33DESIGN METHODOLOGY INSERT3IC LAYOUTThe increasing complexity of the integrated circuit has made the role of design-automa-tion tools indispensable, and raises the abstractions the designer is working with to everhigher levels. Yet, when performance or design density is of primary importance, thedesigner has no other choice than to return to handcrafting the circuit topology and physi-cal design. The labor-intensive nature of this approach, called custom design, translatesinto a high cost and a long time-to-market. Therefore, it can only be justified economicallyunder the following conditions: • The custom block can be reused many times, for instance as a library cell• The cost can be amortized over a large volume. Microprocessors and semiconductormemories are examples of applications in this class.• Cost is not the prime design criterion. This is becoming increasingly rare. Examplesare space-applications and scientific instrumentation.With continuous progress in the design-automation arena, the share of custom designreduces from year to year. Even in high-performance microprocessors, large portions aredesigned automatically using semicustom design approaches. Only the most performance-critical modules such as the integer and floating-point execution units are handcrafted. nCreating a manufacturable layoutVerifying the layoutDMIA.fm Page 33 Monday, September 4, 2000 11:19 AM34 IC LAYOUT Chapter 3Even though the amount of design automation in the custom design process is mini-mal, some design tools have proven to be indispensable. Together with circuit simulators,these programs form the core of every design-automation environment, and are the firsttools an aspirant circuit designer will encounter.Layout EditorThe layout editor is the premier working tool of the designer and exists primarily for thegeneration of a physical representation of a design, given a circuit topology. Virtuallyevery design-automation vendor offers an entry in this field. Most well-known is theMAGIC tool developed at the University of California at Berkeley [Ousterhout84], whichhas been widely distributed. Even though MAGIC did not withstand the evolution of soft-ware technology and user interface, some of its offspring did. Throughout this textbook,we will be using a layout tool called max, a MAGIC descendant developed by a companycalled MicroMagic [mmi00]. A typical max display is shown in Figure 3.1 and illustratesthe basic function of the layout editor—placing polygons on different mask layers so thata functional physical design is obtained (scathingly called polygon pushing).Since physical design occupies a major fraction of the design time for a new cell orcomponent, techniques to expedite this process have been in continual demand. The sym-bolic-layout approach has gained popularity over the years. In this design methodology,the designer only draws a shorthand notation for the layout structure. This notation indi-cates only the relative positioning of the various design components (transistors, contacts,Figure 3.1 View of a max display window. It plots the layout of two stacked NMOS transistor. The menu on the left side allows for the selection of the layer a particular poligon will be placed on.DMIA.fm Page 34 Monday, September 4, 2000 11:19 AMSection 35wires). The absolute coordinates of these elements are determined automatically by theeditor using a compactor [Hsueh79, Weste93]. The compactor translates the design rulesinto a set of constraints on the component positions, and solves a constrained optimizationproblem that attempts to minimize the area or another cost function. An example of a symbolic notation for a circuit topology, called a sticks diagram, isshown in Figure 3.2. The different layout entities are dimensionless, since only position-ing is important. The advantage of this approach is that the designer does not have toworry about design rules, because the compactor ensures that the final layout is physicallycorrect. Thus, she can avoid cumbersome polygon manipulations. Another plus of thesymbolic approach is that cells can adjust themselves automatically to the environment.For example, automatic pitch-matching of cells is an attractive feature in module genera-tors. Consider the case of Figure 3.3 (from [Croes88]), in which the original cells have dif-ferent heights, and the terminal positions do not match. Connecting the cells would requireextra wiring. The symbolic approach allows the cells to adjust themselves and connectwithout any overhead.The disadvantage of the symbolic approach is that the outcome of the compactionphase is often unpredictable. The resulting layout can be less dense than what is obtainedwith the manual approach. Notwithstanding, symbolic layout tools have improved consid-erably over the years and are currently a part of the mainstream design process.Figure 3.2 Sticks representation of CMOS inverter. The numbers represent the (Width/Length)-ratios of the transistors.13InOutVDDGNDFigure 3.3 Automatic pitch matching of datapath cells based on symbolic layout.BEFOREAFTERDMIA.fm Page 35 Monday, September 4, 2000 11:19 AM36 IC LAYOUT Chapter 3Design-Rule CheckingDesign rules were introduced in Chapter 2 as a set of layout restrictions that ensure themanufactured design will operate as desired with no short or open circuits. A primerequirement of the physical layout of a design is that it adhere to these rules. This can beverified with the aid of a design-rule checker (DRC), which uses as inputs the physicallayout of a design and a description of the design rules presented in the form of a technol-ogy file. Since a complex circuit can contain millions of polygons that must be checkedagainst each other, efficiency is the most important property of a good DRC tool. The ver-ification of a large chip can take hours or days of computation time. One way of expedit-ing the process is to preserve the design hierarchy at the physical level. For instance, if acell is used multiple times in a design, it should be checked only once. Besides speedingup the process, the use of hierarchy can make error messages more informative by retain-ing knowledge of the circuit structure. DRC tools come in two formats: (1) The on-line DRC runs concurrent with the lay-out editor and flags design violations during the cell layout. For instance, max has a built-in design-rule checking facility. An example of on-line DRC is shown in Figure 3.4. (2)Batch DRC is used as a
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