Suppose that the first of the three different office

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are the offices. Suppose that the first of the three different office types is a corner office,  the second office type has a window, and a third office type is without a window. We  shall call these office cells: CornerOffice  ,  WindowOffice  ,  and  NoWindowOffice  . These cells are primitive cells as far as the contractor is  concerned. However, when discussing the plans with a client, the architect of our  building will also need to see how each offices is furnished. The architect needs to see a  level of detail of each office that is more complicated than needed by the building  contractor. The architect needs to see the cells that represent the tables, chairs, and desks  that make up each type of office. To the architect the primitive cells are a library  containing cells such as  chair  ,  table  , and  desk  . 9.1.4 Schematic Icons and Symbols Most schematic-entry programs allow the designer to draw special or custom icons. In  addition, the schematic-entry tool will also usually create an icon automatically for a  subschematic that is used in a higher-level schematic. This is a  derived icon  , or  derived symbol  . The external connections of the subschematic are automatically attached to the  icon, usually a rectangle. Figure       9.4  (c) shows what a derived icon for a cell,  DLAT  , might look like (we could  also have drawn this by hand). The subschematic for  DLAT  is shown in  Figure       9.4  (b).  We say that the inverter with the instance name  inv1  in the subschematic is  subcell  (or submodule) of the cell  DLAT  . Alternatively we say that cell  instance  inv1  is a child of the cell  DLAT  , and cell  DLAT  is a parent of cell  instance  inv1  .   FIGURE 9.4  A cell and its subschematic. (a) A schematic library  containing icons for the primitive cells. (b) A subschematic for a cell,  DLAT, showing the instance names for the primitive cells. (c) A  symbol for cell DLAT. Figure       9.5  (a) shows a more complex subschematic for a 4-bit latch. Each primitive  cell instance in this schematic must have a unique name. This can get very tiresome for  large circuits. Instead of creating complex, but repetitive, subschematics for complex  cells we can use hierarchy.
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   FIGURE 9.5  A 4-bit latch: (a) drawn as a flat schematic from gate- level primitives, (b) drawn as four instances of the cell symbol DLAT,  (c) drawn using a vectored instance of the DLAT cell symbol with  cardinality of 4, (d) drawn using a new cell symbol with cell name  FourBit.
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