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

From: Gerrit Heitsch <>
Date: Sun, 28 Aug 2011 18:51:37 +0200
Message-ID: <>
On 08/28/2011 06:20 PM, Segher Boessenkool wrote:
>>> (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.

Too bad... A little blitter in the VIC, wouldn't that have been nice? :)

>> 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.

Hm, Couldn't make out the driver circuit in the vincinity so I thought 
it was a input.

Too bad they didn't use this signal to generate a 'start of frame' 
raster IRQ (maskable of course). Yes, you can do this with a normal 
raster IRQ, but then you have to reprogram the comperator for other 
raster IRQs. A fixed raster IRQ 'start of frame' might have made some 
things easier.

> The 6569 had none of these pads, FWIW.

Maybe they wanted some extra test capabilities to sort out broken chips 
before packaging them.


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Received on 2011-08-28 17:00:18

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