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Show additional new cells and so forth until the design has been completed. Top-down design is the converse of bottom up. The designer starts with a high level specification of the circuit, e.g., a netlist, and proceeds by breaking down each of the high level modules in the description into a set of interconnected submodules, each of which is less complicated than its parent. The design process is completed when all of the submodules are simple enough to be designed with physical design tools. A prototype is created bottom-up when its implementation is created before its interface and the interface is subsequently derived directly from the implementation. There are two methods for doing this. One method is performed automatically for the designer each time a CIF-based prototype is read in. In this case, ACME reads in the annotated CIF and creates a layout implementation, constructs an interface with the same form as the layout implementation, adds ports to this interface based on the port annotations and initializes the cellmatrix with the wire data necessary for route-over. The second method is performed manually when the designer "wraps" cells to create new hierarchies in ACME. This is done by first selecting the cells and wires that will form the implementation of the new prototype. The designer then creates an interface by painting an area that encloses the selected cells and wires. Actually, the interface need not fully enclose the area of the selected cells but typically does. Once the area has been defined, the designer proceeds by adding ports to the interface and, finally, by naming the ports and the prototype. Top-down prototypes are created by the same "wrap" technique except that there are no cells or wires selected, i.e., the implementation is empty. This top-down approach allows a designer to create designs structurally and then refine them later by "pushing into" the prototype (performing an in-place hierarchical edit) and adding implementation details. Both of these design techniques will be demonstrated later in the next chapter. 4.4.5 Physical and Structural Prototypes Cells derive their physical or structural attribute directly from their prototype. The attribute is computed automatically based on interface and implementation characteris55 |
