Routing Serial Peripheral Interface (SPI) on a printed circuit board (PCB) involves specific considerations to ensure signal integrity, especially for higher speeds, but even at lower speeds, good practices are beneficial. Based on routing guidelines, here's how to approach SPI routing.
Key Principles for SPI Routing
When routing SPI signals, two fundamental principles from the provided guidelines are crucial:
1. Route Over a Ground Plane
A primary recommendation is to route your SPI lines over a ground plane. This is important even if the SPI communication is not operating at high speeds. Routing signals over a continuous ground plane provides a return path for the signals, which helps minimize electromagnetic interference (EMI) and improves signal integrity.
- Benefit: Provides a controlled impedance environment and reduces noise coupling.
2. Use Appropriate Trace Widths
Trace width considerations are important, especially depending on the layer the trace is routed on and the distance to the ground plane (dielectric thickness).
-
On Outer Layers: For traces routed on the outer layers of the PCB (top or bottom), use wider traces. The guideline suggests sizing these traces 2-2.5 times the distance to the nearest ground plane on the adjacent layer. This is particularly relevant for impedance control on thinner dielectrics.
-
On Inner Layers: On inner layers, wider traces are often less critical for standard SPI speeds. Traces sized 10-15 mil (thousands of an inch) are often considered acceptable.
-
Context: The guideline hints that these trace width recommendations are related to achieving a certain impedance, though SPI may not always have explicit impedance requirements.
Practical SPI Routing Tips
Beyond the core principles from the references, consider these practical tips for routing SPI:
- Keep Traces Together: Group the SPI signals (SCK, MOSI, MISO, SS/CS) together as much as possible. This helps maintain signal timing relationships and simplifies routing over the ground plane.
- Minimize Via Usage: Vias can introduce impedance discontinuities. Try to minimize the number of vias on SPI traces, especially the clock (SCK) and data lines (MOSI, MISO).
- Trace Length Matching: For very high-speed SPI or when long traces are unavoidable, consider matching the lengths of the SCK, MOSI, MISO, and SS/CS traces to minimize skew between the signals.
- Signal Integrity Simulation: For critical or high-speed designs, consider using signal integrity simulation tools to verify routing strategies.
By following these guidelines, particularly routing over a ground plane and using appropriate trace widths as suggested by the references, you can effectively route SPI signals on your PCB, leading to reliable communication.
