I wrote code for the GameBoyAdvanced years ago. See Circuit Cellar Magazine, February 2006. Back then I had a nice XPort cartridge to link my computer to the GBA. There was no hardware work involved -- just C++ and learning the hardware registers on the GBA.
Gameboy/GameboyColor homebrew development is nothing new. There are lots of tutorials on GB/GBC hacking (see the links below). This repo is a journal of my experience and any code I produced along the way.
I started with a Game Boy Color and several cartridges I bought from ebay.
Nice, concise listing of registers and opcode changes: Tech info
Here is a cartridge like I made (I got the idea here). It explains how to remove the existing ROM and replace with an FRAM. There is even a circuit here to program the FRAM: Gameboy Fram Cart
http://www.reinerziegler.de/readplus.htm#programmable_MBC5_game_cartridge
http://www.loirak.com/gameboy/gbprog.php
http://belial.blarzwurst.de/gbpaper/paper.pdf
http://gbdev.gg8.se/wiki/articles/Main_Page
FRAM datasheet: http://datasheet.octopart.com/FM1808-70-PG-Ramtron-datasheet-8328945.pdf
The Gameboy hardware line is wonderfully backwards compatible. The Gameboy Advanced (GBA) has two cartridge slots: one for GBA games and one for old Gameboy/GameboyColor games you may own.
The Gameboy (GB) has a special Z80 processor with a few opcode differences. There is a boot ROM in the console that draws a splash screen and transfers control to any cartridge plugged in.
The GameboyColor (GBC) is the GB with additional hardware. The processor and memory layout is the same. GB games play fine on the GBC. The GB uses 4 AA batteries (6V). The GBC uses 2 AA batteries (3V) but has a power boot to crank 3V up to 5 to run the console and cartridge.
The cartridge connector is one-sided, 32 pins. The signals are power, ground, clock, read, write, chip-select, reset, audio, 16 address lines, and 8 data lines.
The Z80's address space is split nicely in half. The lower 32K (A15=0) is ROM space in the cartridge. The upper 32K (A15=1) is for hardware registers, system RAM, and an additional 8K RAM area in the cartridge.
Most cartridges include extra RAM (often battery-backed) and ROMs much larger than 32K. Nintendo used a line of bank-switching chips like MBC5 in each cartridge to select banks of RAM and ROM in the cartridge. I'll discuss the MBC more in the programming section below. The chip generates the upper address lines (A14 and up) from internal latches you twiddle by writing to the ROM address space.
Below are the boards from two cartridges. The Tarzan cartridge has a ROM chip on the left and the MBC5 bank switching chip on the right. The Winnie the Pooh cartridge has RAM and ROM (left and right) with the MBC5 chip above the RAM. The 8 pin chip in the top left monitors the cartridge's power and keeps the battery-backed RAM in backup mode to protect it when the power drops. The battery is the big silver disk in the upper right.
The easiest way to start is to modify an existing cartridge: replace the ROM chip with something programmable.
The cartridge is one-sided. It slides into the back of the unit so that the connector pads and parts are facing out (not in towards the unit).
Cartridges are hardware-backwards-compatible. A simple game with only one bank of ROM will work fine in a cartridge that includes battery backed RAM and room for many ROM banks. Thus a modified Zelda cartridge, which has all the possible resources, could be used to play any game.
I decided to use the simplest hardware: the Tarzan cartridge. I used a screwdriver and needle-nose pliars to unscrew the Nintendo screw to open the cartridge. I used diagonal cutters to carefully remove the ROM chip, and I cleaned off the legs with a fine tip soldering iron.
This picture shows the cleaned board. All of the surface mount pads are traced to through-holes making it easy to solder wires. I have labeled the ROM's holes and the cartridge connector.
The MBC5 bank switch divides the 32K ROM space into two 16K banks. The data lines and address lines A0-A13 (16K) are wired directly from the cartridge connector to the ROM chip. The upper two processor address lines A14 and A15 are wired to the MBC5 chip. MBC5 outputs the RA14-RA19 address lines for larger ROMs as banks are switched in and out through software.
Here are the pinouts for the Nintendo ROM chip and the MBC5 bank switcher. The Tarzan ROM is an LN538, and the upper left pin (pin 1) is A19. 20 address lines -- that's a 1M ROM divided by the MBC5 into 64 16K banks.
I found an old 32K FRAM chip in DIP form on ebay. These are getting harder and harder to find. A much better way would be to use a modern FRAM chip, but these are all surface mount. I wanted to start with something easier to work with.
The FRAM chip is a little tricky in its bus cycle. It uses CE to latch in the address, and a little later you can assert OE to read from it. The FRAM is also sensitive to the CE level. If the CE driver tristates then the data can get mangled (and it does in my playing with it). The datasheet strongly suggests using a pullup resistor on the CE line.
On the GB cartridges, the ROM's CE line is connected directly to the A15 line. As soon as the A15 goes low then the remainder of the address is latched into the FRAM chip. Makes sense because the entire A15=0 area is reserved for the ROM chip.
The Z80 tristates the address lines between access cycles. You must add a pullup resistor (10K does nicely) to the CE line in the modified cartridge. Then when A15 tristates between cycles the CE is pulled back up to begin another access cycle.
I used a 32 pin chip socket for the FRAM even though it only needs 28. That leaves room for future expansion if I ever rewire it for a larger FRAM.
The FRAM wiring is simple. Use the labeled board picture above. Wire up all the signals D0-D8, A0-A13, RD, CE, and RA14 directly from the Nintendo ROM through-holes. Wire up VDD and GND. Wire in a 10K resistor between VDD and CE.
Many people connect the WR line from the cartridge connector to the FRAM's WR signal. Then they make a cartridge reader/writer box to program the cartridge.
I went with a socketed FRAM chip. You take it off the development cartridge and put it in a programmer to load new code. Very old-school.
I made my own programmer using a Propeller prop-stick board.
The propeller output pins are only 3.3V. But the FRAM chip recognizes 2.0 as high. The VDD of the FRAM chip must be 5V. The propeller tolerates the 5V outputs from the FRAM chip.
Again, the 10K pull-up resistor is very important. You must pull the CE pin to VDD so that the FRAM does not get mangled during reset when the propeller chip's GPIO lines are tristated.
I downloaded the GB Tetris ROM to my PC. I used my programmer to write it to the FRAM chip. Then I put the FRAM chip in the cartridge and put it in the GBC. Works great!
