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---
title: ATSAM3X8E bare-metal notes
date: 2024-09-16
layout: post
---
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:
<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 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: <a
href="https://git.asciimx.com/bare-metal-arduino-due/commit/?id=318496925ca76668dd9d63c3d060376f489276f8"
class="external" target="_blank" rel="noopener noreferrer">3184969</a>.
|
