
GDSII Net AnnotationA GDSII file created by NETEXG has had all of the physical connectivity traced out and sorted into structures (cells.) However this may not be the way that some CAD tools expect to find the net information. Certain CAD tools depend on a special GDSII record type known as a PROPERTY record. About the Property Record A PROPERTY record can be attached to either SREFs (structure references or insertions) or to geometries such as BOUNDARY and PATH. The record has two parts: an index and a value. The index is a number from 1 to 127 that is used to identify the property. The value is a string of text. One can see that it is possible to simple define a record (say #6) whose value is a net name string. One then attaches this to all geometric entities that belong to that net. This doesn't work well with most GDSII files since those originating from chip designs generally reuse structures many times. An entity cannot have a single net name attached to it. However with the NETEXG output there is no structure reuse so we can safely label every single entity with a PROPERTY that assigns its net. ExampleConsider the section of a 2 layer IC package as shown below. There are three signal nets that start with a bump pad on layer M1, drop to layer M2 through a via on layer D1 where the net ends on a bump pad on layer M2. The GDSII for such a package will look like its ascii representation below: BGNLIB SAMPLE.DB UNITS UM RESOLUTION 1000 BGNSTR TOP SREF NET1 PROP 30: NET1 < the user chose to use PROP 30 for SREFs SREF NET2 PROP 30: NET2 SREF NET3 PROP 30: NET3 ENDSTR BGNSTR NET1 BOUNDARY LAYER 1 XY XY XY XY XY PROP 6: NET1 < user chose PROP 6 for BOUNDARY/PATH BOUNDARY LAYER 2 XY XY XY XY XY PROP 6: NET1 BOUNDARY LAYER 3 XY XY XY XY XY PROP 6: NET1 ENDSTR BGNSTR NET2 BOUNDARY LAYER 1 XY XY XY XY XY PROP 6: NET2 BOUNDARY LAYER 2 XY XY XY XY XY PROP 6: NET2 BOUNDARY LAYER 3 XY XY XY XY XY PROP 6: NET2 ENDSTR BGNSTR NET3 BOUNDARY LAYER 1 XY XY XY XY XY PROP 6: NET3 BOUNDARY LAYER 2 XY XY XY XY XY PROP 6: NET3 BOUNDARY LAYER 3 XY XY XY XY XY PROP 6: NET3 ENDSTR ENDLIB 
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