Using the Virtex DLL appnote, a 4x clock specification was
created. A NET PERIOD constraint was put on the clock pad
net. The PERIOD is adjusted through the first DLL correctly,
but is not adjusted for the second DLL. If gets the same value
as the first DLL.
A fix for this problem is included in the 1.5i Service Pack 2. For details
on this Service Pack see http://www.xilinx.com/techdocs/5887.htm