Digital Electronics: Principles, Devices and Applications

338 Digital ElectronicsDesign EntrySimulationDesign ConstraintsDesign LibrarySynthesisPlace & RouteDownloadFigure 9.34Programmable logic design and development process.The design entry step is either followed by or interspersed with periodic functional simulation. Thesimulator executes the design for a given set of inputs and confirms that the logic is functionally correct.Hardware compilation comes next. It involves two steps. The first step is synthesis, and the resultof that is a hardware representation called a netlist. The netlist is device independent and its contentsdo not depend on the parameters of the PLD to be programmed. It is usually stored in a standardformat called the electronic design interchange format (EDIF). The second step, called place and route,involves mapping of the logical structure described in the netlist onto actual logic blocks, interconnectsand inputs/outputs. The place and route process produces a bit stream, which is nothing but the binarydata that must be loaded into CPLD/FPGA to make the chip execute the intended hardware design. Itmay be mentioned here that each device family has its own proprietary bit stream format.9.11 Programming LanguagesDuring the PLD development cycle, from design entry to the generation of a bit stream that canbe loaded onto the chip using some kind of electronic programming system, two types of softwareprogram are needed to perform two different functions.The first is a hardware description language (HDL), which is needed at the design entry stage.HDL is a software programming language that is used to model or describe the intended operation ofa piece of hardware. In the present case, this is the function that the PLD chip is intended to performafter it is programmed. It may be worth mentioning here that modern computer languages, includingboth hardware description languages and high-level programming languages, almost invariably containdeclarative and executable statements, and the hardware description languages are particularly richin the former. If we compare the results of a high-level programming language such as C++ andan HDL, it will be an executable program in the case of the former and declarative in the caseof the latter. Hardware description languages that have evolved over the years include ABEL-HDL,