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[WIP Concept] GameCube Controller Native Mod for New 3DS XL via USB-C & Internal MCU Decoder

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Riker123

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Hi everyone!

I’m Iker from Nuevo León, Mexico. I’m starting a project I’ve been dreaming about: connecting a GameCube controller to my New 3DS XL using a custom internal hardware mod. I want to be 100% upfront: I am a complete beginner in electronics and coding, but I have a clear vision of the architecture and I'm ready to learn.

The Concept (Modular Dual-MCU Design)My goal is to keep the console looking as stock as possible while having a "plug-and-play" experience using a single USB-C cable for data:

  1. The "Brain Box" (External):
    • A generic GameCube-to-WiiU/Switch adapter inside a box will serve as the physical ports.
    • An Arduino (Nano or Pro Micro) inside this box will read the GameCube controller data.
    • This data is sent via Serial protocol through a USB-C cable that connects the box directly to the 3DS.
  2. The Console (N3DS XL):
    • A USB-C female port installed in the 3DS shell receives the signal from the external box.
    • This port is wired internally to an ATtiny85 (MCU).
    • The ATtiny85 acts as the decoder: It receives the serial data and its output pins are soldered directly to the button test pads on the 3DS motherboard to trigger the inputs (A, B, X, Y, etc.).
  3. Why this approach?
    • It avoids a bulky 15-wire cable; only 4 wires (VCC, GND, D+, D-) are needed through the USB-C connector.
    • The 3DS remains portable and clean when the external box is unplugged.
The Challenges (Where I need your expert help!)Since I'm learning everything from scratch, I’m looking for guidance on:

  • Soldering Points (Test Pads): I need specific maps for the New 3DS XL motherboard. Which pads are the safest to solder to for the main buttons and triggers? I want to avoid lifting any pads or damaging the traces.
  • The Code: How to properly sync the Serial communication between the external Arduino and the internal ATtiny85 to ensure minimal input lag.
  • C-Stick & Circle Pad: Since the C-stick is a strain gauge, what’s the best way to simulate those analog signals using the internal MCU?
  • Voltage Management: Handling the 3.3V logic of the 3DS vs the 5V requirements of the GameCube controller logic/rumble.
My CommitmentI’m not looking for someone to do all the work for me. I am willing to:

  • Be the beta tester and risk my own N3DS XL for the sake of the project.
  • Document the entire process with photos/videos to create a public tutorial once it’s finished.
  • Learn and practice whatever skills are needed (Fine-tip soldering, Arduino IDE, etc.).
I’ve attached a sketch of my diagram to help visualize the flow. I would love to hear your thoughts on this "Serial-to-MCU" approach!

Thanks in advance for your time and help!
 

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I think ATtiny85 is not suitable for Console side with below things:
  1. It does't have enough pins to cover whole button of 3DS: 15 digitals (N3DS with ZL&ZR) and 2 analogs (include c-stick). Dueing the fact that ATtiny85 does not support USB natively, you will have a lot of extra work for that (you can read about LUFA to add USB support for it), and for USB, it costs you 2 more pins => Total pins you need is higher than its physical support.
  2. It runs reliable on 5V, for 3.3V you have to config it bootloader/fuse
  3. It take a lot of juice from your battery => No good for sleep mode.
=> There are so much more limitations of ATtiny85 you have to pass, above is just some my mind when I 1st read your post.

Why do you have to use an extra layer ("Brain Box")? 3DS has bluetooth, have you consider about leverage it? As I remember, there is some project that turn an original Gamecube controller to wireless with dongle.
 
marhalloweenvt said:
I think ATtiny85 is not suitable for Console side with below things:
  1. It does't have enough pins to cover whole button of 3DS: 15 digitals (N3DS with ZL&ZR) and 2 analogs (include c-stick). Dueing the fact that ATtiny85 does not support USB natively, you will have a lot of extra work for that (you can read about LUFA to add USB support for it), and for USB, it costs you 2 more pins => Total pins you need is higher than its physical support.
  2. It runs reliable on 5V, for 3.3V you have to config it bootloader/fuse
  3. It take a lot of juice from your battery => No good for sleep mode.
=> There are so much more limitations of ATtiny85 you have to pass, above is just some my mind when I 1st read your post.

Why do you have to use an extra layer ("Brain Box")? 3DS has bluetooth, have you consider about leverage it? As I remember, there is some project that turn an original Gamecube controller to wireless with dongle.
Click to expand...
Thanks for informing me!
But I think that bluetooth option would be way more harder (still great idea) and the gamecube controller needs to be moded aswell, ading to the final cost .But could it be even posible?

Now, why did I choose the external box?
The new 3ds xl has a so what good internal space to put every thing, but not big enough to put whatever we want and tiny components could be expensive and hard to desine (in case there's the need).
The external box solves almost everything by doing the hard work to transform the gamecube inputs to usable ones for the MCU inside the 3ds. It could also have an internal baterry to charge the 3ds via the usb!

Is there any other MCU that could work?
 
Last edited by Riker123,
This is an awesome project for you! As you probably know, this kind of thing has been done a few times before, however through my research it appears that the original custom PCBs by Loopy are hard to come by. Nevertheless, researching existing designs could help you immensely! It appears people used a custom PCB which connected to a custom pin-slot or USB-C in the 3DS, then a custom female Gamecube port was plugged into that slot. Here are some resources I found:

New 3DS

https://www.ebay.com/itm/266952884366

(Old) 3DS
https://github.com/dekuNukem/gc3ds
 
I'm super interested in this too, personally I think Loopy's controller mod is almost perfect it just doesn't support the c nub and the ZL/ZR buttons. I'm having trouble identifying the chip that was used in the video PurpleGoose linked. If I knew more about that chip and how it was made that would be great for creating a new one that supports the additional controls the N3DSXL has. I'm also not sure where the test pads are for the c stick or the ZL/ZR buttons.
 
Last edited by CyberFusion,
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cearp said:
good luck, interesting project :)


not true!
Click to expand...
That's what i thought, but I still have some doubts.
Post automatically merged:

CyberFusion said:
I'm super interested in this too, personally I think Loopy's controller mod is almost perfect it just doesn't support the c nub and the ZL/ZR buttons. I'm having trouble identifying the chip that was used in the video PurpleGoose linked. If I knew more about that chip and how it was made that would be great for creating a new one that supports the additional controls the N3DSXL has. I'm also not sure where the test pads are for the c stick or the ZL/ZR buttons.
Click to expand...
If you have some more info I would be very thankful, anything works.
Post automatically merged:

PurpleGoose said:
This is an awesome project for you! As you probably know, this kind of thing has been done a few times before, however through my research it appears that the original custom PCBs by Loopy are hard to come by. Nevertheless, researching existing designs could help you immensely! It appears people used a custom PCB which connected to a custom pin-slot or USB-C in the 3DS, then a custom female Gamecube port was plugged into that slot. Here are some resources I found:

New 3DS

https://www.ebay.com/itm/266952884366

(Old) 3DS
https://github.com/dekuNukem/gc3ds
Click to expand...

Thank for this information. I did watch that video in the past, I just didn't really know what he was doing!
I hope that people that watch this post can finally have a controller mod!
 
Last edited by Riker123,
Hi guys. currently workin on something similar. im using a switch pro controller. my microcontroller is an RP2350 tiny dev which lives in the console. it interacts with a 8bitdo v2 wireless adapter for a handfree option for the console. you can also plug it into the back of the 3ds. ill be building a dock for this to. the c stick is also my last thing to map. the 2350 has proven to be a really good controller for this as the dual core 4 ADC outs work great for the circle pad so far
 

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