17.8 USB Interface

The USB interface consists of a differential data pair (D+/D-) and a power supply (VBUS, GND).

Refer to the Electrical Characteristics section for operating voltages which will allow USB operation.

Table 17-11. USB Interface Checklist
Signal Name Recommended Pin Connection Description
D+
  • The impedance of the pair should be matched on the PCB to minimize reflections.
  • USB differential tracks should be routed with the same characteristics (length, width, number of vias, etc.)
  • For a tightly coupled differential pair,the signal routing should be as parallel as possible, with a minimum number of angles and vias.
 USB full speed / low speed positive data upstream pin
D- USB full speed / low speed negative data upstream pin
Figure 17-15. Low Cost USB Interface Example Schematic

It is recommended to increase ESD protection on the USB D+, D-, and VBUS lines using dedicated transient suppressors. These protections should be located as close as possible to the USB connector to reduce the potential discharge path and reduce discharge propagation within the entire system.

The USB FS cable includes a dedicated shield wire that should be connected to the board with caution. Special attention should be paid to the connection between the board ground plane and the shield from the USB connector and the cable.

Tying the shield directly to ground would create a direct path from the ground plane to the shield, turning the USB cable into an antenna. To limit the USB cable antenna effect, it is recommended to connect the shield and ground through an RC filter.

Figure 17-16. Protected USB Interface Example Schematic