USB interface

AN2674

14/22

Doc ID 14204 Rev 2

3.2

USB decoupling and reference resistor

Decoupling capacitors should be placed as close as possible to the power balls, preferably

directly under the power balls using short wide connecting traces. Placing decoupling

capacitors at a distance degrades the performance, which can result in interoperability

problems and compliance violations with the specifications.

Figure 5.

Example of USB decoupling capacitor bottom mounted layout

Legend:

Blue: top layer of PCB

Orange: bottom layer of PCB

Cyan traces: USB data pairs.

Table 4.

USB power, ground, and reference guidelines

Pin

Guideline

trace

USB_RREF

External reference resistor (43.2 ohm). Connect resistor to ground.