GNROM repro Doraemon title screen glitch

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mightydidz
Posts: 74
Joined: Wed Jan 24, 2018 12:04 pm

GNROM repro Doraemon title screen glitch

Post by mightydidz » Thu Oct 08, 2020 6:51 am

I've built myself a GNROM repro (English Doraemon) and it works fine, except that on bootup there is some artifact on screen.
Just a quick touch on reset button and its fine.
I'm wondering what's on my board that need to be fixed, maybe delaying a signal?
Maybe adding a delay cap on the 74'161?
Here is what I'm talking about.
<img src="https://i.ibb.co/CbjGP0M/IMG-20201008-094309.jpg" alt="IMG-20201008-094309" border="0">
<img src="https://i.ibb.co/wRZMr4y/IMG-20201008-094320.jpg" alt="IMG-20201008-094320" border="0">
<img src="https://i.ibb.co/tsGhyGg/IMG-20201008-094340.jpg" alt="IMG-20201008-094340" border="0">

NewRisingSun
Posts: 1287
Joined: Thu May 19, 2005 11:30 am

Re: GNROM repro Doraemon title screen glitch

Post by NewRisingSun » Thu Oct 08, 2020 7:45 am

It's an original game bug. The game reads uninitialized memory. So it's nothing you can correct in hardware. You would have to modify the PRG-ROM content itself to clear CPU memory.

You can reproduce the problem in emulation, for example in FCEUX by setting Config->RAM Init to "Random", then choosing NES->Power a couple of times. In many of the cases, you will get either a black or gray screen, or the one in your pictures, and a soft reset will then correct the problem.

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