3 eUSB2 Trace Routing

Similar to USB 2.0, eUSB2 data lines, eD+ and eD–, should be routed as a pair with differential impedance over a solid reference plane to provide a good return path for current. The nominal circuit board trace length for eUSB2 connections is 10 inches. This provides more routing flexibility than other similar low-voltage inter-chip interfaces.

Figure 3-1 Repeater Mode Channel Topology.

As previously noted, when routing the eUSB2 data lines, eD+ and eD–, as a differential pair the recommended trace differential impedance is 85 Ω. This allows for matching within the maximum 15% variation to the eUSB2 transmitter and the eUSB2 receiver, while also allowing the same impedance to be used for any USB 2.0 differential traces on the same eUSB2 repeater, simplifying the routing and fabrication.

The eUSB2 data lines can be routed as microstrips over a solid reference plane or as striplines on an inner layer. Routing the eUSB2 data signal pair as microstrips allows for direct routing with no vias in the path, and is the preferred solution if a direct routing path with 5 W distances (5 times the width of the eUSB2 data line trace) from possible noisy signals can be maintained.

A stripline approach will reduce the impact of system noise on the differential lines and allow for better impedance control of the traces; however it does add at least two sets of vias into the signal path that can act as discontinuities. The impact of vias can be minimized by keeping routing symmetric, adding ground vias nearby to act as returns paths and adjusting the anti-pad to control parasitic impedance. Via stub lengths have minimal impact at eUSB2 signaling speeds.

For both stripline or microstrip implementations, the eUSB2 data line traces should be kept as short as possible and routed over a solid reference plane, preferably a ground plane.