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VLSI Design 11Synthesis ToolsVLSI design has reached the point at which most circuits are too complex to be specified completely by hand. Although computers can be used to record layout, thus making design easier, they can also be programmed to take an active role by doing synthesis or analysis.Synthesis tools generate circuitry automatically and Analysis tools attempt to verify existing circuitry that has been designed by hand.

VLSI Design 22Synthesis ToolsMost of the techniques used in the synthesis tools are based on the premise that a circuit consists of cells and their interconnection.Some of the tools that should be provided by the good design system Tools that generate and manipulate the contents of cells (PLA generators, gate-array generators, gate-matrix generators, compacters); tools that work with the layout external to cells (routers, placement systems, pad-frame generators);

VLSI Design 33Synthesis ToolsTools work with the interrelation of the cells and their environment (aligners, pitch matchers). There are silicon compilers that combine these techniques to produce cells and complete systems, ready for fabrication. Finally, there are synthesis tools that help to convert completed layout into suitable form for manufacturing (compensation, mask-graphics generators). These are just some of the tools that should be provided by a good design system.

VLSI Design 44Synthesis Tools1. Cell Contents Generation and ManipulationThese synthesis tools work at a local level, within the cells of a chip. These tools generate the cell layout from a more abstract specification of the cell's functionality.To accomplish this task, the layout must typically be restricted to some regular pattern into which the original specification can be mapped.

VLSI Design 55Synthesis ToolsThe most common methodologies includeThus PLA designs have rigid AND and OR planes of transistors,gate-arrays have fixed transistor pairs, and gate matrices have columns of gating material crossing rows of switchable interconnectVLSI Design 66Synthesis ToolsPLAThe PLA is made up of an AND plane and an OR plane. The AND plane produces product terms of input variables selected by the programming taps and the OR plane produces the sums of these product terms selected by a different set of programming taps. The symbolic representation of the places where the programming taps are placed is known as the personality matrix of the PLA.VLSI Design 77Synthesis ToolsFigure shows the generic structure of a PLA that programs these logic functions:f = (a b ( c)) (( b) ( a))g = (a c) (( a) ( c))

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8Synthesis ToolsGate Arrays:Gate Arrays Like the PLA, it contains a fixed mesh of unfinished layout that must be customized to yield the final circuit. Gate-arrays are more powerful, however, because the contents of the mesh is less structured so the interconnection options are more flexible.VLSI Design 99Synthesis ToolsA typical gate-array is built from blocks that contain unconnected transistor pairs, although any simple component will do. An array of these blocks combined with I/O pads forms a complete integrated circuit and offers a wide range of digital electronic options. These blocks are internally customized by connecting the components to form various logical operators such as AND, OR, NOT, and so on. The blocks are also externally connected to produce the overall chip.VLSI Design 1010Synthesis ToolsA typical gate-array. Blocks are in the center, pads are at the periphery.

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11Synthesis ToolsReading assignment: Gate MatricesVLSI Design 1212Synthesis Tools2. Generators of Layout Outside the CellsThese synthesis tools deals with layout external to the cell. Since most active circuitry is inside the cells, these tools handle the tasks of inter-cell wiring (routing), cell placement, and pad layout.Since routing can be very complex and can consume more area than the actual cells do, it is important to plan the connections in advance. Although proper cell planning and placement can make the routing simpler, tools that do this automatically are rare.

VLSI Design 1313Synthesis ToolsRoutersA router is given sets of points on unconnected cells that must be connected. The space between these cells can be as simple as a rectangle or as complex as a maze. When the routing space is rectangular and contains connection points on two facing sides, it is called a channel. When routing is done on four sides of a rectangular area, it is called a switchbox. More complex routing areas usually must be decomposed into channels and switchboxes. Many different techniques exist for routing, depending on the number of wires to run, the complexity of the routing space, and the number of layers available for crossing.VLSI Design 1414Synthesis Tools Fig. Routing areas.

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15Synthesis ToolsMaze RoutingMaze routing is the simplest way to run wires because it considers only one wire at a time. Given the connection points and a set of obstacles, the maze router finds a path. The obstacles may be cells, contacts that cannot be crossed, or wires that have already been run.VLSI Design 1616Synthesis Tools3. Tools that work with Cells and Their EnvironmentThese synthesis tools deals with the general environment of cells.These tools do not concern themselves with explicit layout, but rather ensure that the layout is properly used. Included in this category are alignment and pitch matching tools.

VLSI Design 1717Synthesis ToolsAlignment and Pitch MatchingWhen completed cells are placed together to form a circuit, there are two ways that they can connect: implicitly by abutting with composition rules or explicitly by connection rules. Abutment presupposes that the contents of the cells are precisely designed so that their mere presence near each other forms connecting wires. This has the advantage that less space is wasted between the cells because no complex routing of wires is needed. The disadvantage is that some space may be lost inside the cells in order to get the wires to the right place.

VLSI Design 1818Synthesis ToolsThis process of properly aligning wires for external connection is part of pitch matching.Pitch matching is actually the stretching and shrinking of cells so that their bounding boxes line up. VLSI Design 1919Synthesis Tools4. Silicon Compilerssilicon compilers are systems that construct custom layout from high-level languages.Modern silicon compilers translate structural descriptions into IC layout using many of the same steps as do traditional language compilers. A front-end parser reads the input description and converts it to a structural representation such as the processor floor-plan.VLSI Design 2020Synthesis ToolsThe back end then produces layout in two phases.First, a set of procedurally defined cells are invoked to produce the bulk of the circuitry for data operations. Control sequences are often built with traditional PLA techniques. Second, routers make the necessary connections between the major structural components. There are even optimization passes that can be invoked to compact the layout.VLSI Design 2121Synthesis Tools5. Post-layout GeneratorsOnce a design is complete it must be manufactured or else it will remain just so much wallpaper. This process involves removing hierarchy by flattening the circuit, modifying the layout graphics for the manufacturing needs, making a template or mask for each layer, fabricating the actual circuit, and then packaging it for use. The set of synthesis tools under this group are concerned with the layout modifications.VLSI Design 2222Synthesis ToolsCompensationWhen a circuit is fabricated, differences are introduced between the mask data and the true circuit. For example, contact cuts involve the removal of material from previous layers, and because of the nature of the chemicals used, more area gets removed than is indicated on the mask. In order to get the correct dimensions on the final artifact, the mask dimensions must be compensated, in this case by shrinking.VLSI Design 2323Synthesis ToolsCompensation is highly process-dependent--much more so than design rules. For example, most MOS processes have design rules that prevent polysilicon lines from being closer together than they are wide, but one process will compensate polysilicon by shrinking the geometry, whereas another process will bloat it. One reason that the direction of compensation is so variable is that some masks are negative and some positive, so it is never clear whether a layer's creation is the result of etching away unwanted material or protecting what is wanted.VLSI Design 2424Synthesis ToolsShrinking on the final circuit indicates the need to bloat on the mask, which is called positive compensation. When the mask is to be shrunk, that is negative compensation. This compensation is typically performed immediately prior to mask creation and is therefore never displayed to the designer.VLSI Design 2525Synthesis ToolsThere are special markings that must be placed on the masks of an integrated circuit. These markings are used for mask alignment, process checking, and wafer cutting. In general, they are placed on a mask after compensation because their specified dimensions are indicative of desired mask markings and not wafer markings.

VLSI Design 2626Synthesis ToolsWafer cutting, sometimes called scribing or sawing, is the subject of another piece of mask graphics that a design system must handle. Scribe lanes are fairly wide lines that run horizontally and vertically across the wafer. They typically form a regular grid inside the round area of the wafer and do not extend to the edgesThe last piece of graphics that a CAD system should be able to place on a layout is text. All designers document their circuitry by spelling out messages. VLSI Design 2727Synthesis ToolsThis text will include the circuit name, different letters for each layer, and any other message of interest. This text not only is useful for documentation purposes but also serves to verify the correct side of a mask during fabrication.Mask graphics is a significant layout task that CAD systems often overlook; rather than providing these symbols, the designer is often forced to produce them manually. VLSI Design 2828Synthesis ToolsA good system not only will provide text and graphics but will help with the entire mask-preparation process by understanding what is necessary and where it should be placed.VLSI Design 2929