Chapter 7: DESIGN ENVIRONMENTS

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7-3: 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 (see Section 3-9). To properly control translation, the IO Options command has many subcommands for the different file types.

Unfortunately, many of these formats are pure geometry with no information about the circuit connections. When read, the input formatter creates pure-layer nodes to describe the geometry. This means that transistors, contacts, and other multi-layer nodes are not constructed properly. Although the facet appears visually correct, and can be used to export the same type of file, it cannot be analyzed at a circuit level.

A solution to this problem is coming soon, and a prelininary implementation is available now. The Node Extract subcommand of the Network command of the Tools menu will replace the pure-layer nodes in the current facet with connected nodes and arcs. It isn't guaranteed to work in all situations, and it doesn't recognize transistors yet.

CIF Control

CIF options are controlled with the CIF Options... subcommand of the IO Options command of the File menu.

Figure 7.10

This dialog controls the conversion between layers in Electric and layers in the CIF file. By clicking on an Electric layer, you can type a new CIF layer into the dialog.

By default, CIF output writes the entire hierarchy below the current facet. If you check the "Output Mimics Display" item, 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 item.

Another option is whether or not to merge adjoining geometry. This is an issue because of the duplication and overlap that occurs wherever arcs and nodes meet. The default action is to write each node and arc individually. This makes the file larger because of redundant box information, however it is faster to generate and uses simpler constructs. If you check the "Output Merges Boxes" item, 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.

Another option is whether or not to instantiate the circuit in the CIF. By default, the currently displayed facet becomes the top level of the CIF file, and is instantiated at the end of the CIF. This causes the CIF file to display the current facet. If the CIF file is to be used as a library, with no current facet, then uncheck the "Output Instantiates Top Level" checkbox, and there will be no invocation of the current facet.

The "Normalize Coordinates" option causes the coordinate system of each CIF cell to be centered at zero.

When reading CIF files, the CIF "wire" statements are assumed to have rounded geometry at the ends and corners. If you check the "Input Squares Wires" item, CIF input assumes that wire ends are square and extend by half of their width.

You can request that resolution errors be checked during CIF output. In addition, by selecting "Show Resolution Errors", the errors are can be reviewed, one at a time, after CIF output is done. The "Output resolution" field is the minimum coordinate value that can be safely written in CIF (due to CIF's use of the centimicron as the smallest unit). All geometry of that size or less will be flagged during CIF output. For example, current MOSIS rules require that no boundaries be quarter-lambda or less, so a value of .25 in this field will detect such violations.

GDS Control

GDS options are controlled with the GDS Options... subcommand of the IO Options command of the File menu.

Figure 7.12

This dialog controls the conversion between layers in Electric and layers in the GDS file. By clicking on an Electric layer, you can type a new GDS layer into the dialog.

When reading GDS II files, text annotations can often clutter the display, so they are ignored from the GDS input. If you check the "Input Includes Text" item, the text will be read and displayed.

Another GDS II input option controls whether facet instances are expanded or not. By default, facet instances are not expanded (they appear as a simple box). If you check the "Input Expands Facets" item, facets are expanded so that their contents are displayed. Expansion of facets can always be changed with the subcommands of the Expand Facet Instances and Unexpand Facet Instances commands of the Facets menu.

Another GDS II input option controls whether arrays are instantiated or not. By default, arrays are instantiated fully, but this can consume excessive amounts of memory. If you uncheck the "Input Instantiates Arrays" item, only the upper-left and lower-right instance are actually placed.

Another GDS II input option controls whether unknown layers will be ignored, or placed in the circuit. By default, unknown layers appear as DRC-Nodes (special nodes used to indicate DRC errors, which appear as orange squares). By checking "Input Ignores Unknown Layers", the unknown layers are simply ignored.

When writing GDS files, there is an option to merge adjoining geometry. This is an issue because of the duplication and overlap that occurs wherever arcs and nodes meet. The default action is to write each node and arc individually. This makes the file larger because of redundant box information, however it is faster to generate and uses simpler constructs. If you check the "Output Merges Boxes" item, 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.

GDS II format is only able to handle straight lines, not curves. If your design has any curved geometry, the GDS II output will approximate these curves with straight line segments. To control how this approximation is handled, there are two factors: the maximum angle and the maximum sag. The maximum angle is the number of degrees of curvature between line segments (so a quarter-circle curve, which is 90 degrees, will be broken into 10 lines if the maximum angle is 9 degrees). The maximum sag is the maximum distance between the actual curve and the center of the approximating line segment. No line segment will be allowed to sag more than this distance. These two values can be entered in the "Maximum arc angle" and "Maximum arc sag" areas.

EDIF Control

EDIF options are controlled with the EDIF Options... subcommand of the IO Options command of the File menu.

Figure 7.13

This dialog controls whether EDIF output writes schematic or netlist views (the default is netlist). It also lets you set a scale factor for EDIF input.

DEF Control

DEF options are controlled with the DEF Options... subcommand of the IO Options command of the File menu.

Figure 7.17

This dialog controls whether DEF reads physical and/or logical information.

CDL Control

CDL options are controlled with the CDL Options... subcommand of the IO Options command of the File menu.

Figure 7.21

This dialog control the library name and path information that is written, and it lets you control the conversion of square-bracket characters.

DXF Control

DXF options are controlled with the DXF Options... subcommand of the IO Options command of the File menu.

Figure 7.11

This dialog controls the conversion between layers in Electric and layers in the DXF file. By clicking on an Electric layer, you can type a new DXF layer into the dialog.

By default, Electric flattens DXF input, removing levels of hierarchy and creating a single facet with the DXF artwork. By unchecking the "Input flattens hierarchy", Electric will preserve the structure of the DXF file.

If a layer name in the DXF file is not found in the list that you setup in Electric, it will be ignored. If you check "Input reads all layers", then all layers are read into Electric, regardless of whether the layer names are known.

To control scaling, you can change the meaning of units in the DXF file. The default unit is "Millimeters", which means that a value of 5 in the DXF file becomes 5 millimeters in Electric.


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