When I synthesize a VHDL design with XST, the VHDL parser reports the following error:
"ERROR:HDLParsers:3017 - <file>.vhd: Library unit <my_pkg> in library <my_lib> was compiled before unit <my_entity> that it uses."
This error can occur when compiled libraries are out of date relative to the files that use them.
To resolve this issue, you can clear the work directory and recompile the design using the following command within ISE:
Project -> Cleanup Project Files