How to set up ATSAM3X8E microcontrollers for bare-metal programming in C

HOW TO SET UP ATSAM3X8E MICROCONTROLLERS FOR BARE-METAL PROGRAMMING IN C

05 OCTOBER 2024

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This article is a step-by-step guide for programming bare-metal ATSAM3X8E chips -found on Arduino Due boards. It also includes notes on the chip’s memory layout -relevant for writing linker scripts. The steps described in this article were -tested on an OpenBSD workstation.

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Toolchain

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To interact directly with a bare-metal ATSAM3X8E chips, we must bypass the -embedded bootloader. To do that, we need a hardware programmer capable of -communicating with the chip over the Serial Wire Debug (SWD) protocol. Since -the workstation we upload the program from presumably doesn’t speak SWD, the -hardware programmer acts as a SWD-USB adapter. The ST-LINK/V2 programmer fits this -bill.

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The OpenOCD on-chip debugger software supports -ATSAM3X8E chips. OpenOCD, on startup, runs a telnet server that we can connect to -to issue commands to the ATSAM3X8E chip. OpenOCD translates plain-text commands -into the binary sequences the chip understands, and sends them over the wire.

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Finally, we need the ARM GNU Compiler -Toolchain to compile C programs for the chip. The ARM GNU compiler -toolchain and OpenOCD, as a consequence of being free software, are available -on every conceivable platform, including OpenBSD.

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Electrical connections

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The following photos illustrate the electrical connections between the Arduino -Due, PC, and the ST-LINK/V2 programmer required to transfer a compiled program -from a PC to the MCU.

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Wiring

Arduino Due

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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.

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Uploading the program

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The source.tar.gz tarball at the end of this page contains a sample C program -(the classic LED blink program) with OpenOCD configuration and linker scripts. -First, use the following command to build it:

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$ arm-none-eabi-gcc -mcpu=cortex-m3 -mthumb -T script.ld \
-    -nostartfiles \
-    -nostdlib \
-    -o a.elf main.c
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Then, open a telnet session with OpenOCD and issue the following sequence of -commands to configure the chip and upload the compiled program to it:

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$ 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"
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The first of the above commands starts OpenOCD. In the telnet session, the -first command halts the chip in preparation for receiving commands. Next, we -inspect the current GPNVM bit setting (more on this later). If the bit is unset -(the gpnvm show command returns 0), we set it to 1 and verify the update.

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The final command, issued from outside the telnet session, uploads the program -to the chip. Those are the bare minimum set of commands required to program the -chip. The AT91SAM3 flash driver section of the OpenOCD manual lists all -available commands for the ATSAM3X8E chip.

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GPNVM bits

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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 (the default), the chip boots from the ROM, -which contains Atmel’s SAM-BA bootloader.

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Conversely, 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 in the linker script, we set the -GPNVM1 bit and leave the GPNVM2 bit unchanged to ensure the chip -executes our program instead of the embedded bootloader at startup.

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Linker script

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At a minimum, the linker script must place the vector table at the first -address of the flash. This is mandatory for ARM chips unless we relocate the -vector table using the VTOR register.

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The first entry of the vector table must be the stack pointer. The stack -pointer must be initializes to the highest memory location available to -accommodate the ATSAM3X8E’s descending stack.

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The second entry of the vector table must be the reset vector. In the reset -vector, we can perform tasks such as zeroing out memory and initializing -registers before passing control to the main program.

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Files: source.tar.gz

by W. D. Sadeep Madurange