This uses V-USB to implement a USB joystick with minimal code. It supports up, down, left, right and two buttons (and can trivially handle 6 more).
The code runs on Thomas Fischl's 12MHz atmega8 USBasp programmer hardware, which supports V-USB, has several I/O pins, and is available cheaply. It can be programmed using a second USBasp programmer or by loading something like USBaspLoader on it to allow self-reprogramming.
I wrote this example because none of the other joystick projects I found implemented just a simple joystick with a minimal amount of code. I was able to pare this example down to very little code so that the essence is easier to understand. Thanks to others whose joystick allowed me to write this.
main.c USB joystick
usbconfig.h Configures V-USB
usbdrv/ Software-based USB driver; see http://www.obdev.at/vusb/
Makefile Compiles code and flashes it
README This documentation
License.txt GPL v2 + V-USB additions
schematic.png Schematic of USBasp hardware
usbasp.jpg USBasp hardware this was tested on
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Use second USBasp programmer to reprogram first. Connect them together via ISP ribbon cable. Close JP2 on first one to allow reprogramming its firmware. Connect second one to USB.
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Build and flash firmware:
make flash
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Disconnect second programmer from USB and ISP ribbon.
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Plug first one into USB and verify that it shows up as joystick.
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Run a joystick test program that shows what's being pressed. On Linux:
jstest --event /dev/input/js0
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Connect GND to one of the control pins and verify that button presses show up (be sure to avoid connecting it to +5V!). LED should toggle between on and off each time you close/open a button pin.
Controls (connect to GND to press):
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Down PB3 | 1 2 | +5V
N/C | 3 4 | PD1 Button 1
Up PB2 5 6 | PD0 Button 0
Right PB5 | 7 8 | GND
Left PB4 | 9 10 | GND
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Male connector on USBasp programmer
V-USB implements a low-speed USB device in software. When connected, it tells the host that it's a USB joystick. It also tells the host what controls it has and the format of the report it sends (usbHidReportDescriptor[] in main.c). Then when the joystick changes it sends the new state of the controls to the host. This can only be done every 8ms (1/125 second), when the host asks the device if it has any updates. There are ways to make the host check more often (up to 1000 times a second) by changing its hid polling rate.
-- Shay Green gblargg@gmail.com