Virtuoso Schematic Composer User Guide

Understanding Connectivity and Naming Conventions

The Syntax of an Inherited Net Expression

The syntax of an inherited net expression is a subset of that of an NLP expression. The four components are the search scope, search property name, format instruction, and default global signal name.

Example Net Expression

Example Net Expression Labels

The asterisk ( * ) after a global name shows that this is not a regular wire name but a name that is an overridable net expression. The default global signal name specifies what the wire or pin is connected to by default.

Note: The display of the net expression label has several options.

How Net Expressions Evaluate

The search scope is limited to the @ search, which requires an instance lineage. The search travels up the instance hierarchy looking for an instance property of the given search property name until either an override is found or the instance lineage is exhausted. The default signal name must be global in nature and serves two important purposes: it names the net locally

Virtuoso Schematic Composer User Guide

Understanding Connectivity and Naming Conventions

and is the inherited signal when no override is found during the search. The format instruction is retained here in compliance with the syntax of an NLP expression.

The lookup property is a netSet type and is used only for an inherited connection. The netSet property value can be one of the two possibilities: an override signal name or an overriding, redirecting net expression. The former is the inherited signal name while the latter case redirects the search to a new property name and may introduce a new default signal name to replace the one in the original net expression in the hierarchy below it. The search for the new lookup property starts with the current instance where the netSet property with the override inherited net expression is found.

Net expressions are evaluated for each hierarchical path. The system uses the property name specified in the net expression label to search upward from the cellview containing the net expression label (one instance at a time) to the top of the design. The first instance that is found that has a matching property name terminates the search. If the property is of type netSet and is a legal net name, its value is used as the connecting net rather than the default net name specified in the expression.

The netSet property can be placed on any instance and it will affect all net expression labels with matching property names at all levels below that instance unless overridden by another netSet property on a lower-level instance.

You can create an inherited connection in a schematic by placing an instance of a symbol where one of the symbol pins has a net expression label. When you run the checker program

Virtuoso Schematic Composer User Guide

Understanding Connectivity and Naming Conventions

on the schematic, the net expression label from the symbol pin is propagated onto the net within the schematic.

[@gnd:%:gnd!]

The basic library contains sample parameterized power and ground supply symbols called vcc_inherit, vdd_inherit, gnd_inherit, and vss_inherit.

Virtuoso Schematic Composer User Guide

Understanding Connectivity and Naming Conventions

Sample Net Expression Library

The location of a sample design that uses inherited connections is

your_install_dir/tools/dfII/samples/tutorial/inhConnFlow/top_easy/schematic

The following design shows the top-level schematic cellview.

inhConnFlow / top_easy / schematic

3.3V!

a_gnd!

Inherited Connections Restrictions

You should be aware of some restrictions when using net expression labels and netSet properties:

■The default name in a net expression label must denote a scalar, global name. For example, vdd! and mygnd! are legal; bus!<0:15> and vdd are illegal.

■The property you use to override an inherited connection must be of type netSet.

■The netSet property value must represent an existing scalar signal name (for example, A, clock, or myPower!) or another net expression.

Note: The signal name used in a netSet property must be the name of an existing net in that cellview.

■Within one cellview, you cannot use two different net expression labels for the same default global signal. Therefore, using both [@power:%:vdd!] and [@vdd:%:vdd!] in the same cellview is not allowed.