I'm trying to make my own custom cartridge but I've run into some problems. Here are the steps I took:
1. Got a donor cartridge PCB (Skiing by Activision). This is a 2k game but the layout looked straight forward.
2. I got an Atmel 8-bit 5-volt 64k EEPROM.
3. I desoldered the skiing EPROM and soldered pin headers in instead. (since the Atmel EEPROM doesn't actually fit because it has more pins.)
4. I programmed the EEPROM with the Skiing ROM. I checked the data in Stella and on the programmer and the bytes match.
5. I placed wires from the headers on the cartridge to the EEPROM on a breadboard.
When I turn the Atari on, it usually makes random sounds and scrambled screens. It's almost like the Atari is getting random data every time it turns on. The sounds and screen data changes every time if anything is displayed at all.
Checking with a logic probe, it looks like everything is high. The address pins and the data pins all register as high.
I'm not sure what to do with the sGND and the A12 address bit. If I connect the sGND to the GND the Atari won't start. Do I just connect the A12 to the EEPROM's A12? Does the Atari use A12 or is it always on?
Outside of wiring it wrong, is there anything that stands out? When I programmed the EEPROM, I left the remaining ~62k empty (FF) assuming the Atari can't address it anyway.