The following guidelines are for the digital I/Os:
- Route SPI and UART lines away from any RF traces.
- Keep the length of the high-speed lines as short as possible to avoid transmission line effects.
- Keep the line lower than 1/10 of the rise time of the signal to ignore transmission line effects (required if the traces cannot be kept short). Place the resistor at the source end closer to the device that is driving the signal.
- Add a series-terminating resistor for each high-speed line (for example, SPI_CLK or SPI_DATA) to match the driver impedance to the line. Typical terminating-resistor values range from 27 to 36 Ω for a 50-Ω line impedance.
- Route high-speed lines with a continuous ground reference plane below it to offer good impedance throughout. This routing also helps shield the trace against EMI.
- Avoid stubs on high-speed lines to minimize the reflections. If the line must be routed to multiple locations, use a separate line driver for each line.
- If the lines are longer compared to the rise time, add series-terminating resistors near the driver for each high-speed line to match the driver impedance to the line. Typical terminating-resistor values range from 27 to 36 Ω for a 50-Ω line impedance.