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Old 11-01-2007, 05:20 AM
Peter Alfke
Posts: n/a
Default Re: Ping Jim: The PFD is dead!

On Oct 31, 3:47 pm, "Symon" <[email protected]> wrote:
> "Peter Alfke" <[email protected]> wrote in message
> news:[email protected]> Hi, John
> > I suppose you know about the old Xilinx app note:
> >
> > which would benefit from your diode trick.
> > Cheers
> > Peter Alfke, Xilinx Applications

> Guys,
> Beware of XAPP028...
> From
> Quote:-
> A small note of caution when using Peter's XAPP028 in Virtex II. As
> well as constraining the logic to the CLBs shown in the app note, make

Thanks, Syms, for pointing this out.
I published this in 1990, in the XC3000 era, and I was proud of
packing it so nicely.
Your comment makes me retire the circuit, but it will unfortunately
survive on the internat...

> sure you specify a MAXSKEW attribute on the reference signal and
> feedback signal to the circuit. I use 100ps. Without this the circuit
> can occasionally malfunction depending on the place and route. (These
> are the signals called 'from VCO divided by N' and 'from reference
> frequency'.)
> There was no problem when this circuit was used on older FPGAs
> where the routing to the F and G lookup tables in a single CLB was
> guaranteed to have low skew. In Virtex II this is no longer the case
> and a single signal that goes to both the F and G inputs of a CLB can
> have significant skew if not constrained. This can cause the circuit
> of XAPP28 to misbehave.
> HTH., Syms.

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