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Design007-Nov2020

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48 DESIGN007 MAGAZINE I NOVEMBER 2020 placement is extremely important. If the board is difficult to route, it may just be the result of poor placement, slots/gates positioned all over the board, or perhaps the sequence of pins on components are flipped. We need to assist the router as much as possible by opening route channels and providing space for vias. Secondly, there are six important issues to consider before you commence with the pro- cess of formally routing the board: 1. The stackup should be planned to ensure that controlled impedance signals have been calculated correctly and that the return current for each signal layer has a clear return path.The resulting stackup configuration should then be transferred to the design rules to define the correct trace width and clearance for each layer and to specify the differential pairs. 2. The power distribution network (PDN) should be planned and bypass and decou- pling capacitors placed in the appropri- ate positions. The iCD PDN Planner is an ideal sandbox for this analysis. It is a good idea to color the power nets with individ- ual colors so that they can easily be recog- nized without having to name the net. 3. Design rules and constraints can be passed from the schematic, which auto- matically sets the design rules in the PCB database, though there is always some adjustment to be done on the PCB side. 4. Via sizes for different net classes need to be defined. This is important for route completion. 5. For rules to properly support the design process, they need to be defined in the correct priority so that the most important rules prevail over rules of lesser impor- tance. 6. Set up the routing options. It amazes me that most EDA tools do not come with the router set to the most useful functions straight out of the box. Before routing, one must tweak the route options to get the tool to do what you want. Details vary by the tool, of course, but the nuisance is near-universal. Thirdly, most popular EDA tools have the ability to cross-probe between the schematic and the router. This is a fantastic feature that enables a PCB designer to build up an extremely dense, complex route in a couple of hours by controlling the router from the sche- matic rather than just pushing the autoroute button and hoping for the best. Cross-probing can also be used as a pow- erful search tool, locating parts and nets on the schematic or PCB. And cross-probing is not limited to schematic and PCB. Mechani- cal tools allow the cross-probing between PCB and 3D MCAD database, enhancing mechani- cal visualization of the product. Figure 2: Cross-probing from schematic to router of part of the PCIe bus.

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