AR# 4760: 4.1i XC4000E MAP - To control whether a BUFGP or a BUFGS is used, a BUFG must be LOC'd before MAP.
4.1i XC4000E MAP - To control whether a BUFGP or a BUFGS is used, a BUFG must be LOC'd before MAP.
Keyword: BUFG, BUFGP, BUFGS
General Description: If a BUFG is used in a design but not LOC'd, the mapper could use either a BUFGP or BUFGS site for the clock. Any attempt to add a .pcf file constraint will fail if the site type is not valid for the clock type chosen by MAP.
This problem can be avoided by using .ucf constraints, which will guide MAP to select the correct clock type.
Another option is to use BUFGS or BUFGP symbols in the design, rather than a BUFG symbol.