General Description: When a latch is packed with a RAM or SRL16E, the clock input is inverted for both; this is incorrect for the RAM/SRL16E.
A workaround is to use map constraints to prevent the unwanted pack. The easiest way to do this would be to apply a unique XBLKNM attribute to the problem latch.
Sample UCF syntax:
INST "latch_inst_name" XBLKNM = XLNX_WA ;
This will prevent the latch from from being packed with any other comps. If there are other comps that need to be packed into the same slice with the latch, they should be assigned the same XBLKNM attribute.