Cadence PCB Best Practices

Using Diff Pairs

Issue link:

Contents of this Issue


Page 8 of 17

Using Differential Pairs in Allegro PCB Editor February 9, 2012 9 Product Version 16.5 When routing the differential pair on different layers, the rules you set in Constraint Manager will not have any effect. To route a differential pair on tandem layers, perform the following steps: This procedure assumes that routing will be done on the top layer and layer 2. 1. Pick the first net, for example Net1 of the differential pair for routing. 2. Right-click and select Single Trace Mode from the pop-up menu. 3. Route the single trace just a small segment on the top layer, click to add a corner, and right-click and select Done. 4. Select the second, net of the differential pair, and sw, Net2 for example, at the route layer with layer 2, 5. Route Net2 to where you ended Net1 of the differential pair and add a corner to coincide with the net on the top layer 6. Right-click and select Done. You will now have the segment on the top layer for Net1 and the segment for Net2 on layer 2 exactly on top of each other. 7. Select Net1 or Net2, right-click and uncheck Single Trace Mode. The differential pair will now route together. To complete the routing, use the same method that was used to start the tandem routing. Dynamic Phase Control for Differential Pairs Differential pair technology has evolved where more stringent checking is required in the area of phase control. This is evident on higher data rates associated with parallel buses such as QPI, SMI, PCI Gen 2, DDR, QDR and Infiniband. In the simplest of terms, differential pair technology is sending opposite and equal signals down a pair of traces. Keeping these opposite signals in phase is essential ensures that they function as intended. As the current Static Phase is limited to a one time check across the entire

Articles in this issue

view archives of Cadence PCB Best Practices - Using Diff Pairs