What's Good About Differential Pair Support in Allegro PCB Editor? More Features in SPB16.2

Some very helpful new features for Differential Pair support are available in in the SPB16.2 release.

Differential Pair Regions

A major enhancement made to Diff Pairs in the SPB16.2 release involves the use of Regions to define Diff Pair Line Width and Gap. Prior to 16.2, a change of line width/gap on the same layer was controlled with the Neck Width and Neck Gap parameters. Restrictions associated with the necking implementation limit the number of alternate line/gap combinations across the path of a diff pair to one. BGA breakout and rules used on Rigid-Flex boards often require more than 2 sets of line/gap constraints on a per layer basis.

Coupled with the Region enhancement is the shift of additional Diff Pair Physical constraints, Min Line Space and Tolerance +/- to the "Physical" domain in Constraint Manager. The Electrical domain remains unchanged and should be used to manage the electrical constraints associated with Diff Pairs, namely phase control and uncoupled length. The electrical domain does not support constraints or parameters on a per layer basis.

Differential Pair Swap

The differential pair pin swap feature is an option added to the existing swap pins command. The option has two modes. If Diff Pair Swap is not selected, there is no effect on the existing swap pins functionality. There is also a usability enhancement to the swap pins function, the ability to preview the result of the swap to the user before committing it.

Use Model

The user initiates the swap command from the standard menu location in Place> Swap> Pins. The new option and modes appear on the Options panel. To enable differential pair pin swapping, the user checks the Diff Pair Swap check box and the desired mode.

Two Pair Swap

In this mode, the user selects a pin to swap. Pins within the same swap group as the selected pin AND are connected to differential pair nets are highlighted. The diffpair signal's polarity will be determined by (in this order):

1. The model definition.
2. Attached property Diff_Pair_Pins_Pos or Diff_Pair_Pins_Neg.
3. Last character of the physical net name is + or - .
4. Last two characters of physical net name are _P or _N.

If polarity is undetermined, then normal (pre-16.2) pin swap rules apply.

Swap Polarity

The user selects a pin to swap. The pin within the same swap group as the selected pin AND connected to the same differential pair is automatically selected. At this point the result of the swap is displayed, but not committed. The user is prompted with “Pick DONE/NEXT or pick the first pin of the next swap.” This is the same message as always. The difference is that the user can preview the result of the swap.

Miscellaneous

Changing modes: If the user changes modes after the 1st pick, the selection set of eligible pins will be updated accordingly.

Toggling the option: Changing the Diff Pair Swap option after the 1st pick, will cause the selected pin to be deselected and the user prompted to pick a pin to swap (i.e. redo the 1st pick).

Non-diffpair pins: With the Diff Pair Swap option enabled and a non-diffpair pin is selected to swap (1st pick), then only non-diffpair pins within the same swap group will be highlighted and eligible for swapping.

Reporting: A Pin Swap Report is available with the rest of the Allegro reports under the Tools menu.

All pin swaps will be tracked with the property Last_Pin_Swap. The property will be attached to the pin and the value will be the name of the pin that it was last swapped with. Updating the Allegro design from the Front End or third party netin will cause all pin swap properties to be deleted. 

As always, I'm interested in your results and suggestions in using these new SPB16.2 features.

Jerry GenPart