Recently I started doing some hardware design, just for fun. Anyway, I got my hands on one microcontroller with USB interface so I wanted to see how I can control it from my Linux box. It turns out to be quite easy and with this any general USB HID class device can be controlled (USB rocket launchers, USB toys and such I suppose).
Anyway, this post just lays out basics on how these devices can be used with Python on Linux. (Mind though that I’m Python, USB and Linux noob so don’t take this for granted – and of course no warranties whatsoever.)
First of all we need some library to access USB – libhid there should be Python bindings with it. I got both the library and the bindings with the following command (on my Ubuntu):

sudo apt-get install libhid-dev python-hid 

Then we need some hardware. I suppose any suitable Hid device can be used as long as communication protocol is known. I created my own device using HexWax expandIO-USB which is just a fancy name for (in my case) PIC18F2455 with HexWax’s firmware. I got mine from Farnell . The chip supports 10-bit A/D, digital I/O, counter input, interrupts, comparators, CCP, SPI, I2C so we can control loads of different peripherals if we wanted to.

The board is very simple and I used simplest reference schema from the documentation. It doesn’t follow any good USB design practices, I’m sure, but it can be assembled on a breadboard . Here’s the schema:

mind though, I used PIC18F2455 but it’s the same thing
And as I said it can be assembled on a bread board:

This image was made in Fritzing

Very simple – only 6 capacitors, 2 resistors, a LED, a crystal, the chip itself, USB connector and few wires. I assembled it quickly, and instead of using USB connector I cut one USB cable I wasn’t using and stripped off wires and plugged them into the breadboard – it looked ugly but it worked (of course use this approach on your own risk, as with anything else). Here’s USB pinout . Btw, I added a LED and one resistor just so I have some kind of a peripheral to play with.

After I finished testing the design on breadboard I used stripboard to create a more permanent board:

Basically the same thing, I just added some receptacles on both sides of the chip so I can connect other peripherals later.

Oh yes, since there’s HexWax firmware preloaded there’s no need to do any microcontroller programming, all’s done from PC.

Off to programming part and Python. First of all lets see how this board shows up to our system when we plug it in. Here’s output of lsusb command:

It shows up as a device with ID 0b40:0132. This can be changed in code but I couldn’t be bothered. So lets put libhid into action. Here’s code that is initializing the library, finding the device, reading it’s details (so far all standard code from libhid), then reading device’s firmware, setting our LED pin to be output, and blink 10 times with it with 1 second pause. Simple. Comments are in the code :

This was a quick run through. Hopefully I get some time to play more with this in the near future, and will follow up with new posts.