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Unformatted text preview: Chapter 10 Abstract Generation I N ORDER for the place and route tool to do its job, it needs to know certain physical information about the cells in the library it is using. For example, it needs to know the bounding box of each cell so it can use that bounding box when it places cells next to each other. Note that this bounding box may be described by the furthest pieces of geometry in the layout view of the cell, or it may be offset from the layout so that the cell geometry overlaps or has a gap when it is placed next to another cell. The place and route tool doesnt need to know anything about the layout; it just places the cells according to their bounding boxes. The other critical information for the place and route tool is the signal connection points on the cells. It needs to know where the power and ground connections are, along with the signal connections so that it can wire the cells together. It also needs to know just enough about the layout geometry of the cells so that it knows where it can and cant place wires over the top of the cell. A view of the cell that has only this information about the cell (bounding box, signal connections, and wiring blockages) is called an abstract view. An abstract view has much less information than the full layout view, which is useful for two reasons: its smaller and therefore less trouble to read, and it doesnt have full layout information, so cell library vendors can easily release their abstract views while keeping the customers from seeing the full layout views, thus retaining their proprietary layout. As an example, consider a simple inverter. Figure 10.1 shows the layout view side by side with the abstract view. You can see that the abstract is very simple compared to the layout. The abstract view doesnt need any information about any layers that arent directly used by the place and route program. Because the place and route program uses only the metal layers for routing, only the metal layers show up in the abstract. This example actually has four connection points: A (input), Y (output), and vdd! (the top metal piece) and gnd! (the bottom metal piece). Note that the power and ground connections are 302 CHAPTER 10: Abstract Generation designed to abut (actually overlap) when the cells are placed next to each other with their bounding boxes touching. We, of course, have full layout views because thats what weve been designing. The Were using Abstract from the IC v5.1.41 release. tool that extracts the abstract information from the full layout view is called, appropriately enough, Abstract , and it can be run from your cadence directory using the cad-abstract script, or called directly from the layout view in Virtuoso if youre running it on a single cell....
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This note was uploaded on 02/24/2011 for the course ECE 6710 taught by Professor Dr.erikbrunvand during the Fall '11 term at University of Utah.
- Fall '11