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If you are absolutely sure you can do it, go ahead and try. There are already two projects on the way, one by krikzz and another one by someone else, who I'll keep secret for the moment. (Not me.)

I don't mean to discourage you too much, but if you're not sure you can finish it (given your available time and skill) you may just be wasting your time. The problem for you with doing an open source project is that anyone can make copies and sell (well depending on the license, but assuming GPL) without paying you anything. The problem for anyone producing the hardware is the same, the already thin profit margins are spread even thinner if other people start producing the same hardware.

I would argue that open source works well when the development time matches the user base, ie small user base and low developer cost, and big user base and high developer cost (for example the Linux kernel, Firefox, Apache, MySQL or your favorite large OSS project.) Otherwise the risk is that the project becomes stale because of lack of development effort. This is especially pronounced in a hardware project, since hardware has a higher production cost than the electricity it takes to compile a program.

For this project, I suspect the challenge will be the SD card state machine and FAT interpreter. You want this to both work and be fast enough. And you need a bootloader somewhere that cannot be easily erased so you'll effectively brick your cart. Plus, probably, an endless list of things that need to be fixed.

Well well, enough with the discouragement rant.

A fake Atmega? Are you really sure about that?

The circuit could be modified for use with a GBC/MGB, I'm pretty sure. I personally would have done the circuit a little differently. It now connects just to the battery terminals. I would have used three terminals, ground, regulated +5V Vcc and the sense voltage. The sense voltage trigger point could then be set to something suitable for either a DMG or a GBC/MGB (something using only two 1.5 V batteries).

The problem then becomes fitting the board inside a GBC/MGB. And speaking of that, unless that board is single-sided, it could be made more space-efficient by placing components on both sides of the board. Though that might be problematic when you're going to mount it, but yeah, might be beneficial for the smaller 'boys. But this is just turning into a technical review of the batt_dmg.

You could likely not use an indicator from a GBA SP, as it's probably made using a Li ion battery charging chip which has an internal voltage reference made for that battery chemistry. This mean the threshold for a low voltage is likely set higher than the voltage of two AA/AAA, ie two AA/AAA will always detect as being low. Not to mention that it's easier to design a simple circuit from scratch than it is to reverse engineer the GBA SP's.

Same with the GBA's, which is using AA batteries, but the indicator circuit is likely integrated into the voltage converter.

Did you understand what I meant by the resistors? Do you need an image explaining what I mean?

Someone made it to ridicule hime, and I guess he's taking pride in that notoriety.

And sorry, shawn. This was fun while it lasted. Let's now move on.

This hereby officially only allowed if you pay for it with Dogecoin.

Before possibly wasting your time/money, could you please look inside, take a photo of the board, and note what is written on the chips that are on there?

Unfortunately, the short answer is no. EPROM chips typically require a high voltage (around 12V, compared to the 5V for that the GB is using.) to be present, and that has to be generated somehow. Aside from the practical difficulty of doing this (This might be done with an extra adapter board sitting between the flasher and the cart containing a voltage boost circuit for example.) it might be impossible to this programming in-place on the cartridge. For example, the programming pin may not be routed to the cartridge connector, or it may be shared with one of the other pins, and connected in parallel with the memory bank controller chip in such a way that the MBC would be destroyed.