USB descriptors are used by a USB device to describe its properties such as id, name, manufacturer, serial number and functionality. In CircuitPython, we used to rely on a bunch of C macros from the Advanced Software Framework from MicroChip to generate a descriptor for each board at compile time. C macros, however, aren’t a great tool for generating this because some descriptors reference others. (Specifically name, manufacturer and serial number are stored as indexes with matching string descriptors.) This makes maintaining descriptors a bit of a challenge. So, instead of continuing to use these C macros, we’ve switched to a small Python library we’ve created to generate the descriptor byte array instead.
With this new USB descriptor library for Python, one can use all of Python’s goodness to set up the descriptors as desired and then convert them to bytes. Once the descriptors are converted to bytes, normal Python file writing can be used to generate a file to hold them. Currently, we have a script that generates a C file.
Lastly, since the new library is Python and only uses the struct module, we hope to run the library on CircuitPython itself! That way we can change the descriptor dynamically on boot! We’ll be able to choose between HID, MIDI and other things dynamically.