How To Route USB Tracks

Geoffrey Hunter

mbedded.ninja Author

Overview

The D+ and D- (data) USB traces on circuit boards require impedance controlled routing. The tolerance depends on the exact USB standard used (e.g. USB1.0, USB2.0 full-speed/high-speed, USB3.0).

The USB2.0 spec specifies a 90R differential impedance and a 45R single-ended impedance for the D+ and D- data lines.

For most normal sized PCBs, signal integrity starts becoming an issue at the 480Mb/s USB baud rate.

I recommend visiting the Transmission Lines page before continuing here...

What Is My Bandwidth?

Lets recall the well-known rule-of-thumb:

$$\begin{align} BW \approx \frac{0.35}{t_r} \end{align}$$

where:
$t_r$ is the maximum rise time from 10 to 90% (this is stated in the USB spec.), in seconds (s).

$BW$ the resulting maximum bandwidth the track has to support, in Hertz (Hz). You can also think of this as a maximum frequency, since the bandwidth starts at 0Hz.

The USB specification states a maximum rise time of 4ns. Using the equation above, this gives us a bandwidth of approximately 87.5MHz.

From this, we can calculate the wavelength of an 87.5MHz signal travelling down a track on standard FR-4 PCB.

$$\begin{align} \lambda = \frac{c}{f\sqrt{\epsilon_r}} \end{align}$$

where:
$c$ is the speed of light, in meters per second
$f$ is the frequency that you worked out above (which we called bandwidth)
$\epsilon_r$ is the di-electric of the material the electro-magnetic wave is travelling through, and is unitless. In our case this will be the di-electric of the PCB. For standard FR4, this is about 4.35.

Another general rule-of-thumb is that the impedance of a PCB track is not important if it is at least 10 times smaller than the wavelength of the signal.

Thus, the minimum wavelength of the USB full-speed signal is 1.65m, and the characteristic impedance of the track is only important if the total track length is greater than 165mm.

USB Standard	Minimum Rise Time, $t_r$	Track Length At Which Impedance Matters
2.0 Low-speed (1.5Mb/s)	75ns (USB Spec Rev 2.0, Table 7-10)	3.1m
2.0 Full-speed (12Mb/s)	4ns (USB Spec Rev 2.0, Table 7-9)	165mm
2.0 High-speed (480Mb/s)	500ps (USB Spec Rev 2.0, Table 7-8)	20.5mm

As you can see from the above table, for most PCBs designs, you don't really have to worry about USB2.0 low-speed tracks, you might be concerned about really long USB2.0 full-speed tracks, and you have to control the impedance for all but the smallest USB2.0 high-speed tracks.

For more information, see the Transmission Lines page.

What Should My Impedance Be?

The impedance depends on the USB standard.

USB Standard	Differential Impedance	Single-ended Impedance
1.0
2.0 Full-speed (12Mb/s)	$90 \Omega \pm 15\%$
2.0 High-speed (480Mb/s)	$90 \Omega \pm 15\%$	$45 \Omega \pm 10\%$