[solved] need help with SFROM multicart
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Author:  QuickMythril [ Mon Jun 22, 2015 8:51 pm ]
Post subject:  [solved] need help with SFROM multicart

hello, first time posting here, but probably won't be the last. sorry if this post would belong better in NES Hardware and Flash Equipment. my previous experience: several years working with electronics, two successful working TKROM boards with 27C020s in PRG and CHR.

my information sources: ... -multicart ... t#Variants

my problem/question: so i am trying to make a multicart where you press reset to send the signal from the CIC to a binary counter that switches between the areas of ROM by connecting to the highest address lines. i was using the tutorial above (with some deviation) and so far i have a 27C020 connected in the PRG slot. this contains a 256 KB .bin that i made by joining two different 128 KB .bin files. i would connect the counter to A17 (pin 30) on this eprom to switch between the two 128 KB sections if i understand correctly. i am currently waiting on that part in the mail.

now the CHR slot is only 28 pin and the board i had was 32 KB, but SFROM should also be able to handle 64 KB as well. that still works out because the two CHR.bin files i wanted to switch between are 32 KB each. originally i had written the eprom with the CHR data repeated as 11112222, (8 * 32 KB bins = 256 KB eprom). which brings up the first actual question. can i use a 32 pin 27C020 on this board? looking at the pinout it just has two less address pins. so i left A17 and A16 disconnected, and i have A15 of the CHR wired to A17 of the PRG, so that should also cause the reset button to switch between the 32 KB bin files. before i put soldered it in, i figured i should have the data written as 12121212, that way A16 and A17 would be the same either way and A15 would switch properly, so i erased and reprogrammed that first.

the two eproms do not fit side by side because the CHR slot just doesn't have enough room, so i bent the legs around and soldered it in to the opposite side of the PCB. just to be thorough i have both eprom pins 1/31 (VPP/PRG) not connected, PRG pin 2 to hole 24 (A16), [EDIT: i had removed this but i forgot: PRG pin 30 to hole 1 (A17)], PRG pin 24 (OE) to pin 16 (GND). for the CHR eprom i have the bottom 26 pins (4-29) connected to holes 2-27. hole 1 (A15) is not connected, hole 28 is connected to pin 32 (VCC), so i have 1,2,30,31 all not connected (unused address and programming pins).

i figured even without the counter hooked up this game should work if i tried it in a NES. it just wouldn't be able to switch. the other side of the chips would be inaccessible to me. that's my next question, whether or not that would work. of course for some reason it did not work, well almost. i kept getting graphics from the second half of the data, and it actually started the menu and accepted controller input, but it kept freezing and glitching and at one point i saw graphics from the other half of the data mixed in.

that had me worried and also thinking i might've done something wrong with the setup of that data. the other thing i was thinking was that i read someone saying unused addresses need to be tied to ground. i am hoping it is something that simple. anyway thank you for reading my long story, and extra thanks to anyone who can help me solve this problem and get the game working.

here is a picture of my board, also explaining the wiring for those who are more visually oriented:

EDIT: just tried again and got past the title screen onto level 1 before it froze. it just might be an issue with this top loader not making the best connection. i will try to clean the pins as well, and try another nes.

EDIT2: also tried on a front load NES and got the same issue. i'm going to try putting the files i made back to a .nes file and see if that will run properly in emulators.

EDIT3: grounded the address lines and it plays the first game just fine. tied it to +5V and it played the second game. problem solved. can't wait for my counter to arrive.

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