Interrupt vectors - Porting to STR75x/Rowley

ogradyg wrote on Friday, March 14, 2008:

I’m working on porting FreeRTOS over to an existing STR75x based hardware platform and I’m using the Rowley Crossworks tools.

I’ve got the basics converted over from the STR75x RIDE/GCC port but I’m not sure what the best way to get the interrupt vectoring working is.  I’ve used the Rowley/SAM7 FreeRTOS port on another project and after reviewing it, it appears that the vectoring functions are included in the Atmel supplied libraries.

The work I’ve done in the past with Rowley and the STR75x used no RTOS and in that case, I used the Rowley ctl library functions for configuring vectors.  The STR75x RIDE demo used a startup file from ST with modifications from Raisonance and FreeRTOS and I’m not sure what the beset way to proceed is.

davedoors wrote on Friday, March 14, 2008:

The important thing with FreeRTOS is that there are no instructions prior to portSAVE_CONTEXT being called. At least if there are instructions then all registers are put back to their interrupted state before portSAVE_CONTEXT is called.

There are two methods of doing this on the LPC2000 and SAM7 and I guess the same is true for STR75x.

The first method is to vector directly to the interrupt handler, and have the handler save and restore the context. This is the sort of thing described on the following page Scroll down to the section headed "Interrupt service routines". When this technique is used the IRQ vector must go directly to the handler. So the IRQ vector loads the handler address directly from the interrupt controller. So using the LPC2000 as an example, the vector table looks like this:

    b     _start        /* reset - _start            */
    ldr   pc, _undf        /* undefined - _undf        */
    ldr   pc, _swi        /* SWI - _swi                */
    ldr   pc, _pabt        /* program abort - _pabt    */
    ldr   pc, _dabt        /* data abort - _dabt        */
    nop                                    /* reserved                    */
    ldr   pc, [pc,#-0xFF0]    /* IRQ - read the VIC        */
    ldr   pc, _fiq        /* FIQ - _fiq                */

_undf:  .word __undf                    /* undefined                */
_swi:   .word vPortYieldProcessor       /* SWI                        */
_pabt:  .word __pabt                    /* program abort            */
_dabt:  .word __dabt                    /* data abort                */
_fiq:   .word __fiq                     /* FIQ                        */

__undf: b     .                         /* undefined                */
__pabt: b     .                         /* program abort            */
__dabt: b     .                         /* data abort                */
__fiq:  b     .                         /* FIQ                        */

with the line ldr   pc, [pc,#-0xFF0] being the IRQ vector loading the program counter with the address provided by the interrupt controller.

The second technique is to have a single IRQ handler entry point that calls portSAVE_CONTEXT first, then looks at the interrupt controller to find the address of the application handler, jumps to the handler, then returns to the single entry point to restore the context of the new task. This is also demonstrated in the FreeRTOS download. Look at the crt0_STR75x_FreeRTOS.s file in the Demo\ARM7_STR75x_GCC\SystemFiles directory and you will see how this is done. In this file the vector table looks like this

        LDR     PC, Reset_Addr
        LDR     PC, Undefined_Addr
        LDR     PC, SWI_Addr
        LDR     PC, Prefetch_Addr
        LDR     PC, Abort_Addr
        NOP                          /*; Reserved vector*/
        LDR     PC, IRQ_Addr
        LDR     PC, FIQ_Addr

Reset_Addr      : .long     Reset_Handler
Undefined_Addr  : .long     UndefinedHandler
SWI_Addr        : .long     SWIHandler
Prefetch_Addr   : .long     PrefetchAbortHandler
Abort_Addr      : .long     DataAbortHandler
                  .long 0      /*; Reserved vector*/
IRQ_Addr        : .long     IRQHandler
FIQ_Addr        : .long     FIQHandler

and IRQHandler like this


    portSAVE_CONTEXT                    /*; Save the context of the current task. */

    LDR    r0, =EIC_base_addr
    LDR    r1, =IVR_off_addr
    LDR    lr, =ReturnAddress            /*; Load the return address. */
    ADD    pc,r0,r1                        /*; Branch to the IRQ handler. */
    LDR    r0, =EIC_base_addr
    LDR    r2, [r0, #CICR_off_addr]        /*; Get the IRQ channel number. */
    MOV    r3,#1
    MOV    r3,r3,LSL r2
    STR    r3,[r0, #IPR_off_addr]        /*; Clear the corresponding IPR bit. */
    portRESTORE_CONTEXT                    /*; Restore the context of the selected task. */

This is code that will work on you CPU directly as it is from an STR750 GCC example.

I would just copy the files from the STR750 example.