[17.4] OrCAD PCB Walk-through: Constraints

July 21, 2020 Shannon Henry

This tutorial demonstrates how to set up differential pairs and constraints using OrCAD PCB Designer Professional. After you complete this tutorial you will be able to:  

  • Set up a differential pair
  • Specify electrical, physical and differential pair rules with Constraint Sets

If you would like to follow along with this tutorial, you can visit our walk-through page to view video tutorials and download design files. To follow the instructions presented in this tutorial, continue using the design you completed in PCB Walkthrough 3 or open the provided board file in the folder directory, PCB Walkthrough 4_Constraints.   

Note: If you did not define your differential pairs at the schematic level, use the following steps for definition in the PCB:

  1. Select Setup > Add Differential Pairs from the menu.
  2. Select Auto Generate.

  1. Assign DP_ for the name.
  2. Set the + polarity to +.
  3. Set the – polarity to -.
  4. Click Generate.
  5. Click OK to close the window.

This has generated the differential pair.

  1. In the Design Workflow, select Constraints > Modes.

Note: In the Modes window, select which constraints will be checked by setting the constraints “On” or “Off”. Electrical constraints are available in OrCAD PCB Designer Professional.

  1. Close the Mode window.
  2. In the Design Workflow, select Constraints > Physical.

Note: This opens the constraint manager window. In the constraint manager, you can assign electrical, physical, and spacing constraints. Assign constraints by creating CSets or assigning values to each individual net.

  1. In the constraint manager window, select the Physical Constraint Set > All Layers.

  1. Right click and select Create > Physical CSet.

  1. Name the constraint set POWER_GND and click OK.
  2. Set the following constraint values:

Minimum line width: 0.381

Minimum neck width: 0.127

Maximum neck length: 5.08

  1. Right click and select Create> Physical CSet.

  1. Name the constraint set DP and click OK.
  2. Set the following constraint values:

Line Width: 0.127

Primary Gap: 0.127

  1. In the worksheet selector, select Net > All Layers.
  2. In the Referenced Physical CSet column, assign the POWER_GND CSet to 3.3V and GND.

  1. In the Referenced Physical CSet column, assign the DP CSet to DPD.

Note: Assign the differential pair Cset (DP) at the top level to apply to each net.

  1. In the worksheet selector, select Electrical > Electrical Constraint Set > Routing.
  2. Right click and select Create > Electrical CSet.

  1. Name the CSet DP and click OK.
  2. Set the following constraint values:

Gather Control: Ignore

Uncoupled Length: 5.08

Static Phase Tolerance: 0.508

  1. In the Worksheet Selector, select Net > Routing > Differential Pair.
  2. In the Referenced Electrical CSet column, assign the DP Cset to DPD.
  3. In the Worksheet Selector, select Properties > Net > General Properties.
  4. Assign 0 to GND.
  5. Assign 3.3 to 3.3V.
  6. Close the Constraint Manager Window.
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