General Description: In FPGA Express 3.4 (XC4000 and Spartan architectures only), the option "Use I/O Reg" under the "Ports" tab is TRUE by default. When I change from TRUE to FALSE, FPGA Express makes improper IPAD/Port connections to the inferred BUFG. This causes in the following error in NGDBuild:
"ERROR:NgdBuild:455 - logical net 'xxxx' has multiple drivers."
To avoid this error, instantiate all the BUFGs that you will require.