A period constraint from the UCF (User Constraints File) overrides a period constraint from the NCF (Netlist Constraints File).
For example, the following appears in the NCF:
NET "my_net" PERIOD = 40;
while the information below appears in the UCF:
NET "my_net" TNM_NET = "my_net";
TIMESPEC "TS_my_net" = PERIOD "my_net" 30.0 ns;
Is this the correct precedence?
When the example design above was implemented, the period constraint of 30 ns found in the UCF was analyzed, while the constraint of 40 ns found in the NCF was ignored.
A detailed explanation of Xilinx timing constraints is available in the Constraints Guide.
For more details on timing constraints, please see the Timing Constraints User Guide: http://www.xilinx.com/support/documentation/sw_manuals/xilinx12_1/ug612.pdf