一个很简单的程序实例：
module test1(a,b,c,out);
     input a,b,c;  
    output out;

assign out=(c==0)?a:b;

endmodule
编译时出现两个警告：
WARNING:LIT:701 - PAD symbol "a" has an undefined IOSTANDARD.
WARNING:LIT:702 - PAD symbol "a" is not constrained (LOC) to a specific
   location.
generate programming file时报错：
ERROR:Bitgen:342 - This design contains pins which have locations (LOC) that are
   not user-assigned or I/O Standards (IOSTANDARD) that are not user-assigned.
   This may cause I/O contention or incompatibility with the board power or
   connectivity affecting performance, signal integrity or in extreme cases
   cause damage to the device or the components to which it is connected.  To
   prevent this error, it is highly suggested to specify all pin locations and
   I/O standards to avoid potential contention or conflicts and allow proper
   bitstream creation.  To demote this error to a warning and allow bitstream
   creation with unspecified I/O location or standards, you may apply the
   following bitgen switch: -g UnconstrainedPins:Allow
求各位大侠支招…… 此帖出自小平头技术问答