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LOW-LEVEL 
DESIGN ENTRY

The purpose of design entry is to describe a microelectronic system to a set of electronic-design automation ( EDA ) tools. Electronic systems used to be, and many still are, constructed from off-the-shelf components, such as TTL ICs. Design entry for these systems now usually consists of drawing a picture, a schematic . The schematic shows how all the components are connected together, the connectivity of an ASIC. This type of design-entry process is called schematic entry , or schematic capture . A circuit schematic describes an ASIC in the same way an architect’s plan describes a building.

The circuit schematic is a picture, an easy format for us to understand and use, but computers need to work with an ASCII or binary version of the schematic that we call a netlist . The output of a schematic-entry tool is thus a netlist file that contains a description of all the components in a design and their interconnections.

Not all the design information may be conveyed in a circuit schematic or netlist, because not all of the functions of an ASIC are described by the connectivity information. For example, suppose we use a programmable ASIC for some random logic functions. Part of the ASIC might be designed using a text language. In this case design entry also includes writing the code. What if an ASIC in our system contains a programmable memory (PROM)? Is the PROM microcode, the '1's and '0's, part of design entry? The operation of our system is certainly dependent on the correct programming of the PROM. So perhaps the PROM code ought to be considered part of design entry. On the other hand nobody would consider the operating-system code that is loaded into a RAM on an ASIC to be a part of design entry. Obviously, then, there are several different forms of design entry. In each case it is important to make sure that you have completely specified the system—not only so that it can be correctly constructed, but so that someone else can understand how the system is put together. Design entry is thus an important part of documentation .

Until recently most ASIC design entry used schematic entry. As ASICs have become more complex, other design-entry methods are becoming common. Alternative design-entry methods can use graphical methods, such as a schematic, or text files, such as a programming language. Using a hardware description language ( HDL ) for design entry allows us to generate netlists directly using logic synthesis . We will concentrate on low-level design-entry methods together with their advantages and disadvantages in this chapter.