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Old 11-01-2007, 12:47 AM
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Default Re: Ping Jim: The PFD is dead!

"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

Beware of XAPP028...


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
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
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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