Combinational Design Examples

So far, we have looked at basic principles in several areas—number
systems, digital circuits, and combinational logic—and we have
described many of the basic building blocks of combinational
design—decoders, multiplexers, and the like. All of that is a needed
foundation, but the ultimate goal of studying digital design is eventually to
be able to solve real problems by designing digital systems (well, duh…).
That usually requires experience beyond what you can get by reading a
textbook. We’ll try to get you started by presenting a number of larger
combinational design examples in this chapter.
The chapter is divided into three sections. The first section gives
design examples using combinational building blocks. While the examples
are written in terms of MSI functions, the same functions are widely used in
ASIC and schematic-based FPGA design. The idea of these examples is to
show that you can often express a combinational function using a collection
of smaller building blocks. This is important for a couple of reasons: a
hierarchical approach usually simplifies the overall design task, and the
smaller building blocks often have a more efficient, optimized realization in
FPGA and ASIC cells than what you’d get if you wrote a larger, monolithic
description in an HDL and then just hit the “synthesize” button.
The second section gives design examples using ABEL. These designs
are all targeted to small PLDs such as 16V8s and 20V8s. Besides the general
use of the ABEL language, some of the examples illustrate the partitioning