First homebrew cart attempt

Discussion of programming and development for the original Game Boy and Game Boy Color.
gekkio
Posts: 48
Joined: Fri Oct 16, 2015 6:18 am

Re: First homebrew cart attempt

Post by gekkio » Sat Jul 27, 2019 2:23 am

A0-A15 -> A0-A15 (obviously)
CE -> CS (Pin 5 of edge connector)
This doesn't look right to me. Cartridge connector ~CS is not a chip select signal for a ROM, and if you use it like that, you can cause hardware damage because there will be bus contention with the work RAM chip.
But since you haven't yet booted an actual game, I think you should be ok because I don't think the boot ROM reads work RAM.

Also, if you don't have a mapper chip, you can only have 32 kB of ROM which means A0-A14 pins can be connected, but A15 should be permanently tied to VCC or GND.
So this is not a correct assumption:
As far as I can tell I should be able to load a 64KB game off a W27C512 without issue
You will never be able to have a 64 kB game without a mapper chip / banking method of some sort.



This reply to has some more information:
http://gbdev.gg8.se/forums/viewtopic.php?pid=3625#p3625

And here's a schematic for a 32 kB cart for reference: (note that the flash chip is 1Mbit and unused address pins are tied to VCC!)
https://github.com/Gekkio/gb-hardware/b ... T32K-A.pdf

lidnariq
Posts: 9702
Joined: Sun Apr 13, 2008 11:12 am
Location: Seattle

Re: First homebrew cart attempt

Post by lidnariq » Sat Jul 27, 2019 12:00 pm

Stryder99x wrote:CE -> CS (Pin 5 of edge connector)
Specifically on the game boy, "CS" is roughly an inverted copy of A15. (There are some differences, but it's easiest to think of it that way)

The MBCs use "CS" and A14 to find the region where the cart can place RAM.

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