To resolve this issue, manually add an IBUF to the port where the BUFG is being inserted, as follows:
1. Download the IBUF pcore and place it in your project's pcore directory:
http://www.xilinx.com/txpatches/pub/applications/misc/input_buf_v1_00_a.zip 2. Add the IBUF pcore to your system.
3. Find the external port associated with the BUFG.
4. Connect the input of the IBUF pcore to the external port.
5. Connect the output of the IBUF pcore to the pin of the peripheral.