All VCCO pins on the FPGA must be connected, even if a bank is unused.
the HSWAPEN pin must be either connected directly to GND, or forced Low by another device on the board
Spartan-6 FPGA SelectIO Resources User Guide 笔记4 I/O Pins Note for PCB
原文：http://www.cnblogs.com/zheming/p/4655132.html