Constraints in the Automated Device Placement and Routing Flow

Device groups and constraints are central to the effective placement and routing of custom and analog layouts. Constraint generation is done after initializing the layout. The automated device placement and routing flow supports automatic structure recognition using circuit finders, the organization of these structures into device groups, and the creation of corresponding constraints in the form of Modgens along with symmetry information for symmetric structures.

The Auto P&R assistant is pre-loaded with a set of circuit finders. Each finder analyzes the source data (schematic cellview) and identifies all devices and device groups that match the given criteria. For example, the finder APR Instances (Symmetry by Connectivity) identifies all symmetric instances, nets, and pins in the source cellview and groups them into a device group. The finder names have the APR- prefix, which indicates that these finders are customized for the Auto P&R flow.

For devices to be added to device groups, they need to be registered for the Circuit Prospector assistant.

For a list of circuit finders that the Auto P&R assistant supports, see APR Circuit Finders.

Use the Constraints tab of the Auto P&R assistant to find matching structures and generate constraints in the form of Modgens.

Modgens provide an easy way to manipulate device arrays for matching, for example by changing the aspect ratio and adding dummies.

By default, the Modgens and symmetric structures are listed in the Constraint Manager. The automatic device placer and router honor these constraints during design placement and routing.

The following environment variables control how Modgens are generated:

aprCreateModgens: Generates constraints as Modgens. The default value is t. To create symmetry constraints instead of Modgens, set the value to nil:
```
envSetVal("layoutXL.AP" "aprCreateModgens" 'boolean nil)
```
The newly created symmetry constraints are grouped into figGroups and listed in the View and Edit Constraints section under the various categories.
aprCreateModgenFromTemplate: Applies the default Modgen reuse template when creating the new Modgens. The default value is t.

Automatically created constraints can be overridden by creating groups and constraints manually, which are respected by the automatic tools.

Virtuoso Automated Device Placement and Routing Flow Guide
Product Version IC23.1, November 2023