Input/output characteristics and SelectIO™ logic resources available in Spartan®-6 FPGAs are detailed in this user guide. Chapter 1, SelectIO Resources describes the electrical behavior of the output drivers and
input receivers, and gives detailed examples of many standard interfaces. Chapter 2,SelectIO Logic Resources describes the input and output data registers and their Double-Data-Rate (DDR) operation, and the programmable input delay (IDELAY). Chapter 3,Advanced SelectIO Logic Resources describes the data serializer/deserializer.