Writing reusable USB device descriptors with XML, Python, and C #USB
Kevin Cuzner points out that writing USB device descriptors time and again is rather annoying. A project ripe for a bit of automation.
A recent project required me to reuse (once again) my USB HID device driver. This is my third or fourth project using this and I had started to find it annoying to need to hand-modify a heavily-commented, self-referencing array of uint8_t’s. I figured there must be a better way, so I decided to try something different.
In this post I will present a script that turns this madness, which lives in a separate file.
The project goals:
Fully automatic computation of the wLength fields in descriptors.
Ad-hoc descriptor definition (i.e. I can specify descriptors throughout the code in many places).
Portable to all my machines without any dependencies other than Python. In general I use arch with python installed, so requesting that python be available isn’t a big deal for me.
Fully compatible with my existing USB driver structure (i.e. use the same usb_descriptors table format).
Fairly agnostic of the actual USB driver used. The idea is that this can be used by other people who don’t want to be stuck with my USB driver implementation.
Find all block comments (/* … */) in the source and extract them, stripping off leading “*” characters from each line. The blocks are retained as individual continuous pieces and are each parsed separately.
If the block doesn’t contain text matching the regex “<descriptor+.>”, it is discarded. Otherwise, the contents of the block comment are wrapped in an arbitrary element and then parsed using elementtree.
Each parsed comment block is assumed to declare one or more “descriptors”. The parsed XML is run through an interpreter which begins assembling objects which will generate the binary descriptor.
After every block has been parsed, the script will generate all the descriptors into a C file, automatically tracking endpoint numbers, addresses, and descriptor lengths.
Adafruit publishes a wide range of writing and video content, including interviews and reporting on the maker market and the wider technology world. Our standards page is intended as a guide to best practices that Adafruit uses, as well as an outline of the ethical standards Adafruit aspires to. While Adafruit is not an independent journalistic institution, Adafruit strives to be a fair, informative, and positive voice within the community – check it out here: adafruit.com/editorialstandards
Stop breadboarding and soldering – start making immediately! Adafruit’s Circuit Playground is jam-packed with LEDs, sensors, buttons, alligator clip pads and more. Build projects with Circuit Playground in a few minutes with the drag-and-drop MakeCode programming site, learn computer science using the CS Discoveries class on code.org, jump into CircuitPython to learn Python and hardware together, TinyGO, or even use the Arduino IDE. Circuit Playground Express is the newest and best Circuit Playground board, with support for CircuitPython, MakeCode, and Arduino. It has a powerful processor, 10 NeoPixels, mini speaker, InfraRed receive and transmit, two buttons, a switch, 14 alligator clip pads, and lots of sensors: capacitive touch, IR proximity, temperature, light, motion and sound. A whole wide world of electronics and coding is waiting for you, and it fits in the palm of your hand.
Have an amazing project to share? The Electronics Show and Tell is every Wednesday at 7pm ET! To join, head over to YouTube and check out the show’s live chat – we’ll post the link there.