Matched Net Lengths

Rule Category: High Speed

Rule Classification: Unary

Summary

Specifies the allowable difference in net lengths. The set of nets being targeted is defined by the scope of the rule (as defined by its full query), with the reference length (the longest net in the set) being determined by the rule scope in combination with the checkboxes enabled in the the Constraints settings.

For example, when Check Between Differential Pairs is enabled, the average diff pair length is used for each pair (as displayed in the Differential Pair Editor mode of the PCB panel), with the average length of the longest pair then being compared to the average length of each other pair in the set defined by the scope. If the Check Nets Within Differential Pair option is enabled, then for each pair in the set, the nets within the pair are compared to each other.

Those nets that are found to be outside of a specified tolerance (i.e. are too short) can be lengthened by running the command: Tools » Equalize Net Lengths, or Tools » Interactive Length Tuning or Tools » Interactive Diff Pair Length Tuning.

Constraints

Default constraints for the Matched Net Length rule.

• Tolerance - specifies a length tolerance when comparing each net with the longest net in the set. Any net whose length does not lie within the specified tolerance is deemed to be too short and will have track added to it should the Equalize Net Lengths command be run. (Default = 1000mil).
• Check Nets Within Differential Pair - enables net length checking to occur between nets in a differential pair.
• Check Between Differential Pairs - enables net length checking to occur between differential pair nets.
• Check Between Other Electrical Objects - enables net length checking to occur between every net.

How Duplicate Rule Contentions are Resolved

All rules are resolved by the priority setting. The system goes through the rules from highest to lowest priority and picks the first one whose scope expression(s) match the object(s) being checked.

Rule Application

Online DRC, Batch DRC and the Equalize Net Lengths feature.

Notes

The PCB Editor can add "accordion" sections to nets to equalize their lengths. Having defined the Matched Net Lengths rule, from the PCB document select Tools » Equalize Net Lengths . The matched lengths rule will be applied to the nets specified by the full query of the rule and accordion sections will be added to those whose length falls outside the permissible tolerance.

The underlying algorithm that adds the accordion sections will only do so on vertical and horizontal tracks. If a net has been predominantly routed using 45° track segments, the possibility of successful equalization will be greatly reduced, depending upon the availability and extent of horizontal and vertical track on which to add the equalizing accordion lengths. If nets with such routing do exist, reroute them using short 45° track lengths and more orthogonal sections.

The degree of success depends on the amount of space available for the accordion sections and the accordion style being used. The 90° style is the most compact and the 45° style is the least compact.