Making a 4MB (32Mb) RTC Flash Cart with a MBC30 cart
Channel: makho
Date: 2019-09-28

BIG THANKS TO ALEX FROM INSIDEGADGETS! He sat down and helped me troubleshoot an issue I was having with flashing this cart and helped get it working reliably! More details near the bottom. 

For those who are not aware, MBC3, the only chip that supports RTC in GB/C games, does not support ROM sizes of over 2MB. Some documentation seems to say otherwise. MBC30 (only found in Japanese Pokemon Crystal carts) is thought to be the exact same except that it can address twice as much RAM. Well, apparently, thanks to a collaborative effort and some research and experimentation done by gekkio, we now know that MBC30 can in fact support 4MB ROM sizes. 

Due to this proof of concept from gekkio and then because I could confirm his work, we now believe that 4MB flash carts can be easily made (so gen two Pokemon ROM hack makers, you now have twice as much space to work with if you want your ROM to work fully on actual hardware). 

It appears that the glitch I was experiencing with that one bank is due to a quirk with MBC30 chips and using pin three on the GB cart bus to flash the chip. If you don't want to have your cart look like a Pokemon Crystal cart, you can also get a custom PCB: https://github.com/MrHDR/MBC30-Flashcart

This PCB is sized to fit in regular carts but the MBC30 chip MUST come from a JP Pokemon Crystal. MBC30 is not interchangeable with MBC3 or vice versa for this application. PCB is untested. 

In the video, I soldered the WR pin on the flash chip to the third pin on the cart bus, like standard flash carts. For whatever reason, MBC30 does not like this, but only for one single bank. The solution to this issue is to either flash the chip before soldering it and then give up on that bank indefinitely, or to solder the WR pin wire to the audio pin on the GB cart bus instead (pin 31). From there, the GBxCart RW console flasher can be modified to use pin 31 instead and can then successfully flash every bank in the cart. As of now, only GBxCart RW can fully write this cart: https://shop.insidegadgets.com/product/gbxcart-rw/

This is still being actively investigated so I would highly appreciate any pertinent info if you happen to know something I don't or stumble upon something. If you're the type who just wants a finished product, that's coming too. Eventually.