The XC4000E family of Field-Programmable Gate Arrays (FPGAs) provides the benefits of custom CMOS VLSI, while avoiding the initial cost, time delay, and inherent risk of a conventional masked gate array. The XC4000E family provides a regular, flexible, programmable architecture of Configurable Logic Blocks (CLBs), interconnected by a powerful hierarchy of versatile routing resources, and surrounded by a perimeter of programmable Input/Output Blocks (IOBs). XC4000E devices have generous routing resources to accommodate the most complex interconnect patterns. They are customized by loading configuration data into the internal memory cells. The FPGA can either actively read its configuration data out of external serial or byteparallel PROM (master modes), or the configuration data can be written into the FPGA (slave and peripheral modes).