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74 DESIGN007 MAGAZINE I OCTOBER 2019 individual rules is via a matrix (Figure 1). This example is a spreadsheet view with typical con- trols for replication, group select>copy>paste, drag copy, and row copying. Many of the de- signs we create today require some sort of im- pedance matching. This makes controlling trace widths by layer a must. Having this view as part of the matrix is critical to making de- sign rule entry simple. Next, let's talk about what is—or should be—the most widely used design rule feature: classes, the grouping of nets under a descrip- tive name that rules can be assigned to. Now that we know what a class is, what are the ad- vantages? 1. Control copper-to-copper spacing between nets in a group or against other groups by assigning rules to a large selection of nets versus one by one. 2. Easier control of common net and differential pair impedance. 3. Group control for high-speed constraints. 4. Clearance rule reuse from design to design and within a design. 5. Some not-so-obvious advantages are design review and simplified design segmentation via net color. Creating descriptive classes makes finding special nets in the design far easier. This bene- fits those not familiar with the design and sets up net coloring as visual assistance with place- ment and routing. I also mentioned design rule reuse as a ben- efit to classes. With a comprehensive rule sys- tem, designers create clearance groups instead of assigning copper-to-copper clearance values via the class definition. Separating physical clearance rules from class definitions provides designers the ability to reuse a set of clearance rules between many classes. One can imag- ine that if you're working on a complex design with 25+ classes with many of those classes containing the same rules, it would be difficult to find and modify those rules. By using clearance groups, net classes, and a class-to-class matrix, the clearance rule as- signment process is simplified along with mak- ing future changes. As can be seen in Figure 2, we are given a two-dimensional matrix of the net classes to which we can assign clearance groups. This facilitates rule reuse and stream- lines rule updates. Many modern board designs feature areas where trace widths and clearances need to be modified for a special purpose. Some of these Figure 1: A constraint editor system.