Tag Archives: usb descriptor

Writing reusable USB device descriptors with some XML, Python, and C

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:

Into these comment blocks which can live anywhere in the source and are somewhat more readable:

In most of my projects before this one I would have something like the first script shown above sitting in a file by itself, declaring a bunch of uint8_t arrays and a usb_descriptors[] table constant that would be consumed by my USB driver as it searched for USB descriptors. A header file that exposes the usb_descriptors[] table would also be found in the project. Any USB descriptor that had to be returned by the device would be found in this table. To make things more complex, descriptors like the configuration descriptor have to declare all of the device interfaces and so pieces and parts of each separate USB interface component would be interspersed inside of other descriptors.

I’ve been using this structure for some time after writing my first USB driver after reading through the Teensy driver. This is probably the only structural code that has made it all the way from the Teensy driver into all of my other code.

With this new script I’ve written there’s no more need for manually computing how long a descriptor is or needing to modify the configuration descriptor every time a new interface has been added. All the parts of a descriptor are self-contained in the source file that defines a particular interface and can be easily moved around from project to project.

All the code for this post lives here:


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