Difference between revisions of "How to use inherited connections"

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| <code>vss_inherit</code>          || <code>[@vss:%:vss!]</code>
 
| <code>vss_inherit</code>          || <code>[@vss:%:vss!]</code>
 
|}
 
|}
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Revision as of 17:24, 9 May 2011

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]):

  1. search scope
  2. search property name
  3. format instruction
  4. 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!]


Inherited connections 001.png
Inherited connections 002.png
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Inherited connections 006.png
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Inherited connections 014.png
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Inherited connections 022.png
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