Bus error debugging on arm cortex m4

Question

I am trying to debug a precise bus error on a Arm cortex m4 chip. The board is a teensy 3.1 with a freescale MK20DX256VLH7. The error only happens when i actually send characters with the uart and results in a forced hard fault because i dont have buserror and memory error handlers. The fault happens after a random amount of time between 1 second and 1 minute when sending 30 integers per second with the uart. Also when i print the values in binary, not the uartPutInt() but the uartPutBin() function, the fault does not happen.

I "hacked" the teensy so i can use SWD debugging. I use a arm-none-eabi toolchain gcc, gdb.

I tried to double stack size but that doesn't help. I tried different itoa() approaches, doesn't help.

full code can be accessed here: https://github.com/paulusbrand/tricopter

The problem occurs in uartPutInt() function, its not the prettiest function but i tried different approaches and this one is the easiest to debug and understand.

original:

void uartPutInt(int32_t data) {
    char buf[16] = {0};
    uint32_t tmpData;
    uint8_t neg = 0;
    int8_t tmp = 0;

    if(data<0) { // check negative
        tmpData = -data;
        neg=1;
    }
    else {
        tmpData = data;
    }

    while(tmpData) { // convert to chars
        uint8_t num = tmpData % 10;
        buf[tmp++]=num+48;
        tmpData/=10;
    }

    if(neg) { // add minus sign
        buf[tmp++] = 45;
    }

    while(tmp>=0) {
        uartPutChar(buf[tmp--]);
    }
}

new version:

void uartPutInt(int32_t data) {
    char buf[16] = {0};
    uint32_t tmpData;
    uint8_t neg = 0;
    int8_t tmp = 0;

    if(data<0) { // check negative
        tmpData = -data;
        neg=1;
    }
    else {
        tmpData = data;
    }

    do { // convert te chars
        uint8_t num = tmpData % 10;
        buf[tmp++]=num+'0';
        tmpData/=10;
    } while(tmpData);

    if(neg) { // add minus sign
        buf[tmp++] = '-';
    }

    while(tmp>0) {
        uartPutChar(buf[--tmp]);
    }
}

When the bus error occurs i check the CFSR register in the SCB and find precise bus error and BFAR valid.

The value of BFAR and thus the problematic memory address is 0x01007fd2. Which is, as far as i can tell, in the code area of the memory but far beyond the end of the code. I not really sure what to do with this number.

The program counter PC when the error occurs is 0x1033. Which is in the uartPutInt() function. Disassembled below.

00000f6c :
 f6c:   b580        push    {r7, lr}
 f6e:   b088        sub sp, #32
 f70:   af00        add r7, sp, #0
 f72:   6078        str r0, [r7, #4]
 f74:   f107 0308   add.w   r3, r7, #8
 f78:   2200        movs    r2, #0
 f7a:   601a        str r2, [r3, #0]
 f7c:   3304        adds    r3, #4
 f7e:   2200        movs    r2, #0
 f80:   601a        str r2, [r3, #0]
 f82:   3304        adds    r3, #4
 f84:   2200        movs    r2, #0
 f86:   601a        str r2, [r3, #0]
 f88:   3304        adds    r3, #4
 f8a:   2200        movs    r2, #0
 f8c:   601a        str r2, [r3, #0]
 f8e:   3304        adds    r3, #4
 f90:   2300        movs    r3, #0
 f92:   76fb        strb    r3, [r7, #27]
 f94:   687b        ldr r3, [r7, #4]
 f96:   2b00        cmp r3, #0
 f98:   da05        bge.n   fa6 
 f9a:   687b        ldr r3, [r7, #4]
 f9c:   425b        negs    r3, r3
 f9e:   61fb        str r3, [r7, #28]
 fa0:   2301        movs    r3, #1
 fa2:   76fb        strb    r3, [r7, #27]
 fa4:   e001        b.n faa 
 fa6:   687b        ldr r3, [r7, #4]
 fa8:   61fb        str r3, [r7, #28]
 faa:   e01f        b.n fec 
 fac:   69f9        ldr r1, [r7, #28]
 fae:   4b23        ldr r3, [pc, #140]  ; (103c )
 fb0:   fba3 2301   umull   r2, r3, r3, r1
 fb4:   08da        lsrs    r2, r3, #3
 fb6:   4613        mov r3, r2
 fb8:   009b        lsls    r3, r3, #2
 fba:   4413        add r3, r2
 fbc:   005b        lsls    r3, r3, #1
 fbe:   1aca        subs    r2, r1, r3
 fc0:   4613        mov r3, r2
 fc2:   767b        strb    r3, [r7, #25]
 fc4:   7eba        ldrb    r2, [r7, #26]
 fc6:   b2d3        uxtb    r3, r2
 fc8:   3301        adds    r3, #1
 fca:   b2db        uxtb    r3, r3
 fcc:   76bb        strb    r3, [r7, #26]
 fce:   b253        sxtb    r3, r2
 fd0:   7e7a        ldrb    r2, [r7, #25]
 fd2:   3230        adds    r2, #48 ; 0x30
 fd4:   b2d2        uxtb    r2, r2
 fd6:   f107 0120   add.w   r1, r7, #32
 fda:   440b        add r3, r1
 fdc:   f803 2c18   strb.w  r2, [r3, #-24]
 fe0:   69fb        ldr r3, [r7, #28]
 fe2:   4a16        ldr r2, [pc, #88]   ; (103c )
 fe4:   fba2 2303   umull   r2, r3, r2, r3
 fe8:   08db        lsrs    r3, r3, #3
 fea:   61fb        str r3, [r7, #28]
 fec:   69fb        ldr r3, [r7, #28]
 fee:   2b00        cmp r3, #0
 ff0:   d1dc        bne.n   fac 
 ff2:   7efb        ldrb    r3, [r7, #27]
 ff4:   2b00        cmp r3, #0
 ff6:   d00b        beq.n   1010 
 ff8:   7eba        ldrb    r2, [r7, #26]
 ffa:   b2d3        uxtb    r3, r2
 ffc:   3301        adds    r3, #1
 ffe:   b2db        uxtb    r3, r3
1000:   76bb        strb    r3, [r7, #26]
1002:   b253        sxtb    r3, r2
1004:   f107 0220   add.w   r2, r7, #32
1008:   4413        add r3, r2
100a:   222d        movs    r2, #45 ; 0x2d
100c:   f803 2c18   strb.w  r2, [r3, #-24]
1010:   e00d        b.n 102e 
1012:   7eba        ldrb    r2, [r7, #26]
1014:   b2d3        uxtb    r3, r2
1016:   3b01        subs    r3, #1
1018:   b2db        uxtb    r3, r3
101a:   76bb        strb    r3, [r7, #26]
101c:   b253        sxtb    r3, r2
101e:   f107 0220   add.w   r2, r7, #32
1022:   4413        add r3, r2
1024:   f813 3c18   ldrb.w  r3, [r3, #-24]
1028:   4618        mov r0, r3
102a:   f7ff ff87   bl  f3c 
102e:   f997 301a   ldrsb.w r3, [r7, #26]
1032:   2b00        cmp r3, #0
1034:   daed        bge.n   1012 
1036:   3720        adds    r7, #32
1038:   46bd        mov sp, r7
103a:   bd80        pop {r7, pc}
103c:   cccccccd    stclgt  12, cr12, [ip], {205}   ; 0xcd

Can somebody please help me with this?

Thanks!

Bus error debugging on arm cortex m4

Answers (1)

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