Finding the source of a Hard Fault - C embedded ARM Cortex-M4 32b

Question

I am new in C embedded. Debugging an embedded system for image camera tracking I get the following HardFaultHandler:

The Atollic Debug stops it at this point without apparently indications of specific errors.

__weak void DefaultHardFaultHandle ( void ){
asm volatile(
 " tst lr,#4 \n"
 " ite eq \n"
 " mrseq r0,msp \n"
 " mrsne r0,psp \n"
 " mov r1,lr \n"
 " ldr r2,=HardwareFaultHandler_GetSP \n"
 " bx r2"
 );

I dont have bunch of memory locations, however, how can I make conclusions on which line of code caused the issue according to these locations? This is part of the code, please help me:

uint8_t CameraImageTracker(uint8_t **edgeImage){
    ......

for (y = xRight.yStart; (y < height) && (exit == false); y++)
{
    xRight.yStart = y;

    int x = 0;
    for (x = 0; (x < width) && (exit == false); x++)
    {
        if (edgeImage[y][x] == grayScale)
        {
            xRight.xStart = x;
            xRight.yStart = y;

            CountPixelX(width, height, &xRight, edgeImage, grayScale);

            if (xRight.count > WhiteLinesPixMin)
            {
                exit = true;
            }
        }
    }

    WhiteLinesPixMin = xRight.count;
    WhiteLinesPixMax = (WhiteLinesPixMin + 5);

    if (exit == true)
    {
        exit = false;

        xLeft.xStart = xRight.xStart;
        xLeft.yStart = xRight.yStart;

        CountPixelXleft(width, height, &xLeft, edgeImage, WhiteLinesPixMax, grayScale);

        yLeft.xStart = xLeft.xEnd;
        yLeft.yStart = xLeft.yEnd;

        CountPixelY(width, height, &yLeft, edgeImage, grayScale);

        yRight.xStart = xRight.xEnd;
        yRight.yStart = xRight.yEnd;

        CountPixelY(width, height, &yRight, edgeImage, grayScale);

        ellipseCenter(&xRight, &yRight, &xLeft, &yLeft);

        exit = true;
    }
}
return 0;

so as you commented out this code or used other debug techniques to narrow in on the offending line, what did you see? How much further does this code reduce and still break? — old_timer

user6556709 user6556709 · Accepted Answer · 2019-05-03T17:25:24

By the code you have posted I can't see a source of your hardfault. But the source of the hardfault can be found by inspecting the stack. The Cortex-M4 should push the old register values onto the stack when entering a hard fault. On the stack it should look like this:

sp + 0x00 =  R0
sp + 0x04 =  R1
sp + 0x08 =  R2
sp + 0x0C =  R3
sp + 0x10 =  R12
sp + 0x14 =  LR
sp + 0x18 =  PC <- That is the one you need
sp + 0x1C =  xPSR 
sp + 0x20 =  end of the stack before hard fault

But be aware if you made some funny things the PC may point to the wrong place or to nothing especially if your hardfault was not caused by your program but by DMA, MPU or something like that.

EDIT: Forget to mention: You should first look into the SCB->CSFR to understand what happened. Look in the handbook what the bits mean.

Finding the source of a Hard Fault - C embedded ARM Cortex-M4 32b

2 Answers