Configuring Constraints

This lesson will demonstrate how to configure and assign design rules in OrCAD X Presto. Rules or constraints dictate how critical aspects of the design should be completed. Setting design rules upfront can be used to:

• Adhere to device design requirements
• Guarantee proper functionality
• Ensure safety
• Improve signal integrity
• Adhere to manufacturer’s capabilities and requirements
• Verify the PCB assembly

To follow along, continue with the design from the previous lesson or download the starting materials from the Materials tab of this lesson.

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Defining Differential Pairs

Note: Differential pairs have already been defined in this design. For the purpose of this lesson, we will delete a differential pair and redefine it.

Step 1: Select Tools > Constraint Manager from the menu.

Step 2: Select the Electrical domain from the Worksheet Selector.

Step 3: Select the Net > Routing > Differential Pair worksheet.

Step 4: Right-click differential pair DPD and select Delete. Click Yes when prompted to delete the differential pair.

Step 5: Right-click net D+ and select Create > Differential Pair.

Step 6: The Create Differential Pair window opens. Select Net from the dropdown on top.

Step 7: Select D- from the net list and click the > symbol to add it to the differential pair.

Note: The Auto Setup functionality can also be used to automatically identify and create differential pairs within the design.

Step 8: Enter DPD for the Diff Pair Name and click Create.

Step 9: Click Close to close the Create Differential Pair window.

Defining Physical Constraints

Step 10: Select the Physical domain from the Worksheet Selector.

Step 11: Expand the Physical Constraint Set folder and select the All Layers worksheet.

Step 12: Right-click the DEFAULT constraint set and select Create > Physical CSet.

Step 13: Name the CSet POWER_GND and click OK.

Step 14: Enter the following values for the new CSet under their respective columns.

• Minimum Line Width: 0.381
• Minimum Neck Width: 0.127
• Maximum Neck Length: 5.08

Step 15: Right-click the DEFAULT constraint set and select Create > Physical CSet.

Step 16: Name the constraint set DP and click OK. This will create a group of rules for the differential pairs in the design.

Step 17: Enter the following constraint values for the new CSet under their respective columns.

• Minimum Line Width: 0.127
• Primary Gap: 0.127

Assigning Physical Constraints

Step 18: Select the Net > All Layers worksheet in the Physical domain.

Step 19: Select the POWER_GND CSet from the dropdown in the Referenced Physical CSet column for 3.3V and GND.

Step 20: Select the DP CSet from the dropdown in the Referenced Physical CSet column for DPD.

Note: Assigning this CSet at the top level will assign it to each net of the differential pair.

Defining Electrical Constraints

Step 21: Select the Electrical domain from the Worksheet Selector.

Step 22: Select the Electrical Constraint Set > Routing > Differential Pair worksheet.

Step 23: Right-click in the table and select Create > Electrical CSet.

Step 24: Name the CSet DP and click OK.

Step 25: Enter the following values for the new constraint set under their respective columns.

• Gather Control: Ignore
• Max Uncoupled Length: 5.08
• Static Phase Tolerance: 0.508

Step 26: Expand the Net > Routing > Differential Pair worksheet.

Step 27: Select the DP CSet from the dropdown in the Referenced Electrical CSet column for DPD.

Defining 3D Constraints

Step 28: Select the 3D domain from the Worksheet Selector.

Step 29: Expand 3D Clearance > Constraint Set and select the Component to Component worksheet.

Step 30: Select the plus sign to create a new CSet.

Step 31: Name the CSet PKG2PKG and click OK.

Step 32: Select Add Row to add a row to the CSet.

Step 33: The Create Clearances window opens. Click and drag to select all items in the From Component and To Component tables.

Step 34: Enter 0.254 for the Horizontal and Vertical 3D Clearances. Click OK. The clearances are added to the table automatically.

Step 35: Select the 3D Clearance > Design > Component to Component worksheet.

Step 36: For presto_tutorial, under Referenced Package to Package 3D CSet, select PKG2PKG from the dropdown.

Assigning Net Voltages

Step 37: Select the Properties domain in the Worksheet Selector.

Step 38: Select the Net > General Properties worksheet.

Step 39: In the Voltage column, assign 0 to GND and 3.3 to 3.3V.

Step 40: Close the Constraint Manager.

Confirming the Constraints

Step 41: Back in the Presto canvas, in the Properties panel, select All Objects under Selection Filter to turn all objects off.

Step 42: Select Nets to filter only net objects.

Step 43: Select a 3.3V net. Instead of ratsnest lines, all nets with a voltage assigned have a square with an “X” for easy identification.

Step 44: Select the Constraints mode in the Properties panel. The constraints for 3.3V are shown.

Step 45: Select net D+ from X1 or IC1. The physical and electrical constraints for the differential pair are shown.