Introduction
Field programmable gate arrays (FPGA) are specific integrated circuits that can be user-programmed easily. The FPGA contains versatile functions, configurable interconnects and input/output interface to adapt to the user specification. FPGA allow rapid prototyping using custom logic structures, and are very popular for limited production products. Modern FPGA are extremely dense, with complexity of several millions of gates which enable the emulation of very complex hardware such as parallel microprocessors, mixture of processor and signal processing, etcc One key advantage of FPGA is their ability to be reprogrammed, in order to create a completely different hardware by modifying the logic gate array. The usual structure of a FPGA is given in figure 9-1.
 |
| Figure 9-1: Basic structure of a field programmable gate array |
One example of a very simple function (3-input XOR) implemented in a FPGA is given in figure 9-2. Three pads on the left are configured as inputs, one logic block is used to create the 3-input XOR and one pad on the right is used as output. The propagation of signals is handled by interconnect lines, connected together at specific programmable interconnect points.
 |
| Figure 9-2: Using a field programmable gate array to build a 3-input XOR gate |
 |
| Figure 9-3: Equivalent circuit for the FPGA configured in XOR3 gate |
Three pads are configured as inputs and represent the logical information A,B and C (Figure 9-3). An internal routing path is created to establish an electrical link between the I/O region and the logic bloc. Internally, the logic bloc may be configured in any combination of sequential basic function. Each logic bloc usually supports 3 to 8 logic inputs. In our example, the bloc is configured as a 3-input XOR. Then, other internal routing wires are configured in order to carry out the signal to an I/O pad configured as an output. The global propagation delay of such architecture is evidently very high, if compared to a 3-input XOR gate that may be found in the cell library. This is usually the price to pay for configurable logic circuits.
Notice that FPGA not only exist as simple components, but also as macro-blocs in system-on-chip designs (Figure 9-4). In the case of communication systems, the configurable logic may be dynamically changed to adapt to improved communication protocol. In the case of very low power systems, the configurable logic may handle several different tasks in series, rather than embedding all corresponding hardware that never works in parallel.
 |
| Figure 9-4: FPGA exist as stand-alone Ics or blocks within a system-on-chip |
