[Open-graphics] video controller output clock

[Timothy Normand Miller]

On 3/4/08, Michael Meeuwisse <mickeymeeuw at gmail.com> wrote:
> I continued digging a little, the vid_control module has vid_clock as
>  an input, module video_wrapper uses this module and also has
>  vid_clock as an input. Right now, the s3_top_level maps vid0_clock or
>  vid1_clock to these inputs, but these signals are somewhat odd;
>
>  BUFG Vid0_BUFG (
>      .I(vid0_clock_ext),
>      .O(vid0_clock)
>      );
>
>  vid0_clock_ext is once again, an input:
>
>  // Tempory hack to make everything pass synthesis
>         input mem_clock_ext,
>      input vid0_clock_ext,
>      input vid1_clock_ext,
>
>  Am I deducting correctly that this comment is an elegant "TODO", and
>  the clocks aren't dynamic yet?

Exactly.  Howard's the one who knows what to do here, but he's a
strapped resource, and there's tons the rest of us can do in the mean
time.

>
>
>  On 4 Mar 2008, at 02:24, Timothy Normand Miller wrote:
>
>  > The simplest way is to have a high frequency into a divider.  But
>  > that's kinda limited.  Another thing you can do is take the DCM
>  > output, run that through a divider, and then use the output of that as
>  > feedback.
>
>
> The problem is that the frequencies needed don't really have a common
>  divider, unless you start running several GHz. Say, for example, we
>  want some basic ones;
>
>  640x480 @ 60Hz = 25.175 MHz (say 25, works just as fine)
>  800x600 @ 60Hz = 40 MHz
>  1024x768 @ 60Hz = 65 MHz
>
>  I guess we could do something like, run at 130 MHz. Then we get
>  640x480 - divide by 5.2 (count to 5, every fifth time count to 6?)
>  800x600 - divide by 3.25 (count to 3, every fourth time count to 4?)
>  1024x768 - divide by 2

That's interesting, but it wouldn't leave you with a 50/50 duty cycle.
 This could be a problem for the DVI transmitters, which are DDR.

>
>  Correcting the clock like that all the time is bound to give odd
>  display results. So I don't know how to do it, but a divider sounds
>  imho impossible.

I'm going to get in touch with Howard and find out how he got it to work.

>
>
>  > I think.  I'm really not sure how Howard did it.
>  >
>  > On 3/3/08, Michael Meeuwisse <mickeymeeuw at gmail.com> wrote:
>  >> Quick question.
>  >>
>
>
> Cheers,
>
>  Michael
>  www.projectvga.org
>
>


-- 
Timothy Normand Miller
http://www.cse.ohio-state.edu/~millerti
Open Graphics Project