Chapter VII: DESIGN ENVIRONMENTS

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I/O Specifications

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Electric is able to read and write circuits in a number of different formats. This is done with the Import and the Export commands of the File menu. To properly control the translation, the IO Control command of the Technology menu manipulates information about CIF, GDS II, and EDIF formats. These subcommands display a text editing window in which each layer is shown with its external association. Edit CIF Layers shows a two- or three-character name for each Electric layer. Edit GDS II Layers shows layer numbers for each Electric layer. More information on text editing can be found in the "Text Windows" section of Chapter 4.

Caltech Intermediate Format (CIF) and Calma GDS II are popular interchange formats for manufacturing circuits. To help read and write these files, there are a number of useful subcommands in the I/O Control command of the Technology menu.

By default, CIF output writes the entire hierarchy below the current facet. With the subcommand CIF Output Mimics Display, facet instances that are unexpanded will be represented as an outline in the CIF file. This is useful when the CIF output is intended for hardcopy display, and only the screen contents is desired. To revert to the default state, uncheck the menu entry.

Another default in CIF and GDS II output is to write each node and arc as a separate set of boxes. This produces overlap wherever arcs and nodes meet, and makes the file larger because of redundant box information. However it is faster to generate and it uses simpler constructs. With the CIF Output Merges Boxes or the GDS Output Merges Boxes subcommands, all connecting regions on the same layer are merged into one complex polygon. This requires more processing, produces a smaller file, and generates more complex constructs. To revert to box output, uncheck the menu entry.

When reading CIF files, the CIF "wire" statements are assumed to have rounded geometry at the ends and corners. The command CIF Input Squares Wires changes that assumption sp that wire ends are square and extend by half of their width. To revert to the rounded state, uncheck the menu entry.

Unfortunately, Electric cannot read CIF or GDS II files properly, because the individual layers are not aggregated into nodes and arcs (there is no node extraction done). Instead, these files appear as collections of pure-layer nodes which represent the geometry. This means that transistors, contacts, and other multi-layer nodes are not recognized properly. CIF and GDS II input is therefore only useful for library reference, where the circuitry will not be modified, but simply used in a larger design. If you try to edit a facet that was read from these formats, it appears confusing and nonintuitive.


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