Bought an Arduino Due to play with ARM Cortex chips. Locked out of the chip for days—no dev tools (Arduino IDE, Microchip Studio) for OpenBSD. Need to find a way to bring up the bare-metal ATSAM3X8E.

Three on-chip memories: ROM, flash0, flash1. ARM Cortex-M chips boot into 0x00000. GPNVM bits map one of them to 0x00000:

• GPNVM1=0 → ROM (default).
• GPNVM1=1 and GPNVM2=0 → flash0.
• GPNVM1=1 and GPNVM2=1 → flash1.

On power-up, control jumps to ROM. Without a user program, Atmel’s SAM-BA bootloader traps execution in ROM. Must remap memory via SWD to force chip to boot from flash, bypassing factory bootloader.

Toolchain: ST-LINK/V2 programmer, OpenOCD, ARM GNU Compiler Toolchain.

Connect ST-LINK/v2 to Arduino Due’s DEBUG port:

Wiring

Arduino Due

Remap memory using OpenOCD:

$ openocd -f openocd-due.cfg
$ telnet localhost 4444
  > halt
  > at91sam3 gpnvm show
  > at91sam3 gpnvm set 1
  > at91sam3 gpnvm show

Full command list in OpenOCD manual AT91SAM3 (flash driver section).

At a minimum, vector table must be initialized with stack pointer and reset vector:

__attribute__((noreturn)) void _reset(void) 
{
    unsigned long *dst, *src;
    extern unsigned long _sbss, _ebss;
    extern unsigned long _sdata, _edata, _sidata;

    for (dst = &_sbss; dst < &_ebss; dst++)
        *dst = 0;
    for (dst = &_sdata, src = &_sidata; dst < &_edata;)
        *dst++ = *src++;

    main();
}

extern const unsigned int _sp;

__attribute__ ((section(".vtor"))) const void* _tab[] = { &_sp, _reset };

Linker script places vector table at start of flash0 and initializes stack pointer to top of RAM (descending stack):

MEMORY
{
    rom (rx)  : ORIGIN = 0x00080000, LENGTH = 512K
    ram (rwx) : ORIGIN = 0x20000000, LENGTH = 96K
}

SECTIONS
{
    .text :
    {
        KEEP(*(.vtor))
        *(.text*)
        *(.rodata*)
    } > rom
}

_sp = ORIGIN(ram) + LENGTH(ram);

Getting linker script right wasn’t easy—long hours with the datasheet, OpenOCD manual. David Barrass from OpenBSD and folks at OpenOCD mailing lists (bless them!) helped generously.

Build and upload program:

$ arm-none-eabi-gcc -mcpu=cortex-m3 -mthumb -T script.ld \
    -nostartfiles \
    -nostdlib \
    -o a.elf main.c
$ openocd -f openocd-due.cfg -c "program a.elf verify reset exit"

Chip unlocked. I’m in.

Commit: 3184969.