65816 module :( (2)

From: Ruud Baltissen (g.baltissen_at_hccnet.nl)
Date: 2001-08-06 23:44:56

Hallo allemaal,

The subject says enough. Before removing the 6526 I swapped the 682 
for a 688 only to discover that nothing worked at all. Only to find out after 
half an hour that the pin that served as input for A15 internally was 
shortcircuited with GND :(
But as a good cheap Dutchman I found out that I still could use this 
damaged IC by exchanging with the other as by coincidence that 
particular pin was connected to GND anyway :)

Then I remembered that I could test the theory that the 6526 was to 
blame by removing it. I did, tried to load and...... &^%$#%$^ same 
problem :(
To be more precise:
- it are graphical inverted characters for the most, not graphics as I 
mentioned before.
- the drive stops turning the floppy a short moment after the graphical 
characters appear but the light remains
- the 65816 doen't hang up but when I push STOP, the C64 says 
"LOADING ERROR?" IIRC (JC, I have seen it a zillion times and now I 
cannot remember it? I must be getting old) and says "READY".

I compared the PHI0 coming from the VIC and PHI2. The positive part of 
the 6510's PHI2 looks like a sawtooth:
     /|     /|
__/  |__/ |__

The one of the 65816 looks like a perfect block.

I thought that maybe PHI2 needed some more delay. Replacing the 
74F04 by a 7404, HCT and L-type did not help. So I added a RC-network 
between the two invertors with R=10 and C=1n. The typical blue screen 
appeared but no cursor.
So I tried by connecting PHI0 directly to PHI2. Graphicl characters again 

I assume it must have something to do with the timing and the onboard 
6526's. Next idea: replace the 6526's by their 2 MHz versions: nope :(

Any ideas? I even accept weird ones, you never know
  / __|__
 / /  |_/     Groetjes, Ruud
 \ \__|_\
  \___|       http://Ruud.C64.org

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