[Lancelot] VGA clk

Tommy Thorn tommy at numba-tu.com
Thu Feb 3 18:59:15 PST 2005

Wei Mark Fang said:
> Hi,
> I am wondering what is the VGA clk supposed to be. I
> have come across a few reference designs where the VGA
> clk (pin E_15) has varying freq. Can it be set to the
> on-board reference 50Mhz, since I want to use the
> SDRAM as well (it shares the output clk pin E_15)?

Well, 25MHz maps nicely to 680x480 VESA mode.  I don't know of any modes using
50MHz.   Finially, is the DAC even rated that high?  Why do you care?  You can
just use the DCM to devide it down.

Peter's ABC80 has a VGA interface as does my MIPS.  I find mine particularly
simple, but likely not the most optimal design.

What's your target resolution?  The XFree documentation tells you how to
derive valid timings for any constraint, so assuming the DAC support 50MHz,
you could probably get something like 900x678 going easily.  1024x768 either
needs to be interlaced or you should have a flatpanel that can accept really
low refresh rates.


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