Part Instances and Occurrences

A part instance is a part you have placed on a schematic page. A part occurrence is created each time the part instance occurs in a schematic that is within the design hierarchy. So, for example, placing a part on a schematic page in your design creates both an instance and an occurrence. You can assign properties to the part instance, in which case the property (and its associated value) will "shine through" to each part occurrence unless the value is specifically replaced by a value on an individual part occurrence. The Property editor window graphically illustrates the state of instance and occurrence properties.

The instance property values shines through to the occurrence as long as the occurrence property values have not been edited in any way. When you explicitly edit an occurrence property value or when Capture modifies it via one of its tools, the occurrence values overrides the instance value. Only the occurrence value will be placed in the netlist.

If you...	The result will be...
place a part on an unused page in your design	only instance properties on that part.
place one part in the root schematic	one set of instance properties and one set of occurrence properties on that part.
do not modify the occurrence properties on an object	instance properties will "shine through" on the occurrence. The instance properties themselves may be shining through from the library definition (or design cache if the two differ
edit a part in a library	no effect on the part in any project. Use the Update Cache command or Replace Cache command to bring library changes into a design.

How Capture uses instance and occurrence properties

The type of property you update or use in Capture depends on the type of project in which you are working. If you are working with an FPGA project or a PSpice project, Capture allows you a choice, but defaults to update instances when you use the Annotate, Update Properties or Export Properties commands. It is best that you use instances to create Cross Reference and Bill of Materials reports, as well.
When you work with a PCB or schematic project, it is best to update occurrences when you use the Annotate, Update Properties, and Export Property commands. In these projects, Capture also uses occurrences to create reports with Cross Reference and Bill of Materials.

While modifying occurrence properties, open only one occurrence at a time.

The EDIF 2 0 0, VHDL, and Verilog netlist formats generate true hierarchical netlists. Capture uses the instance property values on nets and parts when it generates a netlist for a design with one of these formats. All other netlist formats in Capture produce flat netlists and use occurrence property values.

Preferred modes for design processing

Capture determines and recommends a preferred mode for processing your design based on the type of project flow, type of design with respect to complex or simple hierarchy, and whether occurrence properties already exist on the design.
The Cross Reference Parts dialog box is an example of where you can choose to use instances or use occurrences as the preferred mode.

Use instances	If you are in an FPGA, PSpice, or digital simulation project, or if your design does not have occurrence properties.
Use occurrences	If your design has at least one schematic used multiple times or a schematic from an external library. Using occurrences is also the preferred mode if the design already has some occurrence properties.