A Hierarchical, Automated Design Flow for Low-Power,

s must be generated from sub-block layout views before the containing block can be routed. Both DFII and Pillar have built in abstract generation routines. We used the one in DFII, because it allows the specification of more complex rules. These rules are needed to reduce the memory size of the abstracts. If nothing is done to reduce the size of the abstracts, then the router can be slowed significantly, almost as if no hierarchy had been added. To reduce the size, we add “blanket obstructions” that cover the entire abstract. This eliminates all of the interior wiring from the abstract and reduces the size tremendously. We add blanket obstructions only for the metal layers that are heavily used, typically metals 1-4 for standard-cell blocks, and leave the higher layers open for routing. The default metal layer of the highest obstruction is 4, but the user has the option of changing this number for any block, reducing it if there are not enough routing resources or increasing it if the abstracts become too large. In order to allow abutment of block abstracts in the floorplan, the blanket obstructions must be shrunk to allow use of routing resources within the blocks. There is no “tried-and-true” formula for how much to shrink these obstructions. Our flow shrinks by one metal pitch (i.e. one minimum metal space plus one minimum metal width) to 5.5 Hierarchical Physical Design 114 allow routing just inside the boundary. To incorporate grid-routers, however, it may be necessary to shrink the boundary more on the left and right sides. Fig. 5.18(a) shows the routing grid superimposed on the upper-left corner of a block abstract. A route on the grid and overlapping the top boundary is legal, but a wire inside the left boundary is not. Fig. 5.18(b) shows the closest wire that can be routed. There is no limit on how much the blanket obstruction can be shrunk. It is not always helpful to shrink the blanket obstruction more, however, since the exposed routing resources tend to be already used by the sub-block.