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

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.

<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>

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](https://git.asciimx.com/bare-metal-arduino-due/commit/?id=318496925ca76668dd9d63c3d060376f489276f8)