CAD of Circuits and Integrated Systems

2Basic Notions on the Design of Digital Circuits and Systems

2.1. Introduction

While the first chapter covered basic notions on computational complexity, this chapter focuses on several aspects of the design of digital circuits and systems. It is our expectation that the reader will thus be better equipped for approaching the third chapter, which presents several optimization problems using notions presented in the first two chapters.

2.2. History of VLSI circuit design

2.2.1. Prediffused circuit

This involves a partial premanufacturing by the foundry, enabling an array of transistors to be embedded without connection (matrix). Circuit design is limited to the design of connection levels, and manufacture concerns only the interconnections (masks of metal levels and contacts). This is followed by finalizing the manufacturing of wafers and encapsulation.

2.2.2. Sea of gates

This is also a prediffused circuit, but in comparison to the first type, the fixed sites of gates and routing channels are suppressed. This enables a better integration density.

A prediffused circuit has the advantage of a short manufacturing time due to several logical functions already being implemented on silicon, while those required by the user application can be conveniently interconnected. This type of circuit unfortunately has several drawbacks: its performances may be limited (already sized transistors, etc.), the complexity of the intended circuit is limited by the network size, flexibility ...