Back when I lived in Portland, I used to commute downtown, typically taking the train to and from work. I was thankful to live in a city with a meticulously timed train system. There must be a valid reason to route train lines as differential pairs because I could always count on the trains being on time. Routing differential pairs on a PCB also requires specific rules and meticulous timing that can only be achieved using the best PCB design software.
Where do you begin in understanding your differential pair routing problems? Signals and traces can impede your design without judicious rule checking, trace width and length can be difficult to work around without a strong routing tool, high-density and high speed circuit boards can have nuance requirements from your traces and signals and impedance is always lurking to watch out for when it comes to differential signalling.
Great PCB design software doesn’t just let you set design rules and classes for differential pairs. The best design software integrates rule and class definitions with automatic routing capabilities and rules checking features. But great design software is about more than just laying out schematics and routing trace between components. You need access to the best simulation tools to confirm your device meets your specifications. These tasks help ensure your PCB works properly the first time.
Piecemeal Software Installations Leave Important Functions Out
One of the most time-consuming tasks in PCB design is routing. Differential pairs route signal and return traces in parallel to prevent crosstalk and EMI. These traces are important in high speed and low power devices, and design software with automated differential pair routing capabilities significantly reduces your routing time. Adding more layers or high-density and your trace arrangement can become time-consuming quickly.
As boards become more complex, your designs will likely include components with high pin counts and high pin density. Translating between schematic and layout almost always creates a rat’s nest of traces. Rather than spending time rearranging hundreds of interconnections by hand, this can be automatically rearranged using pin swapping. Any routing changes that occur in the PCB layout editor should also synchronize with the schematic.
The routing display is also important and can be a major time saver if done properly. Net names should automatically show up on pads and differential pairs in the PCB layout. If your software automatically checks design rules, your layout editor should display rule violations on the PCB, as well as in a summary window. It is always helpful if you can personalize the graphics used to indicate rules violations in your layout. No one wants to scroll through a list of violations and manually search for the offending elements.
All of this is possible when you work in an integrated design environment. Even though some software packages claim to use an integrated design model, critical features are still split between multiple interfaces. Automated routing of single-ended and differential pair connections should be integrated with rules checking, pin swapping capabilities, and design simulations. Your PCB panel should contain all you need for trace routing and signal management.
ActiveRoute tool in Altium Designer
Realizing Where the Pain Comes From
Differential pair routing uses predefined design rules in the layout editor. Problems with differential routing often arise due to conflicting rules settings. Not only should rule conflicts be easy to find and fix, but your software should display conflicts in an intuitive interface. When your design software mixes routing design rules and displays them in different locations, you’ll be left banging your head when your software automatically routes your differential pairs incorrectly.
If you’re using differential pairs, then you’re probably working with high pin count devices like FPGAs, memories, or other components. Once you route your differential I/O pairs, your routing changes should quickly and accurately back annotate and update to the schematic. Non-integrated design software enforces two different processes for annotating at the board level and the schematic level. Why spend time learning two different processes?
Your PCB design software package should contain all the routing tools you need from day one. Piecemeal design software doesn’t include the features you need to design top quality PCBs. If you’re lucky, you can buy these features as add-ons. If you’re less fortunate, these specialized routing and directive features are contained in a different program, and you need to transfer between programs. This reduces productivity and creates a major headache, not to mention introducing the potential for errors.
Differential Pair Routing in a Unified Design Environment
When you work in a unified design environment, differential pair classes and directives defined at the schematic level easily synchronize with the layout. Directive symbol placement and differential pair configuration can be performed in either view, and you can flexibly use the software in the way that works best for you. Because design rules are globally defined, there is no need to translate design rules between a schematic program and a capture program.
Powerful design software gives you the power to automatically define paths for your signal nets. This includes routing between multiple layers and through pin and via arrays. These are by far the most time-consuming portions of routing. Working in a unified environment means you only need to use a single tool to route differential pair and single-ended nets. Rather than routing each pair, you only need to define the path for your net, and the software should automatically finish your routing based on your design constraints.
Once you have routed all your connections, performed pin swapping, and cleaned up any messy traces, a unified design environment gives you the power to check your layout against industry-standard and custom-defined design rules. Working in a unified environment also gives you instant access to simulation and signal integrity tools. Say goodbye to troublesome differential signal and trace placement, impedance and signal issues, and other PCB faults.
Auto-interactive routing in Altium Designer
How Altium’s Unified Design Environment Can Help You
Altium Designer uses a unified interface that makes routing between components intuitive. Differential pairs can be defined for each net in the pair directly on the schematic or layout. All commands are intuitive and easy to locate within the program. Once you’re ready to build your layout, differential pairs quickly synchronize between the schematic and layout. The process of defining design rules that apply to a differential pair is simplified by creating differential pair classes.
When working with high pin density components, designers often need to perform pin swapping or part swapping to clean up crossed connections. Altium Designer includes pin swapping tools that make it easy to switch pins within your nets. Retrace and glossing features give you the capabilities to clean up your routing geometry and confirm your existing traces and differential pairs obey your design rules no matter the length.
Rather than routing your differential pairs by hand, look to a great software package like Altium Designer. The ActiveRoute tool in Altium Designer allows you to define your signal nets, automatically lay traces, and route high pin density components like FPGAs. With automatic design rules checking features, you can meet all routing constraints that are critical for your unique PCB design.
When you work with Altium Designer, you won’t be left to fend for yourself. The documentation for Altium Designer is easy to read, easy to access and is updated with each new product release. Altium also gives you access to the AltiumLive forum, user groups, video library, and webinars provided by industry experts. A strong support network is just a few clicks away.
Now you can download a free trial of PCB layout software to use for your designs. To learn more about Altium Designer and how its routing features can help you design your next product, talk to an expert at Altium.
“We were using Orcad schematic with PADS layout. Keeping our designs in sync was always an issue and extra work for our layout guys. I like the integrated package of Altium. I like the ease of use of the user interface.”
-Design Engineer at Industrial Electronics Company
About the Author
PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.More Content by Altium Designer