If you are planning to
hardrwire a PS2 controller to the psp, there are a couple ways to approach this:
1: As Kink192 said, modify the controller using your own cable that runs individual wires to each of the button points inside the controller. You can use heatshrink to create your own cable(s) or use other multi-conductor cable (old parallel printer cables work well for this.) The number of wires needed in the cable is determined by how many functions you want to control. You will need one ground wire for the controller, and then another wire for each additional button you want to connect, (the analog stick needs three wires). Twenty is a good wire count for all in-game buttons and +/- to add LEDs.
or...
2: Create an adapter to interface an unmodified ps1/ps2 controller using a microcontroller to isolate all of the button presses outside the controller. This is the method used by the PSP2TV adapter. The advantage to this approach it that the controller will still work with a ps1/ps2, and if the parts used are small enough to fit inside the PSP, you will only need to cut out room in the case for a six pin connector. Here are a couple relevant links (
1,
2) which use PIC and AVR microcontrollers.
