Skew on a clock tree on a virtex II : what is the good figure ?

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I am working on a design involving a virtex II -5. I read in a previous post
that the skew one could expect from a clock tree is less than 100 ps.
However when, on the design I am developping, I run the timing analyzer I
get a skew for some clock on a clock tree of about 450 ps or 500 ps (using
the 5.1i F23). How should I interpret this difference ? Any clue ? Which
figure is the right one ?

J.F. Hasson

Re: Skew on a clock tree on a virtex II : what is the good figure ?

The report by the tools is correct.

Different sized parts will have different skews (delays) in the their clock
trees due to size.

Additionally, skew is a function of position on the tree, so from the top left
corner down to the middle of a part all the way to the bottom left corner, the
skew will be less than 130 ps for a 2V6000, wheras from the top left corner over
towards the top middle over to the right corner, it will be about 450 ps for a

Top to bottom: 130 ps.  Right to left: 450 ps.  Smallest in the middles, largest
at the corners.

For a 2V1000, these are all less than 100 ps.

Thus if you wanted to make the highest speed wide parallel interface, I would
use the left or right sides (least skew), rather than the top or bottom edges
(for least skew between bits).

If I had to use the top or bottom edges, I would try to group the IOs near the
center, or near the left and right extremes.

Again the timing reports and FPGA_Editor report the right (worst case) numbers.

Hope this helps,


jean-francois hasson wrote:

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