As of this writing, the ICAP_Clk must be less than or equal to 20 MHz or 12 MHz in case of Spartan-6 FPGAs. The Spartan-6 LX4 to LX75/T devices have a limiting frequency of 20 MHz. The Spartan-6 LX100/T to LX150/T devices have a limiting frequency of 12 MHz.