I wrote up a "brief" explainer on SNES cartridges

Discussion of hardware and software development for Super NES and Super Famicom.

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lidnariq
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Re: I wrote up a "brief" explainer on SNES cartridges

Post by lidnariq » Fri Feb 14, 2020 12:21 pm

If your board had a conventional 8-bit ROM, you could rely on the ROM's enables to only drive the data bus at the right time.

But because your board mixes in the 74'257s, you have to correctly enable them (not the ROM) at the right time in order for things to work correctly.

The only safe time to drive the data bus is when other things aren't driving it, which could be things inside the SNES (such as RAM or PPU) or on the cart (RAM).

In a HiROM cart, it's ok (although pedantically wrong) to just use /CART (p49) to enable the '257s, because nothing will ever try to write to ROM, so we can assume that if the SNES is accessing the cart it's reading from the cart. (There's some marginal behavior at the beginning and end of each memory access cycle, but it's more-or-less ok.)

In a LoROM cart, you can make a similar assumption, using the output of the 74'139 enabling the ROM based on /CART and other address lines.

Finally, if you're using an analog multiplexer, like the '4066, then you can safely use the ROM's output enables and not worry about enabling the '4066 at the right time, because if the ROM's data bus is high-impedance, then the output of the analog multiplexers will also be.

(Note that the 74'3257 is a bad choice specifically in the case of the SNES, because it doesn't pass full 5VCMOS voltages, because it only has nMOSFETs inside instead of NMOS+PMOS full transmission gates)

tepples
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Re: I wrote up a "brief" explainer on SNES cartridges

Post by tepples » Fri Feb 14, 2020 1:23 pm

nocash wrote:
Fri Feb 14, 2020 10:07 am
No, that's nonsense the SNES does draw (consume) power, it does not supply (generate) power.
Both. It draws power through a 7805 regulator, and it uses some of that power to supply power to the Game Pak and controllers connected to it. ("Supply" doesn't necessarily mean "generate".)

poorstudenthobbyist
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Re: I wrote up a "brief" explainer on SNES cartridges

Post by poorstudenthobbyist » Fri Feb 14, 2020 1:31 pm

Ice Man, I should clarify that my write-up was from the perspective of a SNES board that uses the original Mask ROM, not the 27C322. Maybe that's where our disconnect lies. Sorry for the confusion!

Lidnariq, would you mind me asking what your background is in? You've got a lot of insight into these things.

lidnariq
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Re: I wrote up a "brief" explainer on SNES cartridges

Post by lidnariq » Fri Feb 14, 2020 2:16 pm

Although I have an engineering background, all of this is self-taught. It's been obsolete for too long to be things you'd pick up in most professional contexts.

nocash
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Re: I wrote up a "brief" explainer on SNES cartridges

Post by nocash » Fri Feb 14, 2020 2:54 pm

I am afraid that I had misread this sentence "On every SNES board you’ll notice a handful of capacitors." I had interpreted it as referring to the mainboard, but now I think that it does refer to the cartridge board... and then the description makes much more sense than I had thought : )

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