Introduction to Fpga Design 2 Basics of Digital Design 3 Fpga Structure

This paper presents an introduction to digital hardware design using Field Programmable Gate Arrays (FPGAs). After a historical introduction and a quick overview of digital design, the internal structure of a generic FPGA is discussed. We then describe the design flow, i.e., the steps needed to go from design idea to actual working hardware. Digital signal processing is an important area where FPGAs have found many applications in recent years. Therefore a complete section is devoted to this subject. The paper finishes with a discussion of important peripheral concepts essential for success in any project involving FPGAs. 1 Historical introduction Digital electronics is concerned with circuits which represent information using a finite set of output states [1]. Most of the applications use in fact just two states, which are often labelled ‘0’ and ‘1’. Behind this choice is the fact that the whole Boolean formalism then becomes available for the solution of logic problems, and also that arithmetic using binary representations of numbers is a very mature field. Different mappings between the two states and the corresponding output voltages or currents define different logic families. For example, the Transistor–Transistor Logic (TTL) family defines an output as logic ‘1’ if its voltage is above a certain threshold (typically 2.4 V). For the same family, if we set the input threshold for logic ‘1’ as 2 V, we will have a margin of 0.4 V which will allow us to interconnect TTL chips inside a design without the risk of misinterpretation of logic states. This complete preservation of information even in the presence of moderate amounts of noise is what has driven a steady shift of paradigm from analog to digital in many applications. Here we see as well another reason for the choice of binary logic: from a purely electrical point of view, having only two different values for the voltages or currents used to represent states is the safest choice in terms of design margins. Historically, TTL chips from the 74 series fuelled an initial wave of digital system designs in the 1970s. From this seed, we shall focus on the separate branches that evolved to satisfy the demand for programmability of different logic functions. By programmability, we mean the ability of a designer to affect the logic behaviour of a chip after it has been produced in the factory. A first improvement in the direction of programmability came with the introduction of gate arrays, which were nothing else than a chip filled with NAND gates that the designer could interconnect as needed to generate any logic function he desired. This interconnection had to happen at the chip design stage, i.e., before production, but it was already a convenient improvement over designing everything from scratch. We had to wait until the introduction of Programmable Logic Arrays (PLAs) in the 1980s to have a really programmable solution. These were two-level AND-OR structures with user-programmable connections. Programmable Array Logic (PAL) devices were an improvement in performance and cost over the PLA structure. Today, these devices are collectively called Programmable Logic Devices (PLDs).