PCIe & DDR Layout Guidelines

When designing high-speed PCB layouts for PCIe and DDR interfaces, adhering to specific guidelines is crucial to ensure signal integrity, minimize electromagnetic interference (EMI), and achieve reliable performance. Below are the layout guidelines for both PCIe and DDR interfaces:

PCIe Layout Guidelines

Impedance Control:

Maintain controlled impedance for all differential pairs (typically 85 ohms differential).

Use impedance calculators or simulations to ensure the PCB stack-up and trace widths meet the required impedance.

Differential Pair Routing:

Route differential pairs with minimal length mismatch (usually within 5 mils).

Ensure differential pairs are routed parallel and close together to maintain coupling.

Avoid sharp bends; use 45-degree angles or arcs instead of 90-degree turns.

Length Matching:

Match the length of differential pairs to ensure timing synchronization between the signals.

Apply length matching within the pair as well as across pairs if multiple lanes are used.

Via Usage:

Minimize the number of vias in differential pair routing to reduce signal degradation.

If vias are necessary, ensure they are identical for both traces in a differential pair to maintain phase alignment.

Return Path Continuity:

Ensure a continuous return path for signals by avoiding gaps in the reference plane under the differential pairs.

Use ground vias to maintain return path continuity when transitioning between layers.

Isolation from Noisy Signals:

Keep PCIe traces away from noisy power or clock lines to minimize crosstalk.

Implement a minimum spacing between PCIe traces and other signals, typically 3x the trace width.

Power Supply Decoupling:

Place decoupling capacitors close to the power pins of PCIe devices.

Use multiple capacitors of varying capacitances to filter a broad range of noise frequencies.

Connector Considerations:

Ensure that the PCIe connector layout aligns with the standard pinout and that signal integrity considerations (such as impedance control) are maintained up to the connector.

DDR Layout Guidelines

Impedance Control:

Maintain controlled impedance for all signal traces (typically 50 ohms single-ended, 100 ohms differential).

Use a consistent PCB stack-up and ensure the trace width and spacing meet the impedance requirements.

Length Matching:

Address, Command, and Control Signals: Match the length of these signals to within a specified tolerance, typically 25 mils, to ensure timing accuracy.

Data Lines (DQ): Ensure length matching within each byte lane group and between data lines and strobe (DQS) signals. The typical tolerance is 10 mils within a byte lane.

Clock Signals: Ensure that clock signals are matched to the longest address/command/control line.

Trace Routing:

Route traces on the same layer to minimize skew and delay variations.

Avoid using vias for critical signals (especially clock and strobe lines) to prevent added delay and reflections.

Termination and Stubs:

Implement appropriate termination schemes to minimize reflections (e.g., series termination resistors).

Avoid stubs on signal lines as they can cause signal reflections and timing issues.

Fly-By Topology:

For DDR3 and DDR4, use fly-by topology for the address, command, and clock signals, which helps in timing management.

Ensure the layout supports sequential arrival of signals to each memory chip.

Power Distribution:

Use a solid power plane and distribute decoupling capacitors close to the DDR power pins.

Use bulk capacitors near the power supply entry point and smaller decoupling capacitors near each memory chip.

Ground Planes:

Maintain solid ground planes under DDR signals to provide a low-inductance return path.

Avoid splitting ground planes under critical signals to minimize signal integrity issues.

Via and Crosstalk Management:

Minimize the use of vias in DDR routing to reduce potential for signal degradation.

Maintain sufficient spacing between DDR signal lines to minimize crosstalk, especially between data and clock lines.

Reference Designators:

Clearly label and organize reference designators for ease of layout verification and debugging.

Ensure proper orientation of components like resistors, capacitors, and ICs to follow the signal flow logically.

Signal Integrity and Simulation:

Perform signal integrity simulations during the design phase to verify that timing and signal quality meet the DDR standards.

Use tools like HyperLynx, SIwave, or similar for simulations to detect potential issues like reflections, crosstalk, or timing violations.