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24 DESIGN007 MAGAZINE I AUGUST 2025 spacing to control flight time ensures timing con- sistency across clock domains. Proper placement of terminators and power decoupling components, particularly within DDR interfaces, further improves signal quality and overall system reliability. Key Points • Establish the layer stack first. Use the stackup to decide where sensitive nets should live, then place components to ensure clean transitions between those layers. • Partition component groups based on rise-time and operating frequency, positioning the high- est-speed devices closest to the connector. • All analog signals are routed within the ana- log section, while digital signals are confined to the digital section, both sharing a continu- ous common ground plane. • Use keep-out zones across signal layers, effectively guiding the routing process and preventing signals from crossing designated boundaries on specific layers. • Segregate groups of nets based on their signal amplitudes. • Signal flight time and skew are critical con- siderations in high-speed PCB design. • By controlling the placement of devices, the maximum signal delay can be approximated by the longest Manhattan (X + Y) distance within a specific clock domain. B E YO N D D ES I G N • The clock should always have the longest delay so the data and address lines have time to settle before being clocked. • Timing specifications and simulation results guide the establishment of placement and routing constraints. • The DDR circuitry region must be isolated from other signals to prevent interference while maintaining a common ground plane. • The recommendation is to locate these terminators within 200 mils of the signal source. DESIGN007 Resources Beyond Design by Barry Olney: "Critical Place- ment," "The Fundamental Rules of High-Speed PCB Design Part 2," "Interactive Placement and Routing Strategies." Barry Olney is managing director of In-Circuit Design Pty Ltd (iCD), Australia, a PCB design service bureau that specializes in board- level simulation. The company developed the iCD Design Integrity software, incorporating the iCD Stackup, PDN, and CPW Planner. You can download the software at www.icd.com.au. To read past columns, click here.