C64G short board shakeout

Disk drives, Monitors, SuperCPU etc.
rmzalbar
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C64G short board shakeout

Post by rmzalbar »

Moving this off to its own thread..

C64G (west germany WHITE breadbin, short board 250469)

The issue with this machine I will be sorting out are:

1. VSP bug. Will fix by installing SRAM.
2. Grey dot bug (going to experiment with delaying CS to VIC-II)
3. Some disk speeder systems not working - cartridge based as well as software custom loaders

To sort out #3 I will need to socket a few chips. CIA (one of them,) logic, CPU, and maybe even the 64-pin PLA. I am still waiting on the correct size PLA socket to do that. This board is not fun to desolder on because there are large fills with no thermal reliefs, so I try the other chips first.


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eslapion
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Re: C64G short board shakeout

Post by eslapion »

rmzalbar wrote: Wed May 06, 2020 7:20 am 2. Grey dot bug (going to experiment with delaying CS to VIC-II)
Never heard of that before. Could you give some details ?
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Re: C64G short board shakeout

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eslapion wrote: Wed May 06, 2020 4:09 pm
rmzalbar wrote: Wed May 06, 2020 7:20 am 2. Grey dot bug (going to experiment with delaying CS to VIC-II)
Never heard of that before. Could you give some details ?
I assume you're familiar with the HMOS VIC-II grey dots bug, where mid-raster color changes are marked with a single grey pixel (sometimes half of a pixel.) I've personally only seen it on PAL 8565. Certain games and effect show it, such as the intro to Skate or Die, Super Cycle, and the dropship level of Aliens.

If you run VICE SC version there is an option to enable the grey dots bug emulation there.

2 out of 3 of my 8565 show the bug. One of them shows it only until it warms up, for the first 20 minutes or so. Power cycling can cause it to appear or disappear randomly too, the thought there is that the offset of dot clock and pixel clock is random and set at power on.

I saw some discussion that the CS signal from the big PLA to the VIC-II might be a little too early for the data bus compared to the long boards, and that delaying CS to the VIC-II would eliminate the grey dots effect. Someone said they used a capacitor to do that. I tried it and was able to reduce the grey dots by 1/2 pixel, but couldn't go any further because the RC filtering became too great and was reducing the signal below threshold. My next plan was to experiment sending CS through some gates to act as a delay.

https://csdb.dk/forums/?roomid=11&topic ... allposts=1
https://csdb.dk/forums/?roomid=7&topici ... irstpost=2
http://cbm-hackers.2304266.n4.nabble.co ... 61i40.html
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Re: C64G short board shakeout

Post by banman »

Hi rmzalbar,


I was curious if you are able to show a screen shot of the grey dot bug you have.

I tried looking for decent pictures on the internet but was unsuccessful. I know it's a well documented. There was one video on YouTube. I couldn't really see where it was showing up. That is only because I didn't know what to look for.


I would like to see if I have it on the machines I have here. Thank you... :D
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Re: C64G short board shakeout

Post by rmzalbar »

Right now it seems I have the "good" vic-ii in the machine. It has the VSP bug, but not the grey dots. I will take pictures the next time I encounter it.

The Activision game "Aliens" displays the grey dots in the cockpit area of the dropship when you begin the first action sequence in the game. Make sure you test several times. I get the grey dots on my "bad" chip about every third power cycle. My other "bad" chip displays grey dots for about 15 minutes from a cold start, so thermal related.

None of my NTSC VIC-II have the grey dots. I also haven't tried HMOS VIC-II in long boards or vise-versa.
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Re: C64G short board shakeout

Post by eslapion »

rmzalbar wrote: Wed May 06, 2020 5:51 pm I assume you're familiar with the HMOS VIC-II grey dots bug, where mid-raster color changes are marked with a single grey pixel (sometimes half of a pixel.) I've personally only seen it on PAL 8565.
In fact, I was not and I don't have a 8565. I installed a 8562 on my favored 250466 and my 250469 is NTSC so it had the same VIC-2 as the 250466 by default.
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Re: C64G short board shakeout

Post by rmzalbar »

The VICII_reg_timing.prg test from testbench repository

https://sourceforge.net/p/vice-emu/code ... ii_timing/

can display the grey dots. You'll see sparkling white pixels at edge transitions on HMOS/C64C. The screen layout is quite scrambled however on NTSC chips, but would probably still work to show grey dots.
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Re: C64G short board shakeout

Post by banman »

Hi rmzalbar ,



I will try that link with a test program.

I was looking at the pictures posted there.


no bug.jpg
maybe bug.jpg


I was wondering are the red circled areas (I have added these red circles not the original poster) where I should be looking at or am I looking at something entirely different ?

I don't ever recall noticing the issue before on any machines I have used...
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Re: C64G short board shakeout

Post by rmzalbar »

The area you've circled in the 2nd image is showing the grey dots bug, it is indeed the transitions at the edges of the red bars. That looks to be 100% grey dots bug. On my currently installed 8565, I get a lesser amount of that, which fades away completely in about 15 minutes.

You won't encounter it with most software. It only occurs when someone writes a change to a VIC register mid-raster by way of a raster split. Some games, demos, etc. do that. You might also see more commonly it as few jittering pixels occurring in screen areas where there aren't any graphics, where a programmer might use a visually blank area as an opportunity to make register changes.

Apparently there is a race condition that occurs involving the short board's RAM, 64-pin PLA, HMOS VIC-II, or combination thereof.

Not all chips exhibit it. My NTSC HMOS vic-II chip doesn't do it at all.
https://csdb.dk/forums/index.php?roomid=11&topicid=78186&showallposts=1 wrote:The way to control it is stable rasters...

On 856x based machines (i.e newer C64s and C128s) when writing a color register ($D020-$D02E) currently being used to display graphics a grey dot (color 15) appears at the first pixel of the cycle.

The reason for the grey dot appears to be a glitch in the color register bank itself, not in the mapping from color enables to actual 4-bit color. This effect is thus independent of the previous color register displayed.

It has been reported that the dot can be smaller than 1 pixel, and that it does not show or randomly appears on some machines.
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Re: C64G short board shakeout

Post by rmzalbar »

Allright! 64-pin WIDE format sockets finally arrived. I have a complete spare chipset now for C64C, so now I just need to get some work stuff cleared away and I can start trying to sort out the disk-speeder issues.
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