Recently http://tokyodriftpsp.wordpress.com/projects/ps2-controller/ hacked a PlayStation 2 controller and built an adapter that allows you to connect the PS2 gamepad to a Sony PSP console to control it.
For those curious, this adapter didn't require any hardware alteration to the PSP or the PS2 controller, as a plug-in was used on the PSP Firmware side of things.
The adapter then makes use of video out and PS2 controller extension cables, along with an ATmega8 microcontroller to handle the signals between the two devices.
Check out a video of the PS2 controller hack in action with a PlayStation Portable below, via YouTube!
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