If we traveled back through time, we would always find someone using a map or making a map. During 600 B.C., someone scratched a 5 x 3” world map on clay tablets while sitting in the ancient city of Babylon. Ptolemy established elements of the science of cartography when published his 8-volume book titled “Geography” sometime during 150 A.D.
Today, GIS technologies have placed maps at our fingertips. Despite the differences in technologies and methods, maps—whether written on clay or in an electronic format—provide coordinates, use visual references, and apply labels.
In our world of PCBs, schematics and netlists lead us through a design. As you move your PCB design from concept to layout and then to production, you need the intelligence that those documents provide. Each works as part of a framework and provides particular advantages.
Schematic vs Netlist: A Picture is Worth a Thousand Words
The power of a schematic rests within the information about the logical and electrical connectivity between components on a PCB. When you draw and edit the graphics that form a schematic, you also illustrate how signals travel through the circuit. Schematics also work as part of your toolset by making circuitry easier to read and understand.
One of the basic parts of any schematic involves defining the connections between the component symbols on the diagram. When you compile a schematic, you build a comprehensive connectivity visual representation of the design. As an example, a schematic maps both how traces intersect at junctions and form an electrical connection and how some electrical paths cross but do not result in a connection. In terms of best practice, signal inputs always begin on the left while signal outputs end at the right side of the schematic. Power inputs begin from the top while ground and negative voltages remain at the bottom.
With all this, schematics give detailed information about all components and all nets that make up a PCB design. Your consistent approach to drawing a schematic allows other individuals—such as other members of your team or manufacturers—to easily read the schematic. Consistency and organization involve minimizing the number of net connections and using uniform symbols for all devices and labels for all nets, pins, and connectors. In addition, you should always consider the purpose of the schematic. High-level schematics do not require the same level of detail seen with other schematics that show every signal path.
If you have designed a larger, complex circuit, consider separating the schematic into logical blocks that define functional areas of the circuit. Large schematics often use separate sheets for each functional area. Separating the schematic into blocks and pages assists with readability and printing.
Circuit schematics look wildly different depending on the device you’re designing for.
Along with using symbols, schematics also assist with identifying components and parts through the use of unique designations. For example, schematics use “Q” as a designation for transistors and “T” for transformers. Multiple components of the same type should always have a consistent naming sequence. You can also use labels to designate parts that have a special requirement such as shielding, define properties on components, or specify design constraints in the schematic which will be transferred to layout via netlist. The readability of your schematic also depends on the logical and organized placement of text in the same vertical or horizontal orientation.
PCB Netlists: Words Provide All the Information You’ll Need
In contrast to schematics, netlists provide a simple data structure for textually describing every connection in a PCB design. Netlists include reference designators, pin numbers, and signal keywords. The list of connections may appear as: U12 pin 2 connects to Q24 pin 3. You can use the netlist to compare the connection data with the information provided on a schematic. The final set of output files for a circuit should match the netlist and the netlist should match the schematic.
With the use of a netlist, you have all the necessary elements for integrating a successful design with other electronic design applications without generating errors. Netlist changes can occur because of pin number changes, deleted pin numbers, special characters, redundancy, or change in the netlist format. Given this type of information, the netlist also serves as a tool for verifying the design during review and testing. Manufacturers prefer to receive the final netlist to ensure that they produce the PCB to exact specifications.
Schematic vs Netlist for PCB Integration: Which is Better?
When we use Electronic Design Automation (EDA) software and have all the library elements in place, schematics and netlists work as the best tools for integrating information into the PCB layout tool and moving the PCB from design to production. Design integration synchronizes component, routing, and manufacturing within the PCB database.
With the proper choice, you’ll have your circuit boards printed in no time.
EDA tools simplify the process of schematic integration through the creation of an intermediate file read in by the schematic and PCB tools. Because of this factor, schematic integration can only occur within the same EDA family. Even with this stipulation, schematic integration offers the flexibility of allowing for cross-probing, forward-and-back annotation, and the easy pass through of net properties.
However, the complexity of some schematics may require that design teams include additional documentation that describes the intent and functionality of the design. Thankfully, Cadence has enabled IPC-2581 files to integrate and import stackups and other manufacturing specifications with ease.
Although many design teams rely on netlist integration, the different formats of netlists require custom editing and error checking to the netlist before integration can occur. In addition, teams must reformat netlists for use by most EDA tools and then document all changes made to the netlist to ensure design repeatability. Reformatting can occur through spreadsheet or word processing macros.
Working through schematic integration and file importing can be a pain. Fully utilizing the tools available within Cadence’s PCB design toolset will enable you to find these pains negligible. Cadence’s OrCAD PCB designer has a smart and powerful design flow within the tool capable of the specifications and particular designs you need.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.