Tag Archives: hid

Cross-platform driverless USB: The Human Interface Device

During my LED Wristwatch project, I decided early on that I wanted to do something different with the way my USB stuff was implemented. In the past, I have almost exclusively used libusb to talk to my devices in terms of raw bulk packets or raw setup requests. While this is ok, it isn’t quite as easy to do once you cross out of the fruited plains of Linux-land into the barren desert of Windows. This project instead made the watch identify itself (enumerate) as a USB Human Interface Device (HID).

What I would like to do in this post is a step-by-step tutorial for modifying a USB device to enumerate as a human interface device. I’ll start with an overview of HID, then move on to modifying the USB descriptors and setting up your device endpoints so that it sends reports, followed by a few notes on writing host software for Windows and Linux that communicates to devices using raw reports. With a little bit of work, you should be able to replace many things done exclusively with libusb with a cross-platform system that requires no drivers.

Example code for this post can be found here:

https://github.com/kcuzner/led-watch

One thing to note is that since I’m using my LED Watch as an example, I’m going to be extending using my API, which I describe a little bit here. The main source code files for this can be found in common/src/usb.c and common/src/usb_hid.c.

Contents

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Bare metal STM32: Writing a USB driver

A couple years ago I wrote a post about writing a bare metal USB driver for the Teensy 3.1, which uses Freescale Kinetis K20 microcontroller. Over the past couple years I’ve switched over to instead using the STM32 series of microcontrollers since they are cheaper to program the “right” way (the dirt-cheap STLink v2 enables that). I almost always prefer to use the microcontroller IC by itself, rather than building around a development kit since I find that to be much more interesting.

LED Wristwatch with USB

One of my recent (or not so recent) projects was an LED Wristwatch which utilized an STM32L052. This microcontroller is optimized for low power, but contains a USB peripheral which I used for talking to the wristwatch from my PC, both for setting the time and for reflashing the firmware. This was one of my first hobby projects where I designed something without any prior breadboarding (beyond the battery charger circuit). The USB and such was all rather “cross your fingers and hope it works” and it just so happened to work without a problem.

In this post I’m going to only cover a small portion of what I learned from the USB portion of the watch. There will be a further followup on making the watch show up as a HID Device and writing a USB bootloader.

Example code for this post can be found here:

https://github.com/kcuzner/led-watch

(mainly in common/src/usb.c and common/include/usb.h)

My objective here is to walk quickly through the operation of the USB Peripheral, specifically the Packet Memory Area, then talk a bit about how the USB Peripheral does transfers, and move on to how I structured my code to abstract the USB packetizing logic away from the application.

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