From 25615d16f93ffafcb19d0940cfab75e1f374b3b9 Mon Sep 17 00:00:00 2001 From: Sadeep Madurange Date: Fri, 7 Nov 2025 21:07:12 +0800 Subject: Improve writing. --- _site/archive/arduino-due/index.html | 149 ++++++++++++++++++----------------- 1 file changed, 77 insertions(+), 72 deletions(-) (limited to '_site/archive/arduino-due') diff --git a/_site/archive/arduino-due/index.html b/_site/archive/arduino-due/index.html index 13e006e..86f4fb0 100644 --- a/_site/archive/arduino-due/index.html +++ b/_site/archive/arduino-due/index.html @@ -2,12 +2,12 @@ - Bare-metal ARM Cortex M3 chips + Bare-metal ATSAM3X8E chips - Bare-metal ARM Cortex M3 chips + Bare-metal ATSAM3X8E chips @@ -40,34 +40,38 @@
-

BARE-METAL ARM CORTEX M3 CHIPS

+

BARE-METAL ATSAM3X8E CHIPS

05 OCTOBER 2024

-

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.

+

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

-

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.

+

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.

- +

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.

+ +

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.

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.

+

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.

@@ -82,69 +86,70 @@ components necessary to move a compiled program from a PC to the MCU.

-

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.

+

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

-

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.

- -
    -
  1. Start OpenOCD: - 
    $ openocd -f openocd-due.cfg
-
    -
    2. -
  2. Open a telnet session and set the GPNVM1 bit to 1: - 
    $ telnet localhost 4444
+
    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
-
        
    
-
    3. -
  3. 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
-
    -
    4. -
  4. Upload the program using OpenOCD: - 
    $ openocd -f openocd-due.cfg -c "program a.elf verify reset exit"
-
    -
    5. -
+$ 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.

+

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.

-

GPNVM bits and the linker script

+

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 -(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.

- -

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.

+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

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