Homebrew Looking to mod sdcard slot into slot 1.

[Caboosium]

I was thinking I could solder a SDcard ---> MicroSD card adapter to the Slot 1 keeping the Slot 1 working (If a MicroSD card is not connected.) and also have the MicroSD card usable as a Flash Cart (Similar to a R4).

Thought this might be possible after looking at pin-out information online and seeing similarities.

 

[FAST6191]

DS carts have a variety of functions that will need to be taken care of. Some devices use standard protocols (360 NAND but substituted for XD memory, CF cards being IDE by any other name, any number of controller standards over the years....) but not the DS and its DS slot does not act with much of a standard protocol*.
You can program a chip to act as a middleman but at this point you are reinventing a flash cart, many of which have already been made and had years of work poured into them.

*technically the save uses SPI ( http://problemkaputt.de/gbatek.htm#dscartridgebackup ) and SD cards do speak SPI ( https://learn.adafruit.com/adafruit-micro-sd-breakout-board-card-tutorial/look-out ). There was a family of mods way back called the passme2 which used the save of the DS slot device to launch custom code (I imagine you in turn could mod the firmware of the DS to do this rather than do an exploit). For the effort again flash carts are things which are readily available, and for most of what you are going to do with this SPI affair then you could probably get away with the wifi delivered homebrew -- if you can flash your DS with flashme then you can send whatever you like without hassle, if not there is a new exploit for it https://gbatemp.net/threads/haxxstation-ds-download-station-exploit.473648/ .

Short version. Buy a flash cart.

 

[Caboosium]

DS carts have a variety of functions that will need to be taken care of. Some devices use standard protocols (360 NAND but substituted for XD memory, CF cards being IDE by any other name, any number of controller standards over the years....) but not the DS and its DS slot does not act with much of a standard protocol*.
You can program a chip to act as a middleman but at this point you are reinventing a flash cart, many of which have already been made and had years of work poured into them.

*technically the save uses SPI ( http://problemkaputt.de/gbatek.htm#dscartridgebackup ) and SD cards do speak SPI ( https://learn.adafruit.com/adafruit-micro-sd-breakout-board-card-tutorial/look-out ). There was a family of mods way back called the passme2 which used the save of the DS slot device to launch custom code (I imagine you in turn could mod the firmware of the DS to do this rather than do an exploit). For the effort again flash carts are things which are readily available, and for most of what you are going to do with this SPI affair then you could probably get away with the wifi delivered homebrew -- if you can flash your DS with flashme then you can send whatever you like without hassle, if not there is a new exploit for it https://gbatemp.net/threads/haxxstation-ds-download-station-exploit.473648/ .

Short version. Buy a flash cart.

What is flashme?

 

[MlgAllStar]

What is flashme?

FlashMe is a custom firmware for the DS Phat and Lite, and it requires a way to bridge the SL1 and a GBA flashcard. This was back before normal flash carts existed, and in order to install it you will need a PassMe/NoPass, WifiMe, or normal flashcard like the R4. It basically runs roms off your SLOT-2 flashcard.

 

[FAST6191]

Almost
It is a custom firmware for the DS. It is also nothing like you might have seen for the Wii, 3ds, PSP, original xbox, xbox 360... when they got custom firmwares -- the DS firmware is very basic and does not do a lot, still worth attacking though. To install it does indeed require a way to short a point on the motherboard; in both the DS and DS lite you can do it by opening the back cover, though the lite takes more effort to do properly as you can also short another thing if you are not careful and turn off the DS (thus stopping the install).

It has three main functions people care about
1) Removing the health and safety screen from boot. Optional if you want it though.
2) Allowing people to boot DS code from either the GBA slot or the DS slot. Originally we only had GBA slot devices but the the DS slot was figured out and we got DS slot carts, and then the r4 and then the rest is history.
3) Run unsigned wifi/download play code.

In your case if you don't want to pay the few dollars a basic homebrew capable flash cart runs these days then you might be interested in running small amounts of code (which is a sizeable chunk of homebrew, and certainly enough for you to dip a toe into DS coding) up to around 4 megabytes. Flashme provides a way to do this by allowing unsigned code from download play, there was however the other hack linked which exploits a bug in the download play check code on everything which supports it.

You don't need a GBA slot device to do it, indeed that would also mean you need something extra to force it to boot from that slot to begin with. Most then just use a DS slot flash cart for the 5 minutes the average install takes to do as you can run flashme.nds just fine from one.

There is a fourth function some care about which is brick protection but you really have to go out of your way to do that to a DS.

This is all a lot of effort though when again a basic DS slot flash cart which supports DLDI (the library used to access the SD cards in flash carts) and that works on a DS or DS lite can be had for next to nothing. The ones which support each and every game cost a tiny bit more but most went close enough to the end that you would not be missing out on much if you did get one of the lesser ones.