When I attempt to create the Complex Multiplier v2.1 core, the option to use rounding is grayed out and truncation is automatically selected. Why?
Currently, the only architecture that supports rounding in the Complex Multiplier v2.1 core is Virtex-4. All other architectures default to truncation.
This issue has been fixed in v3.0 and beyond.
For a detailed list of LogiCORE Complex Multiplier Release Notes and Known Issues, see (Xilinx Answer 21591).