XST now supports PERIOD and TIG timing constraints, but there is no option in the ISE Project Navigator GUI to apply them to a design. How can I do this?
In XST 4.1i, PERIOD and TIG are only supported in the constraints file.
The syntax for apply these to a design is:
attribute PERIOD of my_signal: entity is "100MHz";
attribute TIG of my_signal: signal is "TIG";
For more information, please consult Chapter 3 of the XST User Guide, which is supplied with the ISE User Manual.