Makefile, Library, and Sketches for the SparkFun SAMD21 Mini board

I only have one Arduino board, so why have tons of dependencies and generic build scripts for tons of targets?

This project builds for the SparkFun SAMD21 Mini Breakout, a SAMD21G18A board compatible with the Arduino Zero. It should be usable for other SAMD21 boards with minimal tweaking.

I started with the Arduino IDE to get the SAMD boards and SparkFun board definitions and the Arduino-Makefile project, then adapted the parts I need into my own Makefile.

Layout and Usage

• Sketch applications get their own directories at the top-level, and all c/cpp/S files within are compiled and linked into the final binary.
• Run make with SKETCH=name or S=name to chose a sketch. For convenience those variables can be exported from the environment or set in a top-level config.mk file.
• Sketch programs should have the .cpp extension (not .ino) and must include Arduino.h (the Makefile doesn't -include any headers automatically)
• lib/core contains the Arduino core libraries (with various modifications)
• lib/variant contains the SparkFun board variant files
• lib/CMSIS and lib/CMSIS-Atmel contain the low-level ARM and SAM header files.
• Other directories in lib/ are specific libraries enabled via LIBRARIES in the Makefile
  • All libraries are usually enabled and compiled, but only functions and objects actually referenced by the application code will be linked in the final binary.
• All c/cpp/S files in the lib directories are put in libcore.a, and each directory gets a -I option on the GCC command-line.
• All objects and the final elf/bin output are put in the obj/ directory
• obj/ has only one level of files and VPATH is used to add all the various source directories. This means there can't be multiple source files with the same basename

Components

lib/core

• Arduino SAMD core v1.6.20
• https://github.com/arduino/ArduinoCore-samd
• SparkFun provides their own core and libraries, but they're not noticeably different than the upstream Arduino code.

lib/variant

• SparkFun SAMD Arduino core v1.4.0
• https://github.com/sparkfun/Arduino_Boards
• variant files only

lib/CMSIS

• ARM CMSIS 5.8.0
• https://github.com/ARM-software/CMSIS_5
• Just the CMSIS/Core/Include directory

lib/CMSIS-Atmel

• https://github.com/arduino/ArduinoModule-CMSIS-Atmel
• The Atmel (Microchip) download link is broken, so I used the Arduino module from github
• Version 1.2.0
• Just the files I need from the CMSIS-Atmel/CMSIS/Device/ATMEL dir: sam.h, samd.h, samd21/

bootloader

• https://github.com/sparkfun/SAMD21_Mini_Breakout
• Firmware directory of that repo
• I rewrote the Makefile to taste and fixed compiler errors so it builds
• Changed the version extended capabilities to [Arduino:XYZ] instead of [SparkFun:XYZ] so that BOSSA recognizes the extra commands like chip-erase and write-buffer.
• Allow compiling the SAM-BA monitor "bootloader" as an application, which enables updating the real bootloader without a JTAG/SWD programmer.

Arduino-Makefile

• https://github.com/sudar/Arduino-Makefile
• The inspiration and reference for the first versions of this repo
• Ran this makefile to see how the toolchains were called and what was compiled so I could write my own makefile
• The ard-reset-arduino python script is copied directly from that repo, though on Linux I wrote a shell script that calls stty and is a bit faster.
• This project is great, but too automagic for my taste and too complex to easily customize

Notable changes to the core

• Fix compiler warnings in the Arduino and CMSIS code (builds with -Wall -Wextra -Werror)
• Fix LTO builds, which enables optimizations like inlining across compilation units. I had to add __attribute__((used)) to a couple functions (the CDC_Setup function was perplexing and got included anyway in LTO builds, but for whatever reason the 1200 baud bootloader reset only works when marking that function with the used attribute.
• Add a printf method for the Print class using vasprintf from newlib, which makes serial use way nicer. Adding -u _printf_float to LDFLAGS should add support for %f, but it hasn't been working for me (based on some brief searching it seems that may be related to heap starvation).
• De-template RingBuffer so that applications can resize the serial buffer
• Sacrifice 4 bytes of memory so that the 1200 baud USB bootloader reset doesn't erase the app start address. Instead it sets the same magic register that the bootloader uses to detect a reset button double-tap, allowing going back to the application without reprogramming.

Arch Linux Packages

• arm-none-eabi-gcc
• arm-none-eabi-binutils
• arm-none-eabi-newlib
• arm-none-eabi-gdb
• bossa-git (AUR) 1.9.1.r2.g3279031-1
  • the internet says that Arduino forked bossa and the upstream version doesn't work, but the AUR package (that I made) worked just fine for me
• jlink-software-and-documentation (AUR)
  • GDB server for JLink SWD programmer
  • Awesome hardware programmer/debugger for only $20!

License

• Most core libraries are licensed under the GNU Lesser General Public License Version 2.1
• Some libraries and header files are licensed under an MIT or BSD-style license
• My application sketches and Makefiles are licensed under the GNU General Public License Version 3
• Refer to individual source files for specific copyright and license details
• Copies of the LGPLv2.1 and GPLv3 license text are avilable in the docs/ directory of this repository