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Schematic constraints are placed at the beginning of the PCF file by MAP. The start and end of this section is indicated with SCHEMATIC START and SCHEMATIC END, respectively. Because of a “last-read” order, all constraints that you enter in this file should come after SCHEMATIC END.
Note: You are not prohibited from entering a user constraint before the schematic constraints section, but if you do, a conflicting constraint in the schematic-based section may override your entry.
Every time a design is remapped, the schematic section of the PCF file is overwritten by the mapper. The user constraints section is left intact, but certain constraints may be invalid because of the new mapping.