I've been planning this particular mod for quite awhile now, but I've finally gotten comfortable enough with my surface-mounting and pcb fabrication skills to actually publicly display my progress. This all began out of wanting to build one of these GBC cart readers for backing up my save files and/or writing saves to retail carts http://www.reinerziegler.de/readplus.htm#Home%20made%20programming%20systems. I didn't have the right tools to properly desolder the cart connector from the Game Boy the right way, or the patience to do it the one-pin-at-a-time-and-slowly-pry-it-off-one-millimeter-at-a-time way, so I decided to try building it in the shell of a broken Game Boy by just cutting the traces to the cart connector and leaving it attached to the board. There is ample room in the battery compartment for building the rest of the circuit, and a USB-B port fits nicely in the bottom of the shell where the headphone port used to be. But then I got about halfway into the build and had to put it away for a long time unfinished, at which point I began thinking of additional mods I could do. My plans have since changed quite a bit from the original vision, but I still really liked the idea of building inside the Game Boy shell and decided to figure out what else I could do with that idea. I had previously done an SNES-to-USB controller mod with an internal USB hub and internal flash drive, so I began to look into doing something similar here using the buttons in the Game Boy. I also realized that if I could get the cartridge connector desoldered, I really had no need for the original mainboard, and if I could learn to fabricate my own pcb, I could build the whole thing on a pcb and it'd be really slick. So anyways, here's the rundown of everything I'm hoping to achieve with this mod, as well as how far I've managed to get so far.

Goals:
GBC cart reader/writer based on the http://www.reinerziegler.de design (the USB version), built into an original DMG shell
USB port for communication with the PC will be mounted in place of the headphone jack
Internal USB hub
Internal flash drive for storing ROMs/saves/software/drivers associated with the cart flasher
Utilize the GB buttons as a USB HID gamepad
The last free USB port on the hub (after the cart flasher, flash drive, and gamepad) will be wired to a standard USB port to replace the link port (which is nearly the exact size of a USB-A port)
Single, bi-color LED to replace the original power LED, hooked up to the cart flasher (which incorporates two separate LEDs)
Leave footprints on the board for the original power switch, volume knob, power adapter, and battery terminals, with traces to pads if I come up with some idea to incorporate them
Use the original ribbon cable for communicating with the daughterboard (for talking to the buttons), giving me access to the LCD leads as well if I ever want to play with them (although from what I've read, controlling the LCD is neither trivial, nor entirely understood as of now, but whatever, I'm already accessing the ribbon cable, no reason not to add the headers just in case...)

With all of these in one package, I will be able to plug it in to my computer, insert a game cartridge, dump its ROM and save file onto the internal flash drive, load the dumped files into an emulator which will also be installed on the flash drive, and then control the emulator using the Game Boy's original buttons. I will also be able to load saves onto retail carts, and if I follow the other guide on the http://www.reinerziegler.de site, I can even build a cart with a reprogrammable ROM as well.

For now, the first two things that I am tackling separately are prototyping the cart flasher on a breadboard and figuring out how I want to go about the gamepad conversion. I'm fairly certain that I will be rewiring the gamepad buttons to a 4021 IC to run them as an NES controller, because the project I used to convert the SNES controller to USB also supports NES controllers, so if I can make it behave as an NES controller, which is trivial, I won't have to do any of the USB gamepad conversion myself. I'm pretty close to getting the gamepad working, so I'll post back as soon as I have some more info.

My current plan for the gamepad portion of the mod is, as I said, to first wire up the buttons as an NES controller, then convert that to USB using the conversion circuit here http://raphnet.net/electronique/snes_nes_usb/index_en.php. For that, I'm using a slight modification on my micro NES pad that I posted about here http://forums.benheck.com/viewtopic.php?f=57&t=49986. By removing the 4 diodes from the existing button circuit on the LCD/gamepad daughterboard, I have access to one side of all 8 buttons. By merging the other side of the buttons and wiring it to ground, I will have all the connections I need to hook up my micro NES pad circuit, and by cutting the traces that I merged just past the point where I merged them, I can reuse those traces to give me the 5 connections I need to transmit the NES controller circuit across the ribbon cable (the GB originally uses 6, I only need the 5 for the NES controller wires). Here's a pic to give an idea of what this is gonna look like (I'll be making a PCB with the contacts for the buttons lining up with the holes left by removing the diodes, and I *should* be able to simply mount the whole thing through-hole)

This may be good reading material?

http://www.rival-corp.com/2011/05/08/us ... ge-dumper/
http://www.rival-corp.com/2011/03/20/ga ... om-dumper/
http://www.rival-corp.com/2010/12/03/wh ... g-pokemon/
http://www.rival-corp.com/2010/12/02/ga ... adapter-2/

I've actually seen that guy's version of the dumper, but that was awhile ago and he wasn't very far along. I like how he's using an Atmel with a USB transceiver on-chip rather than relying on a separate USB<->UART chip. Hopefully he posts some more details about his build and I'll definitely look into it. The LCD screen is still beyond me for now, but thanks for the links, I'll keep them in mind (still not even sure what I'd want to do with the screen even if I could...). That guy also had a link to the PCB prototyping service he used which is very affordable, so when I actually get this thing ready to build the final product I'll probably go with a professional service like that (heck, they're really cheap, why not?).

I just finished the NES controller daughterboard. Haven't done the last wiring on the GB board itself, but I'm close to having the gamepad portion of the mod complete. Here's what it looks like (the board is mounted to the Game Boy board through the holes indicated in the photo I posted earlier, so it's not just sitting there, it is actually mounted)

Very nice. I like it. Mainly because you did a decent job, where as it is very rare to see someone actually make a PCB because all the others just use a breadboard and CAT5 cable

If I ever get this all done, I plan to actually order a professional PCB, but for now, I'm having fun making my own... It makes things like hooking up the Game Boy cart connector to a breadboard a lot easier. It also allows me to break out those QFP USB chips to a breadboard, which would have been a nightmare any other way.

Same. I make a PCB to break out QFP and SMD/T devices as well. Much more professional.