Inherited Connections and Net Expressions
Inherited Connections
Inherited connections are an extension to the connectivity model that allow you to create global signals and override their names for selected branches of the design hierarchy. This flexibility allows you to use:
- Multiple power supplies in a design
- Overridable substrate connections
- Parameterized power and ground symbols
Net Expressions
A net expression is a special property placed on a net or terminal to define its connectivity. A net expression consists of a property name and a default net name. You use a net expression to override the connection made by a signal or terminal. This allows you to use, for example, multiple power supplies in your design. The signal is redefined due to the value assigned to it. Redefining the signal eliminates the problem of global nets being merged into a single, electrically-equivalent signal, which occurs when the signal traverses the design hierarchy.
You can create, edit, and delete net expressions for pins by changing the net expression information on the Connectivity tab of the Edit Properties form. Once the net expression information is applied to a terminal, the net expression is displayed in the text labels of pins. The following must be true for a net expression to display:
- The pin is created with the Create Label option enabled in the Create Pin form.
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The Net Expressions option is enabled in the Display Options form.
If Net Expressions is disabled, the terminal name is displayed.
Net expressions are not interpreted by the display code in the layout editor. Net expressions are displayed as is, but other tools such as the Layout Versus Schematic (LVS) program use and interpret net expressions. The net expressions on layout terminals must match those found in schematic views.
When you view an instance containing a pin whose terminal has a net expression, the label display is of the terminal name, not the net expression. To see the net expression in a hierarchical design, you must descend into the cellview containing the net expression to see the full text display of the net expression.
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