Atmel Xplained Pro Evaluation Kit for the ATSAMD21J18A Microcontroller ATSAMD21-XPRO ATSAMD21-XPRO Data Sheet

987

Atmel | SMART SAM D21 [DATASHEET]

Atmel-42181C–SAM-D21_Datasheet–07/2014

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

Refer to the 

 for operating voltages which will allow USB operation.

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.

D+

z

The impedance of the pair should be 
matched on the PCB to minimize 
reflections.

z

USB differential tracks should be routed 
with the same characteristics (length, 
width, number of vias, etc.)

z

Signals should be routed 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

VBUS

USB

Connector

VBUS

D+

D-

GND

Shield

GND (Board)

USB_D+

USB_D-

USB 

Differential

Data Line Pair