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----
-title: How to set up ATSAM3X8E microcontrollers for bare-metal programming in C
-date: 2024-10-05
-layout: post
----
-
-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.
-
-## Toolchain
-
-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 <a
-href="https://www.st.com/en/development-tools/st-link-v2.html" class="external"
-target="_blank" rel="noopener noreferrer">ST-LINK/V2</a> programmer fits this
-bill.
-
-The <a href="https://openocd.org/" class="external" target="_blank"
-rel="noopener noreferrer">OpenOCD</a> 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.
-
-Finally, we need the <a
-href="https://developer.arm.com/Tools%20and%20Software/GNU%20Toolchain"
-class="external" target="_blank" rel="noopener noreferrer">ARM GNU Compiler
-Toolchain</a> 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.
-
-## Electrical connections
-
-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.
-
-<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.
-
-## Uploading the program
-
-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:
-
-```
-$ arm-none-eabi-gcc -mcpu=cortex-m3 -mthumb -T script.ld \
-    -nostartfiles \
-    -nostdlib \
-    -o a.elf main.c
-```
-
-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:
-
-```
-$ 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"
-```
-
-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.
-
-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.
-
-## GPNVM bits
-
-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.
-
-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.
-
-## Linker script
-
-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. 
-
-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.
-
-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.
-
-Files: [source.tar.gz](source.tar.gz)