How to use inherited connections
Help resources
- IC-6146.14 → Virtuoso Schematic Editor → Virtuoso Schematic Editor L User Guide; → Understanding Connectivity and Naming Conventions → Inherited Connections
- IC-6146.14 → Virtuoso Layout Suite → Virtuoso Layout Suite L User Guide → Editing and Defining Properties → Using Net Expressions and Inherited Connections
Inherited connections provide a convenient way to use multiple power supplies in a design by defining a single power supply in a low-level schematic that is overridable. This can reduce the need to draw power and gnd pin connections explicitly on each symbol (useful for digital gates). In this how-to, a method for using and defining inherited connections will be given.
Note, the NCSU Design Toolkit replaces the basic library from Cadence with its own which may not have the vdd_inherit or gnd_inherit cells. To add this library to your CDS project with the name basic_cds, add the following line to the cds.lib file:
DEFINE basic_cds $IC/tools/dfII/etc/cdslib/basic
Inherited connections use Net Expressions in Cadence which provide a way of renaming a net. There are four components of a net expression (ex. [@search property name:%:default global search name]):
- search scope
- search property name
- format instruction
- default global search name - shows what the wire or pin is connected to by default
When a net expression is formed, an asterisk (*) is shown after the global name indicating that it is a net expression.
In the basic_cds library, there are four inherited supply symbols. These are given in the table below along with their associated net exprssion label:
| Inherited Supply Symbol | Net Expression Label |
|---|---|
vcc_inherit |
[@vcc:%:vcc!]
|
vdd_inherit |
[@vdd:%:vdd!]
|
gnd_inherit |
[@gnd:%:gnd!]
|
vss_inherit |
[@vss:%:vss!]
|
