Chapter 3: Hierarchy
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3-4: Examining Cell Instances
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When instances are initially created, they are drawn as black boxes with nothing inside. This form of instance display is called unexpanded. When the instances show the actual layout inside of them, they are expanded. This distinction applies only in layout; schematic icons never show their actual contents.

To expand a cell instance, select it and use the commands of the Cell / Expand Cell Instances menu. The One Level Down command opens up the next closed level; the All the Way command opens up all levels to the bottom; and the Specified Amount... lets you type a number of levels of hierarchy to expand. These commands expand all highlighted cells. If a highlighted cell is already expanded, this command expands any subcells inside of the instance, repeatedly down the hierarchy.

Once expanded, a cell instance will continue to be drawn with its contents shown until the commands of the Cell / Unexpand Cell Instances command are used. These commands return cell instances to their black-box form, starting with the deepest subcells that are expanded at the bottom of the hierarchy. The One Level Up command closes up the bottommost expanded level; the All the Way command closes all levels from the bottom; and the Specified Amount... lets you type a number of levels of hierarchy to close.

Figure 3.25
You can also use the expansion (opened eye) and unexpansion (closed eye) icons from the tool bar to expand and unexpand by one level.

The expansion information can also be controlled by using the Object Properties... command (in menu Edit / Properties) and clicking on the "Expanded" or "Unexpanded" buttons.

There are times when you want to see the layout inside of a cell instance, but only temporarily. The Look Inside Highlighted command (in menu Cell) displays everything in the highlighted area, down through all hierarchical levels. This is a one-shot display that reverts to unexpanded form if the window is shifted, scaled, or redrawn.

There is a slight difference in specification between the Expand Cell Instances commands and the Look Inside Highlighted command. The Expand Cell Instances commands affect cell instances only, and thus any instances that are highlighted or in the highlighted area will be completely expanded. The Look Inside Highlighted command affects layout display in an area, so only those parts of instances that are inside of the highlighted area will be shown. Thus, the command Look Inside Highlighted is more precise in what it expands and can be used, in conjunction with Area selection, to show only a specific part of the circuit (see Section 2-1-3 for more on area selection).


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