Cadence PCB Best Practices

Using Diff Pairs

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Using Differential Pairs in Allegro PCB Editor February 9, 2012 16 Product Version 16.5 Troubleshooting Issue #1: Some Differential Pair Nets Do Not Slide Together I have many differential pairs set up with three components assigned to the nets. Some parts of the pairs will slide in tandem as expected while others will not slide together. The pin pair matching function for differential pairs gathers as much data as possible on the nets (including component class and pinuse information), to try and determine the correct set of pins to match. If a coherent set of driver/receiver pins can be determined, side branches of the net are included too. But if the data is ambiguous (no explicit driver/receiver pins), the matching code falls back on the longest path which does not include the side branch. This can be determined by selecting Show Element on the clines that are not sliding as a pair. If the differential pair constraint information is not shown in the Show Element window, that branch is NOT considered. For the branch to be considered, the Show Element window should show diffpair constraint information similar to: Constraint information: (DPTol) R2.1 to U1.13 min= 1272.43 MIL max= 1472.43 MIL actual= 1343.12 MIL DPData: gap=8.00 (--0.01 +0.00) tolerance= 100.00; max uncoupled= -0.01 Note: If the Find Filter is set to Nets instead of Clines, all branches of the net should show the differential pair constraint information to consider the entire net. If all branches of the net do not show differential pair constraint information, the missing branches will NOT be considered a pair and slide together. Solution: Change the pinuse of the pins on the net so there are explicit driver (OUT)/receiver (IN) pin pairs on the differential pairnets. To change the pinuse on the pins: 1. Choose Edit > Property. 2. Under the Find tab, select only Pins. 3. Select all the pins in the net. 4. In the Edit Property form, select Pinuse and change to the appropriate pinuse. Note:

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