AR# 8904: 3.1i XST - How do I add the IOB=TRUE attribute to output flip flops in XST?
3.1i XST - How do I add the IOB=TRUE attribute to output flip flops in XST?
Currently, the IOB=TRUE attribute must be applied to the internal signals driven by flip-flops in order to be processed. (If the attribute is placed on the port signal, it gets applied to the I/O buffer). This methodology works for input FFs, output 3-state FFs, and output FFs that will be placed in a 3-state condition, but not for direct output FFs.
NOTE: With the 4.1i software release, the IOB attribute can be placed directly onto the port.
In the case of an output flip-flop, a work-around is to assign the "IOB=TRUE" attribute to an intermediate signal that connects to the output of the flip-flop and to the output port.