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Actually the answer was pretty simple. I did a long time ago but I forgot to reply, so I apologize. Read about instantiation in VHDL. You must connect the ports of the FFT block to another entity that you built. Be sure that you connect scale_sch_we, fwd_inv_we and start ports. The scale_sch and the fwd_inv port can have a constant value. When you do the simulation be careful with the timings of each signal. Let the signal start to take a High value for at least 1us. But then again that depends of the core's architecture that you chose.