Bare-metal ARM Cortex M3 chips

BARE-METAL ARM CORTEX M3 CHIPS

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05 OCTOBER 2024

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This post is about programming bare metal SAM3X8E Arm Cortex M3 chips found on +Arduino Due boards. I had to learn how to do this because none of the +high-level tools for programming Arduino Dues are available for OpenBSD, which +I use for much of my personal computing.

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Toolchain

+ +

Since we will not be using pre-packaged development tools, we need to assemble +our own toolchain. As usual, we need a compiler toolchain to build programs for +the target chip. As we will be bypassing the embedded bootloader, we will also +need a hardware programmer and an on-chip debugger to flash programs to the +chip. I used the following toolchain.

+ +

Arm GNU compiler +toolchain.
OpenOCD on-chip debugger.
ST-LINK/V2 +programmer.

+ +

Electrical connections

+ +

The following diagram outlines the electrical connections between the different +components necessary to move a compiled program from a PC to the MCU.

+ + + + + + +

Wiring

Arduino Due

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Arduino Due exposes the SAM3X8E’s Serial Wire Debug (SWD) interface via its +DEBUG port. The ST-LINK/v2 programmer uses the SWD protocol to communicate with +the chip.

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Uploading the program

+ +

Follow the steps below to upload a program to the SAM3X8E chip. The +source.tar.gz tarball at the end of the page contains a sample program with a +OpenOCD config file and a linker script.

+ +

Start OpenOCD: +
```
$ openocd -f openocd-due.cfg
+
```
+

Open a telnet session and check that the GPNVM1 bit is set. Otherwise +set it to 1:

+ +

$ telnet localhost 4444
+  > halt
+  > at91sam3 gpnvm show
+  > at91sam3 gpnvm set 1
+  > at91sam3 gpnvm show
+

Build the program using the custom linker script. +

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

Upload the program using OpenOCD: +

$ openocd -f openocd-due.cfg -c "program a.elf verify reset exit"
+

+ +

Refer to the OpenOCD manual (AT91SAM3 flash driver section) for a complete list +of commands supported for the ATSAM3X8E.

+ +

GPNVM bits and the linker script

+ +

By design, ARM chips boot into address 0x00000. ATSAM3X8E’s memory consists of +a ROM and a dual-banked flash (flash0 and flash1), residing in different +locations of the chip’s address space.

+ +

The GPNVM bits control which of them maps to 0x00000. When GPNVM1 is cleared +(default), the chip boots from the ROM, which contains Atmel’s SAM-BA +bootloader. So, the chip runs the embedded bootloader instead of our program.

+ +

When the GPNVM1 bit is 1 (and the GPNVM2 bit is 0), flash0 at address 0x80000 +maps to 0x00000. When both GPNVM bits are 0, flash1 maps to 0x00000. Since we +place our program in flash0 using the linker script, we set the GPNVM1 bit and +leave the GPNVM2 bit as it is.

+ +

The linker script places the vector table at the first address of the flash. +ARM chips expect this unless we relocate the vector table using the VTOR +register. The first entry of the vector table must be the stack pointer, and +the second must be the reset vector.

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Finally, the ATSAM3X8E uses a descending stack. So, in the linker script, we +initialize the stack pointer to the highest memory location available. In the +reset vector, we zero out memory, initialize registers, and perform other tasks +before passing control to the main program.

+ +

Files: source.tar.gz

+ +

by W. D. Sadeep Madurange