micro:bit c toolchain explained, part 2. yotta, file locations

December 2020: I wrote this post when only v1 of the micro:bit existed. I haven’t yet checked to see if the contents are still valid for the toolchain used to program v2 of the micro:bit.

This article follows on from part 1 of my attempt to explain the chain of tools used to build your C code into something that can run on the BBC micro:bit. In this part, I will cover the yotta build tool and how the make build tool is used as the top level component.

Here’s the final tool chain. Admire the beauty.

Offline tool chain for building an executable from C code for the micro:bit

So what is yotta and why do companies choose names like ‘yotta’ and ‘ninja’ for their products when this will make it hard for us to find them on the web?

yotta

Yotta is another build tool. Yotta takes the build file module.json and the configuration files target.json and config.json. Yotta produces the build file for cmake called CMakeLists.txt. If you read part 1 of this explanation of the micro:bit C tool chain, you will be getting used to the idea that one build tool creates the build file for the build tool underneath it. It took me a while to get my head around this. The reason for using these layers of build tools is that there are many other files than the ones that we write to pull into the project to enable our C code to be built to run on the micro:bit hardware.

In your yotta examples directory, which I represent with <yotta_examples> below, have a look in the file <yotta_examples>/yotta_targets/bbc-microbit-classic-gcc/target.json. Almost at the end of the target.json file, you will see where yotta tool is configured to produce a build file for CMake:

"toolchain": "CMake/toolchain.cmake",
  "scripts": {
    "debug": [
      "valinor",
      "--target",
      "NRF51822",
      "$program"

Here ends the description of build tools used to create the hex file that goes onto your micro:bit from the source C code.

make

This section is a bit of an extension/digression. The ‘make’ tool is not used in the programming stack for micro:bit. I use it to automate building and loading a hex file to a micro:bit.

Make is the Grandad of build tools. I often use this tool for building C projects outside of micro:bit programming. ninja may be faster for large C projects, but make is fast enough for my projects and has proven reliability.

The build file for make tool is called makefile, or sometimes Makefile. Linux does not distinguish between the two names, but Windows will. Typing ‘make’ at the command line will cause the contents of makefile to be processed by the make tool. You will be used to the idea of a build file being processed by a build tool by now.

I use make to automate building the binary and loading it onto my micro:bit. Here’s a listing of a typical makefile for one of my projects:

build_microbit:
	yt build
	# script to mount a microbit
	~/data/infolab2/progs/scripts/microbit_mount.sh mount
	cp ./build/bbc-microbit-classic-gcc/source/microbit-c-combined.hex /media/bill/MICROBIT/

yt build obviously builds the project. The line after the comment checks to see if the micro:bit is mounted and if not, mount it. The final line copies the newly generated hex file to the mount point for the micro:bit, which is specified in the script microbit_mount.sh.

I use a script to mount the micro:bit, as this is unmounted after flashing the new code. I use Linux; this may not be necessary with Windows. Details on the script I use can be found on my blog post. The micro:bit is mounted at /media/bill/MICROBIT. You need to change the mount point in the makefile to the location that you choose to mount your micro:bit.