The subct Button: Set Subcircuit Connections

The subct button, available in the electrical side menu, allows electrical connection terminals to be added to a circuit. The terminals are points at which electrical connections are defined, as in the SPICE subcircuit definition. Terminal definition is necessary if the circuit is to be used as a subcircuit in another circuit with connections to the instance (it is possible for a subcircuit to connect to global nets only (see 7.11), in which case the master and instances would have no terminals). The terminals are also used by the extraction system and can provide an initial association of a particular schematic net and physical conductor group.

Terminals can only be created in electrical mode. Once created, a terminal's flags may be edited so as to enable a corresponding terminal location in the physical layout. The extraction system will most often find suitable physical terminal locations automatically, however there are times when the user may need to place terminals manually, which can be done with the Edit Terminals button in the Views and Operations page of the Extraction Setup panel from the Setup button in the Extract Menu, while in physical mode. In electrical mode, this same button is equivalent to the subct button in the side menu.

Subsequent to creation with the present command, terminals can be made visible with the terms button in the electrical side menu. While in physical mode, the terminals will be visible in electrical windows when either the All Terminals or Cell Terminals Only check boxes in the Show group in the Views and Operations page of the Extraction Setup panel is checked.

The terminals must be defined in the schematic representation of the cell, whether or not the cell will ultimately be symbolic (see 7.25). The terminals can be created and deleted only in the schematic. Once created, they will be visible in the symbol view, but must be moved to the desired location by hand. In the symbol view (only) each terminal can have arbitrarily many copies or itself at different locations, each one of which is an equivalent connection point for the subcircuit. This facilitates, for example, tiling. If an equivalent connection point appears on either side of the instance, then placing a row of these instances side-by-side will automatically connect this node to all of the instances. This applies only to the symbolic representation. In the schematic, each cell terminal has a single connection point.

In Xic, there are two types of cell contact terminals.

Scalar terminals
These are the ``normal'', single-contact terminals. These terminals actually convey the connectivity information between the parent and subcell schematics, and are the only terminals that may have corresponding terminals in the physical layout. A scalar terminal is associated with a node property, of a cell or cell instance.

Multi-contact ``bus'' terminals
These terminals reference the scalar terminals and provide a means for connecting a number of these terminals to a multi-conductor net in the schematic. The use of multi-conductor nets and multi-contact terminals can greatly simplify a schematic visually. Be advised that a multi-conductor terminal only references existing scalar terminals, which must exist. These terminals are associated with a bnode property, of a cell or cell instance.

In the schematic, by default ordinary scalar terminals can only be located at connection points of the underlying geometry. These are the vertices of electrically-active wires, and device or subcell connection points. Clicking on such a point, if no terminal already exists at the point, will create a new scalar terminal at the location. The Terminal Edit panel will appear, which can be used to apply a name for the terminal and edit other terminal properties. The new terminal will be shown highlighted to indicate that it is the target of the Terminal Edit panel.