So, i bought a FM16W08 and i've installed it in a gameboy game : Wario Land Super Mario land 3 (MBC1 and 64kbit sram)
I've uploaded a 100% save in the cartridge.
Tried on my gameboy and works but then i got some glitches. "Wario" is invisible, impossible to play.
So i found this topic 3 month earlier and i decided to post here today.
I've contacted BeenVenn, if you don't know him check his shop
https://bennvenn.myshopify.com.
He makes cartridges with fram chips but later i've noticed he makes his own mbc chip.
I've explained the problem and he told me to try to multipex the /CE signal with the cartbus /CLK signal. I've tried with a CD4011, didn't work, black nintendo logo. In the process i've swapped pin 1 and 3 by mistake so this could have damaged the MBC1.
Later on i found this :
https://www.reddit.com/r/Gameboy/comments/4rkoyz/bennvenns_gbgbc_high_quality_reproduction_pcb/
In a post BeenVeen says :
RAMTRON brand FRAM has an unusual requirement where the /CE line must be strobed before any read or write in order to 'precharge' the magic within, and update the address matrix (It's all in the datasheet). On the older GB ROM's the gameboy does single byte reads and writes which due to the nature of the GB bus the /CE line automatically strobes.
The GBA is a little lazy in this regard and holds the /CE line low and just increments the address bus. On a real ROM/SRAM this is a legitimate way of reading data (assuming you adhere to timings) but with the FRAM it simply repeats the same data from the address where the /CE line first became active.
Gold/Silver etc are a lot more optimised and also perform multiple RAM address reads/writes without de-asserting the /CE signal. This is why they glitch. You can fix this by multiplexing the /CE signal with the cartbus CLK(PHI) signal.
As I code my own MBC in a CPLD (An enormous amount of logic gates which I can arrange in any particular order) I can ensure the /CE signal is strobed at ever access request.
Could be great if someone tries to multiplex /CE and /CLK cartbus, even if i already did. (My cart could be damaged)
I'm gonna buy a Logic analyser soon...