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CombinaTIonal Design Examples
So far, we have looked at basic principles in several areas—numbersystems, digital circuits, and combinaTIonal logic—and we havedescribed many of the basic building blocks of combinaTIonaldesign—decoders, mulTIplexers, and the like. All of that is a neededfoundation, but the ultimate goal of studying digital design is eventually tobe able to solve real problems by designing digital systems (well, duh…).That usually requires experience beyond what you can get by reading atextbook. We’ll try to get you started by presenting a number of largercombinational design examples in this chapter.The chapter is divided into three sections. The first section givesdesign examples using combinational building blocks. While the examplesare written in terms of MSI functions, the same functions are widely used inASIC and schematic-based FPGA design. The idea of these examples is toshow that you can often express a combinational function using a collectionof smaller building blocks. This is important for a couple of reasons: ahierarchical approach usually simplifies the overall design task, and thesmaller building blocks often have a more efficient, optimized realization inFPGA and ASIC cells than what you’d get if you wrote a larger, monolithicdescription in an HDL and then just hit the “synthesize” button.The second section gives design examples using ABEL. These designsare all targeted to small PLDs such as 16V8s and 20V8s. Besides the generaluse of the ABEL language, some of the examples illustrate the partitioning
