OrCAD X Resources

Contents Part 2: Practical Example - Constraint Management for High- Speed Designs .................................................................................. 3 Project Overview .............................................................................. 3 Key Design Parameters ......................................................... 3 Schematic Capture ......................................................................... 4 Constraint Definition (Schematic Side) ............................ 4 Electrical Constraints .....................................................................8 Electrical Wiring Topology ....................................................8 Stub Limits (Stub Length) ................................................... 11 Max Exposed Length ........................................................... 12 Layer Sets ............................................................................... 13 Exercise ............................................................................... 18 Set Impedance Targets ....................................................... 18 Jitter ......................................................................................... 18 Set Minimum and Maximum Propagation Delays ....... 19 Total Etch Length ................................................................. 20 Differential Pair Constraints (Electrical)........................ 20 Notes: ................................................................................... 21 Create Net Classes ...............................................................22 Define Differential Pairs ......................................................23 Copying Constraint Sets .....................................................25 Applying Electrical Constraints ........................................ 27 Net Groups ..............................................................................29 Setting Up Length Matching .............................................29 Electrical Constraints Conclusion ................................... 31 Physical Constraints (Schematic Phase) ............................... 31 Trace Width Constraints .....................................................32 Differential Pair Constraints (Physical) ......................... 34 Spacing Constraints .................................................................... 35 Critical Signal Spacing Constraints ................................ 36 Region-Specific Spacing Constraints............................ 38 Differential Pair Spacing Constraints..............................39 PCB Layout ..................................................................................... 43 Introduction ........................................................................... 43 General Solution ................................................................... 44 Application in OrCAD X Presto: ................................... 44 Constraints for Signal Integrity ................................................. 47 Constraint Modes ................................................................. 48 Signal Integrity ......................................................................49 Impedance Control: Specification for controlled impedance traces for high-speed signals ....................49 Crosstalk Mitigation: Maximum parallel trace length ..... .................................................................................................... 51 EMC and EMI .......................................................................... 53 Stub Length ...................................................................... 54 Maximum Stub Exposed Length ................................. 57 Layer Restrictions ........................................................... 58 Return Path ....................................................................... 60 High Speed Constraints: ..............................................................62 Min/Max Relative Propagation Delay: Controlling Signal Timing.......................................................................................62 Critical Trace Length........................................................... 64 Length Matching .................................................................. 65 Differential Pairs (High-Speed) ........................................ 68 Other High-Speed Constraints ..................................................70 Wiring Topology .....................................................................70 Maximum Via Count ............................................................. 72 High-Speed and Advanced Constraints Conclusion ..........73 2 www.cadence.com OrCAD X High-Speed Digital Design Guide

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