Introduction to VLSI Design Flow

We have earlier seen that a cell is the basic building block of application-specific integrated circuits (ASICs) and cell-based designs. A cell is a circuit that delivers a specified functionality, such as logic gates, flip-flops, arithmetic logical units (ALUs), and memory blocks. We carefully design and verify the function of a cell at the transistor level. We also measure characteristics such as delay, power dissipation, area, and the impact of process-induced variations for each cell. Subsequently, we organize these cell-specific attributes in a widely accepted database known as library. We instantiate and connect these cells in our design by utilizing the information contained in the library. We also need a library to verify whether a design delivers the desired functionality.

In VLSI design flow, we employ primarily two types of libraries: technology library and physical library.

• 1. Technology library: Historically, technology libraries were introduced for logic synthesis for defining the functionality of each cell. However, during the last three decades, technology libraries have evolved to support various design tasks such as physical implementation, timing verification, and test activities such as scan chain insertion. Nevertheless, a large portion of a technology library defines the timing attributes of the cells. Therefore, we also refer to a technology library as timing library. We represent a technology library in Liberty format [1]. The library files in the Liberty format are American Standard Code for Information Interchange (ASCII) files, typically with an extension of .lib. However, since library files are voluminous, sometimes we compress and store these files. Moreover, some tools can convert and store the library information in their proprietary format.

• 2. Physical library: A physical library contains information about the geometry or layout of the cells, including design rules, in some abstract form. Physical libraries are popularly represented in library exchange format (LEF) [2].

In this chapter, we will discuss technology libraries in detail. We will discuss physical libraries in Chapter 24 (“Basic concepts for physical design”).