Dedicated cascade routes cannot be accessedin fabric.They cannot be connected to constant values or FPGA fabric, such as LUTs for flip-flops. The dedicated routes can only be connected between adjacent DSP48 slices.
Unused input cascade pins should be tied to '0' so that the tools will know that they are unused.Unused output cascade pins should be left unconnected so that the tools will know that they are not being used.
For more details on how to connect up the PCIN input, see (Xilinx Answer 44205). For more information on a particular DSP48 slice, please see the appropriate product DSP48 slice device userguide: