Issue link: https://resources.pcb.cadence.com/i/1534759
Total Etch Length Traces have critical lengths necessary to avoid signal reflections on the PCB. There are different rules of thumb, such as 1/20th or 1/6th of a wavelength, but sometimes these are overly conservative or too lenient. The only way to know is through simulation. But for now, you can use the manufacturer's data sheet information for the protocol implemented by their specific chip or family of chips. Let's set the Total Etch Length for an ECSet. Steps: 1. In Constraint Manager, go to Electrical > Routing > Total Etch Length. 2. For the DIFF ECSet, set the Maximum Total Etch value to 3500 mil (when you hit Enter on your keyboard, Constraint Manager will automatically convert it to the target units, which is mm in our case). For DDR3 data lines, you would set a minimum and maximum Total Etch according to your design guide. For example, from a design guide for DDR3 you may get values like 4500 mils from the controller. Note: Why set DIFF to 3500 mils? That's a rough estimate given for USB 3.1 as a critical length allowed by some semicon- ductor manufacturing companies for their chips. We cannot know the critical length in truth until we simulate the differ- ential pair signals in a PCB. To do that, you may use a tool like TopXplorer for signal integrity analysis. Set the other Electrical Constraint Sets to Maximum Total Etch Lengths (critical lengths) according to what signal integrity analysis engineers determined from simulation or is mentioned by manufacturing documentation. Differential Pair Constraints (Electrical) Let's set rules for differential pairs in general. To do that: 1. Go to the Constraint Manager, Electrical Constraint Set > Routing > Differential Pair worksheet. 2. Choose the following parameters and values for the DIFF ECSet. a. Uncoupled Length: i. Gather Control = Ignore ii. Max = 0.254 mm (10 mils) b. Static Phase Tolerance = 10 ps (picoseconds) c. Dynamic Phase: i. Max Length = 0.2540 mm (10 mils) ii. Tolerance = 0.1 mm (3.937 mils) d. Min Line Spacing = 0.0889 mm (3.5 mils) e. Primary Gap = 0.1016 mm (4 mils) f. Primary Width = 0.1016 mm (4 mils) g. Neck Gap = 0.0889 mm (3.5 mils) 20 www.cadence.com OrCAD X High-Speed Digital Design Guide