The SAMD21 is a microcontroller developed by Atmel which runs at 48mhz with a Cortex M0+ core. It's used in the Arduino Zero, Metro M0 Express, Feather M0s, Gemma M0 and Trinket M0. Many of our Arduino and CircuitPython libraries use C to interface directly with the hardware and when things go wrong it can be hard to figure out why.

Unlike more common web or server software development the code isn't being run on the same machine as its written on. When code is compiled for a different architecture, such as ARM for the SAMD21, than the architecture the compiler is running on, usually x86, its called cross compiling. There are many different ways to organize how your code compiles we can cover in another guide. What you need to know is that there are a few different toolchains that convert human readable code to machine code. Today we'll focus on the GNU toolchain that is made up of the GNU Compiler Collection (better known as gcc), the GNU Debugger (gdb) and number of other programs. Since the GNU toolchain is open source, ARM, the designer of the Cortex M0+ core, is able to ensure it produces well working code for their cores. So, we'll be using the ARM version of the GNU toolchain to compile and debug the C code.

While I won't cover compiling the code, I'll be using the CircuitPython in the examples. Its source is here and it uses make to run the compiler. I also use an Arduino Zero for all of my debugging because it has a builtin debug chip which converts USB commands to commands the Cortex M0+ core understands.

There are a couple aspects of debugging we'll talk about. First we'll cover a few ways to stop the program at an interesting spot. Second, we'll cover how to inspect the current program state using backtrace. Lastly, using the Micro Trace Buffer, we'll inspect the history of the program execution. Lets get setup.