[Open-graphics] How we compute video clocks

[André Pouliot]

>
>> For the DCM feedback, we don't care about the phase of this clock
>> relative to anything else, but we do need a feedback.  I think you can
>> just wire CLK0 directly to CLKFB.
>
> I'll find some docs about what this feedback is all about. But a wire 
> connecting CLK0 to CLKFB will do the trick?
Actually to use the ClkFx output with a frequency input below 18MHz 
there shouldn't be any feedback present on the dcm.

For the documentation it's in the document ds099.pdf page 88 table 59 
for the timing specification for the Digital Frequency synthesizer.