original address)
	ldr     r0,=$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)
b	HandlerSWI
HandlerSWI      HANDLER HandleSWI
; Setup SWI handler
	ldr	r0,=HandleSWI
	ldr	r1,=vPortYieldProcessor
	str	r1,[r0]

In portASM.s:

	SWI 0
	; 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.
	ADD	LR, LR, #4
	; 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
	BX	R0								; Call the contedxt switch function
	portRESTORE_CONTEXT				; restore the context of the selected task

Thanks in advance for any help!!!

davedoors wrote on Monday, September 05, 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, SWI_Addr
                LDR     PC, PAbt_Addr
                LDR     PC, DAbt_Addr
                NOP                            ; Reserved Vector 
                LDR     PC, [PC, #-0x0FF0]     ; Vector from VicVectAddr
                LDR     PC, FIQ_Addr
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.

imkaush wrote on Monday, September 05, 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……

imkaush wrote on Monday, September 05, 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
	SWI 0
	bx lr

Thanks a lot davedoors!!!! Kudos!