Finalizing a PCB System Schematic

In the 1991 movie “City Slickers,” a couple of the characters comment that; “there’s nothing like bringing in the herd.” Getting the cattle safely home was their goal, and like any project, finishing the job can bring a huge sense of relief. The trick is to make sure that you are really done with the task before you pop the Champagne corks in celebration. You should make sure that all the details of the project have been completed before you say that it is done.

Finishing a schematic is the same way, especially a large system level schematic. The temptation is to celebrate its completion when you are done drawing all the circuitry, but are you really finished yet? There may still be lots of little details that need to be finalized before you can say that you’ve brought in the herd of schematics. Here are some important ideas to keep in mind before you consider your next PCB system schematic completely finished.

Complete all the Individual Designs of a System Schematic First

A system level schematic is composed of individual schematics for each system circuit board. Before you can complete the design at the system level, you need to make sure that each individual design is done first. So let’s review what it takes to finish up a board level schematic:

• Design Complete: The first thing to do is to make sure that you have created all the required circuitry for this design. This may seem like a big oversimplification, but you may be surprised at how many designs go through layout with an important part of the circuitry missing. A full design review at this point is very helpful to make sure that all circuitry is included, and has been verified by circuit simulation.

• Labels: Make sure that all of the required nets and net groups are named and labeled, visible, and arranged according to your design standards. You will also want to make sure that all components have their reference designators scaled and rotated so that they are easily readable.

• Company Data: Have you included all of the necessary titles, names, dates, part numbers, copyright information, and company logos in the title blocks? Are all pages of the schematic numbered and named correctly?

• Schematic Clean-up: Have you cleaned up the schematic so that nets aren’t confusing and components aren’t crowded? You will also want to look for wires that wander more than they need to, or other ways to simplify the look of the circuitry. Remember, the goal here is to come up with a usable, readable document.

• Final Synchronization: Make sure to run a final synchronization between the layout and the schematic. All to often this doesn’t get done, and the board layout ends up not matching the parts and netlist in the schematic.

• Final Checks: Have you run the DRC’s a final time? Check your design for electrical and physical violations. If your synchronization has changed anything or your cleanup has inadvertently caused circuitry problems, you will want to make sure that those issues are checked for and corrected before you release the final schematic.

   

Once you can safely say that all of the individual system schematics have been finalized, you are ready to go on and do the same thing at the system level.

Finalizing a PCB system schematic is part of completing this system design

Follow the Same Check-Off Process to Finalize the System Schematic

Your system level schematic is essentially the same entity as an individual schematic, except that it has blocks representing the individual system designs instead of components or hierarchical schematic sheets. Since the individual designs are often worked on by other design team members, it is important that there is a structure in place to manage the entire project from the system perspective. This will make sure that every design goes through the same design and checking steps so that nothing is left out of the process.

WIth all of the individual designs of the system now finalized, you will be ready to do the same to the main system level schematic. To do this you will go through essentially the same check-off process that you went through on an individual design, but with some slight differences:

• System Design Complete: Have you included all of the system designs in your system schematic, and are all of those designs completed?

• System Labels: Are the different individual designs labeled with names and part numbers, as well as connectivity names for the design interconnects?

• Company Data: Does your system design have all of the necessary titles, names, dates, part numbers, copyright information, and company logos in it?

• System Schematic Cleanup: Is the system design neat and organized so that it is usable and readable by others? Remember that at the system level you can add graphical objects such as arrows or text to help identify specific circuit flows and part numbers.

• Final Checks: Have you maintained the correct system connectivity, and created a connectivity report?

At this point you have brought your system level schematic through the steps needed in order to consider it finalized and complete. With the completion and finalization of the individual PCB layouts, you are now ready to build your boards to test and debug the entire system.

Each individual design of a system schematic needs to be finalized before the main schematic

Use Your PCB Design Tools to Help

The key to success with finalizing a PCB system schematic is to start with the right tools that will give you the power and flexibility to design at the system level. To do this, your schematic tools need to be able to work with multiple designs allowing you to connect those designs together in a way that is usable and readable for all who will be building and working with the design. And while you need the schematic capture tools that will allow you to interconnect all of your designs at the system level, you will also need the layout tools that will bring these different boards together for 3D viewing and checking.

The PCB design tools that you need for advanced design capabilities like this are from the Cadence suite of high-powered EDA software. The Allegro Multi-Board PCB System Design tools will give you the ability to define your system architecture with Allegro’s system capture. With them you will be able to define your system level schematic with all of the features that we have discussed here, as well as complete all design aspects of your system printed circuit boards.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.