Gameboy Development Forum

In a bout of frustration over not being able to find a single chip board that uses a PLCC32 chip, I downloaded EAGLE (apparently the developers are very enthusiastic about the name because it's all capital letters) and began learning what I needed to design a cartridge myself, but I've hit a few snags from limited documentation. I've needed to use Wayback Machine to get some information while some just can't be found.

In my designs, I'm currently using SST39SF010A by Microchip Technology which seems to have similar specifications (or something similar, may have grabbed the wrong one this time.) It's a PLCC32 packaged chip, but I don't quite know how some of the pins line up with the leads on the chip.

Do I need to do anything with the pin 32 on the cartridge (ground) to ground it or does that lead into grounding built into the Gameboy itself?

Which pin do I connect to Chip Enable (CE#) to? I have conflicting information on this. Two sources show I'm supposed to connect it to pin 21 on the cartridge, but another two sources says that is for address 15 and don't specify what should be connected to CE#. Since I don't have an MBC, I would think I would need to use every address pin available.

And, for future projects, would I connect pin 3 (Write) to Write Enable (WE#) if I was designing a cartridge for a game that used save data? Or would that be routed through another chip first?

I just released GB-CART32K-A, which sounds like what you need: https://github.com/gekkio/gb-hardware#gb-cart32k-a
It's all open source and kicad+gerbers+schematics are available in the repository. I haven't actually had this design manufactured yet, so if you decide to use it, maybe manufacture 5 first instead of 100 to be safe
But I've designed and succeeded with some carts and other GB hardware before, so this should work perfectly as well.

For the flash, you need a 5V PLCC-32 flash ROM that uses the standard JEDEC pin layout. For example, any SST39SF series chip will work (but remember that only 32kB will be usable).
In addition, you need one 0.1 uF SMD capacitor (0603, 0805, or 1206. Use X5R or X7R, rated 16V or higher), and one 47k resistor in SMD package (0603, 0805, or 1206).

Ardis wrote:

I would rather learn what makes these chips suitable

In case of flash chips, I can think of at least two things: VCC voltage and pinout. The supported pinout depends on the board, but let's take my board and some PLCC-32 flash chips as an example...

Firstly, Game Boy is 5V and there's no level shifting on my board, so only PLCC-32 chips that work with 5V are ok.
AT49F040: OK, because datasheet says "Single-voltage Operation 5V Read 5V Reprogramming"
AT49F001T: OK, because datasheet says "Single-voltage Operation 5V Read 5V Reprogramming"
SST39SF010A: OK, because datasheet says "Single 4.5-5.5V Read and Write Operations"
SST39SF020A: OK, same datasheet as previous
SST39SF040: OK, same datasheet as previous
SST39LF010: not OK, because datasheet says "3.0-3.6V for SST39LF010/020/040"
SST39VF010: not OK, because datasheet says "2.7-3.6V for SST39VF010/020/040"
GLS27SF010: seems OK, because datasheet says "4.5-5.5V Read Operation". Spoilers: it's actually not going to be ok
SST27VF256 (in DIP-28) that you mentioned earlier, is not a 5V chip so it's not OK in a Game Boy cartridge.

Secondly, the pinout. This requires understanding the board you're using. My board is for JEDEC pinouts, and while some signals aren't really used, none of them are left floating. This means in practice that you can use different sizes of flash ROMs and they all work the same (32 kB is usable).
On boards that have floating address pins, you can't use a larger ROM chip than what the board is designed for.
My board supports the pinout used by AT49F040, so basically you can look for PLCC-32 ROMs with the same pinout (some signal names might differ slightly), and this requirement will be satisfied.
AT49F040: OK
AT49F001T: OK, but it's not immediately obvious why it's OK. The pinout is otherwise the same, but this chip has a #RESET pin where A18 is on some other chips. My board pulls the unused A18 pin high, so this chip would not be in reset mode and would work fine. Some other boards would probably not be compatible with this chip if they leave this pin floating or pull it low.
SST39SF010A: OK, correct pinout. This is a 1Mbit chip instead of 4Mbit like AT49F040, so some address pins are NC but this is fine.
SST39SF020A: OK, correct pinout
SST39SF040: OK, correct pinout
SST39LF010: OK, correct pinout
SST39VF010: OK, correct pinout
GLS27SF010: at a quick glance seems to be OK, but the problem is pin 1, which is normally A18. On these chips it's Vpp, which is programming voltage. If you want to flash this chip, you need 12V on that pin, but my board isn't designed to support this. -> not OK

Here's all the cartridge pins using names that I use:

1: +5V
2: PHI (clock generated by the CPU chip, but it's not the main clock)
3: #WR
4: #RD
5: #CS
6-20: A0-A14
21: A15
22-29: D0-D7
30: #RES (active-low reset signal. This is an open drain signal, which in practice means either the Game Boy or the cartridge can pull this low to reset both)
31: VIN (analog audio input, but this pin is often reused as flash #WR signal)
32: GND

Now, the main thing when designing anything that connects to these pins is to fully understand the purpose of the pins you connect to. The data bus pins are shared, which means multiple chips are connected to them and chip select signals are used to control which chip is allowed to drive the data bus.
This has two implications: 1. only one chip select signal must be active at a time, and 2. you must not drive the bus when your chip select signal is not active.
On the Game Boy, there are two memory areas from the CPU point of view that are reserved for the cartridge: $0000-7FFF, and $A000-BFFF. You'll notice in the pin list that there's a pin called #CS, but this name is slightly misleading, because the Game Boy pulls #CS low when the CPU is accessing $A000-$FDFF.
When accessing $0000-7FFF the correct chip select signal is actually A15, because A15 is low when accessing that memory area and high at all other times. So pin 21 is A15 but also fulfills the role of #ROM_CS at the same time, and you must use A15 as the chip select if you intend to respond to reads/writes in $0000-$7FFF.
(as a side note, if you wanted to respond to $A000-BFFF, you can't just use #CS because it's also active during $C000-$FDFF accesses)
If you're curious about how the different signals behave during reads and writes, check out appendix C of gbctr: https://gekkio.fi/files/gb-docs/gbctr.pdf

Now, if A15 goes to #CE, what about A15-A18 on the flash chip? Since you're only using 32kB, A0-A14 are the address pins and A15 is the chip select, and other address pins can't be used.
In chips based on CMOS technology (basically everything nowadays), you should not let pins float (= left unconnected). This is a bit technical but good source (from 1994!) that explains the topic: http://www.ti.com/lit/an/scba004d/scba004d.pdf
So, you're not really using A15-A18 for anything useful, but they must be connected to something. On CMOS chips you can connect inputs directly to +5V or GND to "lock them" to either low or high value. In my design I used high value, because this gives me compatibility with PLCC-32 chips that have #RESET in pin 1. But if you're using SST39SF010A, you could connect A15-A18 to GND instead if you wanted.
Note that you have another unused input on the flash chip: #WE. This *MUST* be connected to +5V or you will have problems. The reasoning is the same, but this time we can't connect it to GND, because that would basically keep the flash chip in "write mode" all the time since it's an active-low signal.


TL;DR
A15 is the ROM chip select
Connect unused inputs to +5V or GND
Connect flash chip #WE to +5V

Hi everyone,
Sorry first time poster, I hope it's fine to post on this old thread but content wise it is fit. Please tell me if I should repost elsewhere.

So I made my own PCB (32kB only noDMGx chip) and it works fine after a few iterations but only on GB Color. DMG and pocket recognize it and read it (proper Nintendo logo on boot), I get the Di-ding! sound and blank screen.
I tried with my own game or commercial roms, same: works nicely on GB color, not on DMG nor pocket. Conclusion something is wrong with the PCB, not the EEPROM not the way I write it.
I tried tying AUdio-in to VCC via a 40kOhm resistor, no change (as expected but being systematic).
My next suspects are :
* /CS that I did no connect: I tied the /CE of the EEPROM to GND since I thought that it would make no difference since there's no DMGx chip
* /Reset that I did no connect: I tied the /Reset of the  EEPROM to VCC

The rest is wired "as expected" I think, but I listed it on the webpage of the project.
What is your bet guys? Any suggestion please?
Thanks for any help, it's getting frustrating (3rd iteration of that board). I will update once I find the solution, for the good of future generation of tinkerers

http://electrogeek.cc/gameboy%20homebre … ridge.html

As pointed out, CS is really the CS for the RAM portion of the memory map. A15 works as CS for ROM, since it happens to be 0 when the address bus is in the range 0-0x7FFF. As your board is connected currently, there's a bus collision with the RAM chips in the Gameboy which are on the same physical address and data bus, and are now active at the same time! (Just repeating what has been said.)

Here's a suggestion for anyone you or anyone else making a 32k cart using bigger EEPROMs. Please make the cart hackable by connecting the through upper address bits through cuttable jumper links with solder pads, instead of connecting them directly to ground. The links can then be opened and connected for example to a set of DIP switches to make the cart a multi 32k cart in the future if desired. Refer to the jumper library in Eagle, parts SJ_2 and SJ_2W. Ground to one side, +5V to the other and the address pin to the middle. Those are supposed to be soldered in one or the other position, but you could also connect a thin trace between the address pad and the ground pad as a default that can be cut so you don't have to solder those pads on each cart for default use.

Edit: I typed that for no good reason since it seems your (Alan's) cart does do that. I was looking at the screenshot.

Last edited by nitro2k01 (2020-05-24 03:25:15)

Hello there I am new to the forum and could do with some help.

Is there anywhere I can purchase Game Cart Copper Clad PCB's with the Gold fingers ready to be etched?

Thanks

Brian