Re: Blurry picture

From: Segher Boessenkool <>
Date: Sat, 19 Nov 2011 23:05:11 +0100
Message-Id: <>
>> 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).

Also Sharp; pretty much the same design if you ask me (well, the non- 
part that is); "B" is the speed grade.

>> 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?

The internal data bus is precharged during the first halve of each  
cycle.  I explained in detail in a previous post.

> 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.

Before doing such major surgery, maybe we should get some more data  
get scope traces of all the relevant signals (clock, VIC #CS, R/#W,  
#RAS, #CAS, some A and D pins) on various different machines.  I  
find a single bus trace online.

>> 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.

None of my C64C do (and no dots either!)


       Message was sent through the cbm-hackers mailing list
Received on 2011-11-19 23:00:03

Archive generated by hypermail 2.2.0.