spartan 2的IOB不能用来用作边沿输入

很奇怪，请各位大侠指点。
verilog源文件：
always @ (posedge oscx)
begin
i=1+i;
end
结果报错：
ERROR:OldMap:56 - The LOC constraint \"P69\" (a IOB location) is not valid for
   IPAD symbol \"C31\" (pad signal=oscx1), which is being mapped to the following
   site types:
   CLKIOB
我该怎么办呢？
 :(