Notes on programming ATSAM3X8E chips (Arduino Due) without bootloader. Tested on OpenBSD.

Toolchain

Need to bypass embedded bootloader—requires hardware programmer that speaks Serial Wire Debug (SWD). ST-LINK/V2 works as SWD-USB adapter.

OpenOCD translates commands to binary sequences the chip understands. Runs telnet server on startup for issuing commands.

ARM GNU Compiler Toolchain for compiling C programs. Both OpenOCD and ARM toolchain available on OpenBSD.

Electrical connections

Arduino Due exposes ATSAM3X8E’s SWD interface via DEBUG port. ST-LINK/V2 connects there.

Wiring

Arduino Due

Arduino Due exposes the ATSAM3X8E’s SWD interface via its DEBUG port. The ST-LINK/v2 programmer connects to that to communicate with the chip.

Upload procedure

Sample LED blink program with OpenOCD config and linker scripts in tarball below.

$ arm-none-eabi-gcc -mcpu=cortex-m3 -mthumb -T script.ld \
    -nostartfiles \
    -nostdlib \
    -o a.elf main.c

Upload:

$ openocd -f openocd-due.cfg
$ telnet localhost 4444
  > halt
  > at91sam3 gpnvm show
  > at91sam3 gpnvm set 1
  > at91sam3 gpnvm show
$ openocd -f openocd-due.cfg -c "program a.elf verify reset exit"

First command starts OpenOCD. Telnet session halts chip, checks GPNVM bit. If unset (returns 0), set to 1 and verify. Final command uploads program. See OpenOCD manual AT91SAM3 flash driver section for full command list.

GPNVM bits

ARM chips boot into address 0x00000. ATSAM3X8E has ROM and dual-banked flash (flash0/flash1) at different addresses. GPNVM bits control which maps to 0x00000.

GPNVM1 cleared (default): boots from ROM (Atmel SAM-BA bootloader). GPNVM1=1, GPNVM2=0: flash0 (0x80000) maps to 0x00000. Both cleared: flash1 maps to 0x00000.

Program goes in flash0, so set GPNVM1=1 to boot our code instead of bootloader.

Linker script notes

Vector table must be at first flash address–mandatory for ARM chips unless using VTOR register for relocation.

First vector table entry: stack pointer. Initialize to highest memory location (ATSAM3X8E has descending stack).

Second entry: reset vector. Place initialization code here (zero memory, set registers) before jumping to main().

Commit: 3184969