Famicom part of sprites are below where they should be

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nbisbo
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Joined: Fri May 14, 2021 5:50 am

Famicom part of sprites are below where they should be

Post by nbisbo »

famicom displaying sprites wrong with part of the face below the sprite like Mario in the fds bios or the hero in dragon quest 2-4
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Quietust
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Re: Famicom part of sprites are below where they should be

Post by Quietust »

nbisbo wrote: Fri May 14, 2021 12:50 pm famicom displaying sprites wrong with part of the face below the sprite like Mario in the fds bios or the hero in dragon quest 2-4
In the FDS BIOS, Mario is drawn entirely using sprites (and those sprites are also non-overlapping), so there's nothing I can think of that would cause only some of those sprites to display in the wrong place.

Are you using a real Famicom, a third-party hardware clone, an FPGA-based emulator, or a software-based emulator?
Also, what do you want us to do about it? Are you trying to figure out why it's happening, or perhaps how to fix it?

Pictures would be useful, as well as more careful use of the English language so we can actually understand what you're trying to say - perhaps you could also include your question in your own native language, in case there are any bilingual readers here who can provide a better translation.
Quietust, QMT Productions
P.S. If you don't get this note, let me know and I'll write you another.
nbisbo
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Re: Famicom part of sprites are below where they should be

Post by nbisbo »

Real Famicom with a av modImage
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mario pic.jpg
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Jeroen
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Re: Famicom part of sprites are below where they should be

Post by Jeroen »

To me that looks like either the OAM ram inside the ppu is broken. Or your famicom RAM chip is broken, and the broken data is copied to the OAM.

Hopefully it is the latter, because RAM chips can be bought easily, PPU chips not so much.
Pokun
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Re: Famicom part of sprites are below where they should be

Post by Pokun »

It looks like the RAM adapter problem that some people have which could be fixed with pull-downs on the data lines in the RAM adapter. But since Dragon Quest also has problems it might be your Famicom.
Fiskbit
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Re: Famicom part of sprites are below where they should be

Post by Fiskbit »

I suspect this is the same issue I begin discussing here. Specifically, the PPU appears to be sensitive to activity on the CPU data lines, and this can cause it to suffer OAM corruption during vblank. It occurs on some systems with certain kinds of carts, FDS included, and is believed to be related to signal reflection. You can add pulldown resistors to the CPU data lines, as Pokun suggested, but instead, putting low value resistors (100 ohms or so) in series on these lines to act as termination resistors should also solve the problem without eliminating open bus behavior that some games rely on. The resistors-in-series method was used to solve this in the Everdrive N8 Pro.
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