PADS Professional Constraint Manager: Advanced Topologies and Formulas

PCB Layout Schematic and Constraint Tools


• Constraint Manager • Advanced Topologies • Applying Constraints • Using Formulas


Q&A From the Webinar:

Question: How do I delete constraint templates?    
Answer: Using the “View > Navigator > Constraint Templates” will display the list of templates in the Navigator window. These may then be selected and deletedNOTE: The templates cannot be deleted while the templates are assigned to elements. 

Question: Can one use both variables and constants in the same formula.     
Answer: Yes. 

Question: Can one mix the units within a formula?    
Answer: Yes. However, it is suggested to keep your units the same to reduce confusion.  

Question: How do I delete constraint templates?    
Answer: Using the “View > Navigator > Constraint Templates” will display the list of templates in the Navigator window. These may then be selected and deletedNOTE: The templates cannot be deleted while the templates are assigned to elements.       

Question: Are there limitations on tolerance settings?        
Answer: Yes. If you specify very small tolerances, it is important for these values to be no smaller than 0.012 th or 0.002 ps. Using tolerance values below these limitations can cause errors to occur while Constraint Manager solves a formula. Tolerance limitations are a general rule, but there are exceptions. 

Question:  Does the virtual pin have any special properties?  
Answer: Not really. It is a virtual pin and can be routed to and moved around - and vias can be added to it. But it’s not necessarily a guide pin for routing but rather a location to balance the legs of the topology. 

Question:  Is it mandatory to have the pins defined as Sources, Loads and Terminations?  
Answer:   You will need a source pin for T-Shape, H-Tree and Star topologies. The software needs a starting point to balance out all the other legs to be able to match in lengths. It does not need it if you are using Balanced and Unbalanced virtual pins. 

Question:   Can I set the topology manually for Source, Loads and Terminations?  
Answer:   Yes – I actually kind of showed this earlier – so you need to go to the Parts Tab and that’s where you can Manually define the topology pin types to either Source-Load or Termination. 

Question:  Is via length included in the trace length?  
Answer:  In Setup Parameters - you can add you’re via lengthsThey are also included in the lengths while tuning custom ordered netsBut I would also say - Don't forget about the Pin Package Lengths and Delays. They can also play a big role in getting the correct lengths. 

Question: Is it possible for a net to be assigned to different net classes?    
Answer: No, each net belongs to one net class, cannot be assigned to different net classes. 

Question: How can I delete a net class?    
Answer: It’s easy, you should expand the net class and right click then click delete, it’s important to mention that the nets of the deleted net class will pass to default net class. 

Question: Is it possible to include the length of a via or a pin in the trace length or TOF?    
Answer:  If you want to include the length of a via that is part of a net in the minimum or maximum length value, you must increase the total length or time with the approx. value of the via, however the pin length is a different situation this should be included inside the Parts tab pin package length or pin package NS. 

Question: Can a Match group only include tolerances?    
Answer:   of course, in this case defining only the tolerance of the match group, the length of    each net must be within tolerance of each other, if you don’t set a tolerance for the match group, the tool uses the default tolerances for the design. 

Question: Are Variables and Constants case sensitive? 
Answer: No but it is suggested that all formula variables/constants be consistent in case for ease of reading. 

Question: Is there a way to get constraints from one project to another?    
Answer: Yes. The constraints may be exported from one design to another similar design by using the “File > Export > Constraints...” command in Constraint Manager. 

Question: Is it required that Pin-Pairs be generated for canned topologies such as “Star”?    
Answer: No. Manually creating pin pairs, that can be done for any net with more than two pins, allows the user to define constraints for those specific pin pairs. NOTE: The balanced part of predefined topologies cannot be made unbalanced. 

Question: The netline ordering shown early in the webcast could be done ‘manually’ in route mode with Netline Manipulation? 
Answer This can be done after setting the topology type to Complex or Custom and clicking the Net Order button on the topology toolbar, you have to define  From – To's 

Question: Is it possible to match the nets inside a Constraint class and on top of that setup that matching between two different Constraints Classes? 
Answer: Once the constraints are established for one set up constraints these constraints may be defined as a ‘Constraint Template’ which may be applied to other nets in other constraint classes. Care must be taken to make sure the net to pin matching is correct. 

Question: There’s some mismatch in Units setup. Under setup parameters you have an option to setup via delay only in ns. But in CM you can choose between ns,ps etc. So you need always convert one to another? 
Answer Constraint manager can handle different units so there is no need for conversion. 

Question: What makes formula different from match group? both have tol? 
Answer: Using variables in a formula with a tolerance would be the same as setting up a match group with a tolerance. However, with formulas you could then manipulate one of the nets in the group to have an additional length or a different tolerance. So, the formulas give the user more granularity. 

Question: Is there a good way to compare the data from two constraint manager designs? 
Answer The constraint manager data may be exported as a CSV file (“Edit > Export> ...”), these CSV files may then be opened in Excel for comparison. 

Question: Is there any training material that you can share where it shows how to deal with series resistors included on the traces contained within a match group? 
Answer: A net with discrete components in series is called an “Electrical net”. There are many Knowledge Based Articles (KBAs), on handling these nets. Such as : 

Question: Is there a "Rule of Thumb” where we should use a formula or a match group for tunning traces? 
Answer: Either may be used. Formulas allow for finer manipulation within the group of nets that are to be matched. As an example, one of the nets could have an added length or a different tolerance. 

Question: Once you create a Formula and add a tolerance, it overrides the one define on the Netclass ? 
Answer: The constraints added by formulas are for electrical characteristics and do not affect the physical characteristics defined in the Netclass 

Question: I have an old design that has the topology for differential pairs listed as CUSTOMI can’t change the topology to MST; it is no longer in the drop down listIs this normalIf this isn’t normal what could be causing this?   
Answer: That’s correct. Nets defined as differential pairs must be of type ‘Custom’ or ‘Complex’. The From-Tos must be defined, and the nets must be routed in the defined order to avoid DRC violations. 

Question: Can you go over importing package delays? 
Answer: There are several KBAs on package lengths/delays on Support Center. Here are links to a few of them: 

Question: Does CM have built-in templates for DDR memories (daisy chained) that are already have constraints/equations populated for things like byte lane matching, strobe matching, etc. 
Answer: No. A user could define cosntraints for their DDR circuits then export these constraints to be used in another similar design. (File > Export > Constraints...) 

Question: You need to expand your answer for adding via lengths under the setup parameters? Set Length factor to 1 
Answer: Setting a length factor to “1” will utilize the stackup depth of the board. Typically, the board (stackup and via definitions), will be simulated in a tool like HyperLynx to determine the correct value to enter for ‘Via Length Factor’.  



Additional Resources:

Formulas, Constants and Variables in the Constraint Manager

DDR Routing and Tuning, Actual Design:


DDR Setups:



DDR setups for Branch and H-Tree Topologies in Constraint Manager:


Creating Custom Order Topologies


Creating Min/Max Lengths, Match Groups or, Formulas for T-topology


Custom Topologies and Hi-speed Rules


How to control DDR Memory Lengths with balanced topologies


How to control DDR Memory lengths which contain balanced or equal connections

Using Constraint Templates in the Constraint Manager Webinar

KB Article ID# KB000130625_EN_US



Associated Components

PADS Pro Layout