Hi, It depends on how the data is going to be shared between arudino board & SPARTAN 3E FPGA. Here the Arudino board has predefined I/Os & Bus timing protocols. Where as in FPGA you can configure the GPIOs as per your requirement. Make sure that whether you want to interface Data bus (Uni/Bi-Directional) of Arudino to FPGA, or simple i/o port of Arudino to FPGA. After confirming that, refer the timing specs of Arudino i/o and FPGA, create the timing events based signal for handshake between two devices. hope you understand what i mean,..