[Lancelot] VGA clk

David Brown david at westcontrol.com
Fri Feb 4 00:36:27 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)?

50 MHz works well for 800 x 600 x 72 Hz, which is fine for most
monitors and gives a more stable display than a 60 Hz frame rate.  Of
course, it also needs about twice the memory bandwidth of a 25 MHz
pixel clock...

> Well, 25MHz maps nicely to 680x480 VESA mode.  I don't know of any modes
> 50MHz.   Finially, is the DAC even rated that high?  Why do you care?  You
> just use the DCM to devide it down.
> Peter's ABC80 has a VGA interface as does my MIPS.  I find mine
> 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
> you could probably get something like 900x678 going easily.  1024x768
> needs to be interlaced or you should have a flatpanel that can accept
> low refresh rates.
> Tommy
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