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Conrad
Registered: Nov 2006
Posts: 849 |
Controlling that colour change bug thing
Would any hardware gurus/coders care to explain what causes this bug to appear and is it possible to control its visibility, code wise?
Thanks. |
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Conrad
Registered: Nov 2006
Posts: 849 |
WVL: On my C64C the grey dots only appear occasionally. When switching the machine off/on, those dots are either visible or gone. It seems bizarre to me why this happens and just wondered if those dots can be forced to be visible or hidden via some machine code, but in any case I'll try what tlr mentioned. I agree it is annoying to see these grey dots in demos that make use of $d020-$d02e for raster colour splits and stuff similar to that, but I plan to use them for something useful (even it it's pointless for people with an old VIC or emulator ;)) |
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Ninja
Registered: Jan 2002
Posts: 411 |
I have one C128 where those dots are usually(!) stable when the machine is cold. After some minutes they start flickering and sometimes they disappear completely after some more time. All too random for my taste... |
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TPM
Registered: Jan 2004
Posts: 110 |
Very annoying indeed..
On my c64 the dots/stripes appeared and were increasingly present more and more when the VIC got warm, usually after a couple of hours or so. Cool down the c64 for an hour and it was okay again. |
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Ed
Registered: May 2004
Posts: 173 |
Grey dots for the emulator would be nice!
Eventhough they could be a pain in the arse (Microtop's Starion demos) or beautiful (Wizball loading screen) I never experienced them as unstable. Well, not more unstable than a unstable rasterbar for example.
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Martin Piper
Registered: Nov 2007
Posts: 722 |
It is bus timing bug in the VIC hardware where it can output the wrong signal and the wrong time for a very short amount of time. It depends on the exact hardware version of the VIC, how hot it is etc.
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tlr
Registered: Sep 2003
Posts: 1790 |
Quote: Grey dots for the emulator would be nice!
Eventhough they could be a pain in the arse (Microtop's Starion demos) or beautiful (Wizball loading screen) I never experienced them as unstable. Well, not more unstable than a unstable rasterbar for example.
Grey dots have been emulated in x64sc since r22786.
Note that the grey dot is not the only thing that differs color reg-wise between a 656x and a 856x.
On a 656x (the one without the grey dots) color changes appear one pixel later than on a 856x, i.e one pixel into the cycle.
To avoid differences between the machines you should never change a color register when it's visible in the first pixel. |
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Skoe
Account closed
Registered: Jan 2008
Posts: 34 |
tlr, I don't know how well the reason for the grey dot effect is known already. I'm sorry if I repeat known facts here.
Segher is working on reversing the VIC-II photos. We discussed about how the color registers work today.
I can't explain the exact technical background, would be full of mistakes if I did it with my words :) As far as I understood, the registers are only simple latches made of two inverters. When the register is written, the (VIC internal) data bus is connected to the first gate.
Apparently the address decoding and R/W stuff seems to be faster then the data bus drivers. There for an undriven databus (all 1s) is put to the latch. After a while the data bus gets its final state and the right color appears.
With this explanation we had the idea to delay the CS line a bit to make it slower than the data bus. I brutally took a 10 nF capacitor (don't try this at home!) and really: The grey dots can be made disappear. If I put the capacitor there or remove it while the C64 is running I can switch the dots on and off easily.
Segher will try to write it down more detailed or possibly make a drawing later.
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Devia
Registered: Oct 2004
Posts: 401 |
interresting.. |
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Mr. SID
Registered: Jan 2003
Posts: 424 |
Awesome! |
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Krill
Registered: Apr 2002
Posts: 2980 |
There's no way to turn it off by software. |
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