1.8 FPGAs

FPGAs are Field Programmable Gate Array devices. Simply put, they are electrically programmable gate array ICs that contain multiple levels of logic. FPGAs offer high gate density and performance, along with a large number of user-definable inputs and outputs, a flexible interconnect scheme, and a gate-array-like design environment. They are not constrained to the typical AND-OR array. Instead, they contain an interior matrix of configurable logic blocks (CLBs) and a surrounding ring of I/O blocks (IOBs). Each CLB contains programmable combinatorial logic and storage registers. The combinatorial logic section of the block is capable of implementing any Boolean function of its input variables. Each IOB can be programmed independently to be an input, an output with tri-state control or a bi-directional pin. It also contains flip-flops that can be used to buffer inputs and outputs. The interconnection resources consist of a network of lines that run horizontally and vertically in the rows and columns between the CLBs. Programmable switches connect the inputs and outputs of IOBs and CLBs to nearby lines. Long lines run the entire length or breadth of the device, bypassing interchanges to provide distribution of critical signals with minimum delay or skew. Designers using FPGAs can define the logic functions of a circuit and revise these functions as necessary.