32Mbit Multirom 2in1 diagram

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poorstudenthobbyist
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Re: 32Mbit Multirom 2in1 diagram

Post by poorstudenthobbyist » Tue Feb 09, 2021 6:24 pm

What I'm trying to get at is you can't put two 32 Mbit games on one 32 Mbit EPROM.
So how can you store two 64 Kbit saves on one 64 Kbit SRAM chip?
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Kayumba
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Re: 32Mbit Multirom 2in1 diagram

Post by Kayumba » Tue Feb 09, 2021 6:32 pm

poorstudenthobbyist wrote:
Tue Feb 09, 2021 6:24 pm
What I'm trying to get at is you can't put two 32 Mbit games on one 32 Mbit EPROM.
So how can you store two 64 Kbit saves on one 64 Kbit SRAM chip?
:lol: Oh, i see...
So what solution can be ?
As I know , some of games check if there is a right size of SRAM on board and its won't load game.

Can we have some universal schematic for all kind of games (with any size of SRAM) that it will always be able to use 62256 SRAM chip ?

tepples
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Re: 32Mbit Multirom 2in1 diagram

Post by tepples » Tue Feb 09, 2021 8:35 pm

Say you have two 8 Mbit games on one 16 Mbit EEPROM or two 16 Mbit games on one 32 Mbit EEPROM, with some means of selecting one. Now you want to store the two games' 64 kbit (8 KiB) saves on a 62256 SRAM.

A14 = GND
A13 = game select

If the games' saves are both 2 KiB, also connect A12 and A11 to GND.

If the games have different save sizes, with one 2 KiB and the other 8 KiB, it's a bit more complicated. You'll need a dual OR, NOR, AND, or NAND gate on A12 and A11. For game 0 using 2 KiB and game 1 using 8 KiB, use something like this

RAM A12 = game select AND CPU A12
RAM A11 = game select AND CPU A11

You can replace this AND (74x08) with NAND (74x00) if cheaper. Replace this with an OR (74x32) or NOR (74x02) gate for game 0 using 8 KiB and game 1 using 2 KiB. I don't know if or how the gates need to be connected to the battery.

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poorstudenthobbyist
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Re: 32Mbit Multirom 2in1 diagram

Post by poorstudenthobbyist » Tue Feb 09, 2021 8:58 pm

tepples wrote:
Tue Feb 09, 2021 8:35 pm
I don't know if or how the gates need to be connected to the battery.
Using a 62256 to do this is tricky, because you only have one enable pin to work with - the added logic makes it complicated to wire up. I have found that logic devices allow a small but significant leakage current through the output pins when powered off, which drops the voltage on the /CE line (due to a voltage drop across a pull-up resistor), usually enough to take the device out of a low-power state. You'd need a transistor buffer to make it work properly, to decouple the /CE line from the logic output pins.

You could use a 1008 SRAM, as that's more like a 6264 with a positive CE pin as well, for the data retention requirements. Then you can have the logic on the regular /CE pin without worrying about leakage currents.
Check out my website for NES, SNES, and Genesis tutorials here. And visit my store for some custom tools and boards for making games here.

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Kayumba
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Re: 32Mbit Multirom 2in1 diagram

Post by Kayumba » Wed Feb 10, 2021 8:43 am

tepples wrote:
Tue Feb 09, 2021 8:35 pm
Say you have two 8 Mbit games on one 16 Mbit EEPROM or two 16 Mbit games on one 32 Mbit EEPROM, with some means of selecting one. Now you want to store the two games' 64 kbit (8 KiB) saves on a 62256 SRAM.

A14 = GND
A13 = game select

If the games' saves are both 2 KiB, also connect A12 and A11 to GND.

If the games have different save sizes, with one 2 KiB and the other 8 KiB, it's a bit more complicated. You'll need a dual OR, NOR, AND, or NAND gate on A12 and A11. For game 0 using 2 KiB and game 1 using 8 KiB, use something like this

RAM A12 = game select AND CPU A12
RAM A11 = game select AND CPU A11

You can replace this AND (74x08) with NAND (74x00) if cheaper. Replace this with an OR (74x32) or NOR (74x02) gate for game 0 using 8 KiB and game 1 using 2 KiB. I don't know if or how the gates need to be connected to the battery.
OMG !
Thank you to explain.
What you call "game select" = its /RESET ? ou "Q0" on Flip-Flop chip ?

Its looks like it will be more simpler to use Multiroms only with games without SRAM (or maybe only one of two game with SRAM).
That will avoid me to modifie my PCB and to have million of jumpers etc.
I'am afraid that if I will make connections for Multiroms that will support 64+64 SRAM (for example),
and if once I would like to use the same PCB for single EPROMs with SRAM, it will not work.

Thank you anyway guys.

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