1.The VCCO supplies, one for each of the I/O banks
2.VCCINT is the main power supply for the internal FPGA logic
3.VCCAUX is an auxiliary（辅助） source of power used for various Spartan-6 FPGA functions
NOTE: The reference voltage VREF can be connected to multi-purpose VREF pins. All VREF inputs
on a bank must be connected to the same voltage.
Spartan-6 FPGA SelectIO Resources User Guide 笔记3 Supply Voltages for the IOBs
原文：http://www.cnblogs.com/zheming/p/4655054.html