i''ve just started programming in vhdl and was trying to use xsa board v1.2 ( xilinx), I wanted to use internal clock for the simple counter i designed . But in "user constraint file" of xilinx pace (in xilinx ise7.1e) , I tried using p88 as clk to my counter. However, it does not take it and sends "invalid port address " message. How can I get the clock??