p88

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??

bishesh