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Old 24 August 2011, 22:57   #187
Join Date: Jun 2010
Location: out in the wild
Posts: 1,129
change of specs!

Hi everyone,

I just pushed the DVI encoder to the limit, and it seems to violate it's own specifications. I previous postings, I have claimed that the DVI encoder can support up to 165MHz pixelclock, but if you read the application notes and technical bulletins "between the lines", it's only safe to go up to 135MHz pixelclock.

I have just verified that 128.7MHz pixelclock works (4.5 times the Amiga pixelclock), and the picture is rock-solid. However, anything higher makes the monitor lose sync, because the PLL inside the DVI encoder chip goes out of sync. The analogue output of the chip still shows a picture, so it's purely the DVI PLL that is over the limit.

The chip vendor actually recommends to supply the chip with 3.6V instead of 3.3V to get up to the 165MHz barrier, but I don't feel like doing that. There's plenty of screenmodes available with up to 135MHz, the biggest being 1400 x 1050 pixels at 60Hz. It's just the 1600x1200 pixels that we won't have. If you expected that, you might want to cancel your pre-order now :-)

Another change of plans is the connector and the cable inside the computer: I don't have room for the 2x10pin connector on Indivision, because there's an electrolytic cap in the way in the A4000T. I will probably go for an edge connector with standard 1.27mm cable and a 2x10 connector with 2.54mm raster at the other end. The beauty of differential signalling lets me make that cable much longer than the VGA cable of the old Indivision AGA (50cm enough? that's what I'm testing with here).

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