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

J.F.,

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
2V6000.

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,

Austin

jean-francois hasson wrote: