Re: 6561 (PAL VIC-I) interlace mode bit

From: Segher Boessenkool <>
Date: Sun, 28 Aug 2011 18:20:06 +0200
Message-Id: <>
>> (167MB, more complete;
>> uploading a new one)
> I'm looking at that one and am wondering about a few pad designations:
> R/_W seems to be an 'input-only' pad since I cannot see any driver  
> logic next to it. Does that mean that the VIC does not actively  
> pull R/_W to high when accessing RAM?

Yes.  It only ever does reads from RAM (or ROM, or whatever), of course.

> The circuit diagram for the 250469 board where this VIC was used  
> shows a 3.3KOhm pullup for R/_W.

[Half that actually, two resistors in parallel, if I'm not blind]

That doesn't seem to have anything to do with the VIC.  Probably bootup

> Then, below _CS and above LP I see 4 unlabeled pads which seem to  
> have drivers associated to them but, according to the pinout don't  
> seem to be used and they have a different form. The lines leading  
> to them lead towards the video output logic. Testpads?

They output the four bits of current pixel colour.  Probably test  
pads yes,
they are driven quite weakly.

> Same for the pad below LP which suggests an input.

It's an output, start of frame (or end of frame) pulse.

> Then there is the pad between A5 and A6, it looks as if it was used  
> but you didn't put a label on it and the VIC pinout doesn't show a  
> pin between A5 and A6.

That's the bias circuit (a charge pump, basically); I think the pad  
is to
add an external capacitor to tune it.

The 6569 had none of these pads, FWIW.


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Received on 2011-08-28 17:00:03

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