1.Location Constraint 用于分配I/O端口
NET <I/O_NAME> LOC = "<EXTERNAL_PORT_IDENTIFIER>";
Example:
NET MY_IO LOC=R7;
2.IOSTANDARD Attribute 用于选择IO标准如LVCMOS25，LVDS_25等
NET <I/O_NAME> IOSTANDARD=”<IOSTANDARD VALUE>”;
3.Output Slew Rate Attributec
NET <I/O_NAME> SLEW = "<SLEW_VALUE>";
SLEW_VALUE can SLOW (Default) FAST QUIETIO
4.Output Drive Strength Attribute 驱动能力（mA）
NET <I/O_NAME> DRIVE = "<DRIVE_VALUE>";
DRIVE_VALUE can 2 4 6 8 12 (Default) 16 24
5.PULLUP/PULLDOWN/KEEPER for IBUF, OBUFT, and IOBUF
NET <I/O_NAME> <PULLUP/PULLDOWN/KEEPER>;
Spartan-6 FPGA SelectIO Resources User Guide 笔记2 SelectIO Attributes/Constraints
原文：http://www.cnblogs.com/zheming/p/4654992.html