For Spartan-3/-3E and Virtex-4 designs, 3-states are not replaced by logic. What are the reasons this might occur?
Solution 1:
There are 4 situations where XST is not able to replace a 3-state by logic:
1. The 3-state is connected to a black box.
2. The 3-state is connected to the output of a block, and the hierarchy of the block is preserved.
3. The 3-state is connected to a top-level output.
4. The "tristate2logic" constraint is set to "no" on the block where 3-states are placed or on the signals to which 3-states are connected.