---
title: ATSAM3X8E bare-metal notes
date: 2024-09-16
layout: post
---

Bypassing ATSAM3X8E (Due) bootloader via Serial Wire Debug (SWD). 

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

ARM chips boot into 0x00000. GPNVM bits map one of ROM, flash0, flash1 to
0x00000: 

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

By default, control jumps to Atmel's SAM-BA bootloader in ROM. To bypass, set
GPNVM1=1 and place vector table at 0x80000 (flash0).

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

<table style="border: none; width: 100%;">
  <tr style="border: none;">
    <td style="border: none; width: 50%; vertical-align: top; background-color: transparent;">
      <img src="schematic.png" alt="Pinout" style="width: 100%">
      <p style="text-align: center;">Wiring</p>
    </td>
    <td style="border: none; width: 50%; vertical-align: top; background-color: transparent;">
      <img src="connections.jpeg" alt="Circuit" style="width: 100%">
      <p style="text-align: center;">Arduino Due</p>
    </td>
  </tr>
</table>

Remap memory:

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

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

Compile 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"
```

Commit: <a
href="https://git.asciimx.com/bare-metal-arduino-due/commit/?id=318496925ca76668dd9d63c3d060376f489276f8"
class="external" target="_blank" rel="noopener noreferrer">3184969</a>.