AR# 12208: 4.1i Virtex MAP - Registers that are LOC'd to slices end up in IOBs.
4.1i Virtex MAP - Registers that are LOC'd to slices end up in IOBs.
Keywords: IOB, slice FF, register, LOC
General Description: My design contains several flip-flops that are range LOC'ed to a set of slice locations. However, in the NCD file produced by MAP, these flip-flops have been mapped into IOBs rather than slices. No warnings are printed about this, and the .pcf file produced by MAP is empty.
It was found that the flip-flops had IOB=TRUE attributes, which overrode the slice LOCs.
MAP will be changed in the next major release after 4.1i to warn about conflicts between LOC constraints and IOB attributes.