High-speed signals and impedance
For USB, Ethernet and fast SPI, a trace behaves as a transmission line. The basics of differential routing, impedance matching and length matching.
When to treat a signal as high-speed
The criterion is the rise time of the signal, more than its frequency. Reflections become a problem when the trace is longer than 1/6 of the distance the signal travels during its rise time. With a 1 ns rise time a signal travels about 15 cm in FR-4, so take care from 2.5 cm of trace upwards.
Characteristic impedance
It is set by trace width, dielectric thickness and copper thickness. Typical targets:
| Signal | Impedance |
|---|---|
| Ordinary single-ended signal | 50 Ω |
| USB 2.0 differential | 90 Ω |
| Ethernet differential | 100 Ω |
Get the width and spacing from a calculator using the PCB manufacturer's stackup, and request impedance control when ordering.
RoadmapSee where this note sits in PCB Design→Differential routing rules
- Route the two traces side by side at constant spacing, with matched lengths (within about 0.15 mm for USB 2.0)
- Vias in pairs, the same number on each
- No breaks in the ground plane below
Reflections on single-ended signals
For a fast SPI clock leaving the MCU, a 22 to 33 Ω series resistor close to the output pin reduces reflections and ringing. If the edges ring on the oscilloscope, it is the first measure to try.