Figure 1711 an expanded view of part of a cell based

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FIGURE 17.11 An expanded view of part of a cell-based ASIC. (a) Both channel 4 and channel 5 use m1 in the horizontal direction and m2 in the vertical direction. If the logic cell connectors are on m2 this requires vias to be placed at every logic cell connector in channel 4. (b) Channel 4 and 5 are routed with m1 along the direction of the channel spine (the long direction of the channel). Now vias are required only for nets 1 and 2, at the intersection of the channels. Figure 17.12 shows an imaginary logic cell with connectors. Double-entry logic cells intended for two-level metal routing have connectors at the top and bottom of the logic cell, usually in m2. Logic cells intended for processes with three or more levels of metal have connectors in the center of the cell, again usually on m2. Logic cells may use both m1 and m2 internally, but the use of m2 is usually minimized. The router normally uses a simplified view of the logic cell called a phantom . The phantom contains only the logic cell information that the router needs: the connector locations, types, and names; the abutment and bounding boxes; enough layer information to be able to place cells without violating design rules; and a blockage map—the locations of any metal inside the cell that blocks routing.
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FIGURE 17.12 The different types of connections that can be made to a cell. This cell has connectors at the top and bottom of the cell (normal for cells intended for use with a two-level metal process) and internal connectors (normal for logic cells intended for use with a three-level metal process). The interconnect and connections are drawn to scale. Figure 17.13 illustrates some terms used in the detailed routing of a channel. The channel spine in Figure 17.13 is horizontal with terminals at the top and the bottom, but a channel can also be vertical. In either case terminals are spaced along the longest edges of the channel at given, fixed locations. Terminals are usually located on a grid defined by the routing pitch on that layer (we say terminals are either on-grid or off-grid ). We make connections between terminals using interconnects that consist of one or more trunks running parallel to the length of the channel and branches that connect the trunk to the terminals. If more than one trunk is used, the trunks are connected by doglegs . Connections exit the channel at pseudoterminals .
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FIGURE 17.13 Terms used in channel routing. (a) A channel with four horizontal tracks. (b) An expanded view of the left-hand portion of the channel showing (approximately to scale) how the m1 and m2 layers connect to the logic cells on either side of the channel. (c) The construction of a via1 (m1/m2 via). The trunk and branch connections run in tracks (equispaced, like railway tracks). If the trunk connections use m1, the horizontal track spacing (usually just called the track spacing for channel routing) is equal to the m1 routing pitch. The maximum number of interconnects we need in a channel multiplied by the horizontal track spacing gives the minimum height of a channel (see Section 17.2.2 on how to determine the maximum number of interconnects needed). Each
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  • Fall '15
  • prasad
  • The Land, Router, Hierarchical routing, nets, Global Routing, global router

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