Project overview

For some projects, a circuit making it possible to connect 12 buttons (including 4 directional buttons) to a PC using an USB port is very useful. Here's a few examples:

• Home-made Arcade style controller and small arcade cabinets
• Simple controller (1 wire per button) to USB. Eg: Neo-Geo, Atari...
• Using appropriate software on the PC side, the inputs can be used for other purposes. Eg: Switches, limit-switches, alarms... A cleaner solution than modifying an existing game controller

On this page, you'll find out how to build such a circuit using a microcontroller. The firmware implements a standard USB joystick with 4 directional buttons and 8 general purpose buttons.

Depending on your skills, you can build the circuit on a breadboard using thru-hole components or build the surface-mount version using my small multiuse PCB2 circuit board. I also sell ready to use circuits and pre-programmed Atmega8 chips (in DIP package only). Visit my online shop for more details.

No drivers required!
That's right, since the USB standard defines device classes. I'm using the human input device (HID) which allows me to tell to the computer that the connected USB device is a joystick and has 2 axis and 4 or 8 buttons. Another nice thing about this is that the adapter should work with all operating systems supporting HID devices. (I tested and it works at least on Win98, Win2K, WinXP and Linux)

This project uses an ATmega8 microcontroller from Atmel. This microcontroller does not support USB in hardware so I used the software-only usb driver from Objective Development. This driver allows an AVR microcontroller such as the ATmega8 to talk USB with minimal external components. As a result, the interface can be built cheaply and easily

Ready to use circuits available

Schematic

Here is the main schematic:

And here is a wiring example:

Additionally, a wiring table for NeoGeo controllers is shown lower on this page.

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Component list:

Ref	Description
U2	Atmega8 microcontroller. ATMEGA8-16PC, ATMEGA8-16PI, ATMEGA8-16PJ or ATMEGA8-16PU. Dont use an ATMEGA8L-*, the 12Mhz clock would be too high.
R1	1.5k resistor. Ordinary carbon film 1/4 watt resistors will do.
R2, R3	68 ohm resistors. Ordinary carbon film 1/4 watt resistors will do.
D2, D3	3.6 volts zener diodes.
R4	Do not install, not used anymore.
D1	Do not install, not used anymore.
Y1	12 Mhz crystal.
C2, C3	27 pf capacitors. If the crystal datasheet recommends another value, use it instead.
C1	10uf capacitor. Install it near the ATmega8.
JP1, JP2	Jumpers. You can also use dip switches, ordinary switchs or solder bridges.
J2	6 pin header, 2.54mm spacing. Needed for programming the ATmega8.

For the USB connection, just strip the USB cable and solder the wires directly to the board. USB uses standard wire colors:

	Color	Description
	Red	+5 volts
	Black	Ground
	Green	D+
	White	D-

Programming

A microcontroller is a component which must be programmed in order to do something useful. So here is the hexfile which must be flashed into the microcontroller:
usb_game12-1.0.hex

Many microcontrollers have what is called 'Fuse bytes'. In the case of the ATmega8a, there are two bytes: The high byte, and the low byte. Those bytes are used to configure some aspects of the microcontroller. What type of clock to use? Crystal? Resonator? Internal RC clock? Allow programming via ISP? It's very important to set the fuses to the right values. Using the wrong values can render your MCU unusable.

For this project, here are the appropriate fuse values:
high byte = 0xc9, low byte = 0x9f

For details about how to program an AVR, visit my AVR programming page.

Pre-programmed Atmega8 (DIP package) available

Source code

The source code is released under the GPL license and compiles with avr-gcc. To prevent conflicts, please do not distribute modified version where the USB report descriptor has been modified without replacing the USB Vendor ID and Product ID by yours.

usb_game12-1.0.tar.gz

PCB for surface-mount

The surface-mount version uses my Multiuse PCB2 circuit. Here's what it looks like:

Multiuse PCB2:

Wiring diagram for this project on 'Multiuse PCB2':

Ready to use circuits available

NeoGeo adapter wiring

Neo Geo controllers uses standard DB15 connectors. This connector is also used by old PC Joysticks but it incompatible. Here's a table describing how to wire a Neo Geo controller connector to this circuit:

DB15 pin	Neo Geo name	USB Game12 name	Comments
1	Common	Common
3	Select	Button 5
4	D Button	Button 3
5	B Button	Button 1
6	Right	Right
7	Down	Down
8	+5v	+5v	Only for some controllers
9	D Button	Button 3	Duplicates pin 4
11	Start	Button 4
12	C button	Button 2
13	A button	Button 0
14	Left	Left
15	Up	Up

(DB15 pins 2 and 10 are not connected)