Re: Blurry picture

From: Gerrit Heitsch <>
Date: Sat, 19 Nov 2011 19:10:39 +0100
Message-ID: <>
On 11/19/2011 12:51 PM, Segher Boessenkool wrote:
> The C64C uses a LH5062 Sharp CMOS part. It might well be faster, but
> who knows.

There are 2 different 64pin Chips in the C64C, one that includes the 
color RAM (252535-01, LR38125 from SHARP) and one that doesn't 
(252715-01, LH5062B).

> My current theory is all the components are just a little different,
> and it all conspires to make R/#W or #CS too early :-)

The problem is that the data bus doesn't carry all 1's when idle, so 
where do the 1's that video sequencer sees when hitting the 'grey dots' 
come from?

> Another way to check is to just
> replace the pullup on a machine and see what that does to the dots, of
> course.  Much better than stupid calculations :-P

Another way I have been thinking about is to rig up a +12V regulator and 
use a 6569R5 in a board made for the 8565R2 and supply pin 13 with +12V. 
Shouldn't be too hard, the voltage doubler circuit is still there, just 
needs a 7812 plus some capacitors. If we still don't see grey dots then, 
this suggests it's something with the 856x-VIC.

> This hypothesis can be tested, yay!  Anyone want to try?  Put a scope
> on the analog Vdd, and/or try a bigger bypass capacitor on it.

I could, but the C64C I have access to at the moment (the others are 
stored away) doesn't show the vertical bars. Most C64C only show them 
somewhat, they are really visible on a C128 though.


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Received on 2011-11-19 19:00:08

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