UCF :
NET "clk" TNM_NET = "clk";
TIMESPEC "TS_clk" = PERIOD "clk" 20 ns HIGH 50 %;
The 9.2iSP4 has translated the constraint into the following PCF line:
TS_dll_clk2x180 = PERIOD TIMEGRP "dll_clk2x180" TS_clk / 2 PHASE 5 ns HIGH 50%;
But, in 10.1, it has been translated into the following PCF line without PHASE :
TS_dll_clk2x180 = PERIOD TIMEGRP "dll_clk2x180" TS_clk / 2 HIGH 50%;
This problem has been fixed in the latest 10.1 Service Pack available at:
http://www.xilinx.com/support/download/index.htm
The first service pack containing the fix is 10.1 Service Pack 1.