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Virtual Terminals

If one holds the Ctrl key while clicking anywhere except over another terminal, a scalar terminal will be placed, whether or not it is over a circuit connection point. This is useful if the BYNAME flag is to be set for the terminal, which indicates that it will not connect by location, but by name matching only. It is also useful for implementing ``virtual'' terminals which connect to nothing, but satisfy connectivity references in layout vs. schematic testing, and for other purposes.

Suppose one has a subcell with physical layout only that one wishes to include in a full design hierarchy. It may not be convenient to create a schematic for the subcell, but it is desired that the connections to the subcell be included in the LVS checking of the overall design. It is possible to assign ``virtual terminals'' to the subcell. Virtual terminals are treated like ordinary terminals in connecting to instances of the subcell, but are ignored when creating netlists for the subcell itself.

A virtual terminal is created in the subct command by holding the Ctrl key while clicking on locations in the electrical schematic (even if the schematic is empty). They can be placed anywhere except on top of another terminal; location is not important. Once created, they can be moved or deleted like ordinary terminals.

Once placed, they will be considered in establishing the connectivity to instances of the cell, but will be ignored when establishing connections within the cell. Thus the cell looks like a ``black box'' with terminals. Virtual terminals can be used along with ordinary terminals if only part of the internal circuit is to be visible from the outside.

In SPICE netlists, virtual terminals will appear in the subcircuit connection list in .subckt and call lines, but will not be connected in the .subckt definition. One can use a spicetext label to add a .include line to bring in a circuit definition from a file, for example, to satisfy the references.

In the graphical display, virtual terminals of the current cell are shown with a beer-barrel outline for differentiation from the standard terminals which are square. The cell bounding box is expanded to contain all virtual terminal locations. The center of a virtual terminal is a ``hot spot'' for hypertext node references, i.e., clicking on the terminal center will add the associated node to the prompt line edit string in the plot and iplot commands and when creating labels or properties.


next up previous contents index
Next: Multi-Contact Connectors Up: The subct Button: Set Previous: The subct Button: Set   Contents   Index
Stephen R. Whiteley 2017-11-08