AN2674

USB interface

Doc ID 14204 Rev 2

13/22

3

USB interface

3.1

USB routing

USB signal trace routing should follow good high speed PCB rules and meet the

specifications for differential impedance and maximum trace delay between connector and

the SPEAr3xx device.

●

USB data traces should be routed with the shortest most direct path possible to their

connectors.

●

USB data traces should have no resistors or magnetic components.

●

USB data traces should be routed only over ground planes.

●

USB data traces should never cross gaps or breaks in the return plane.

●

USB data traces should never be routed under other devices or between the pins of

other devices.

●

USB data vias should be widely separated from other signal vias.

●

If USB data traces transition layers to a different return plane, then ground vias for the

return current should be placed very close to the signal vias.

Figure 4.

Cross section of PCB showing minimum spacing dimensions for USB

signal traces

Table 3.

USB signal routing constraints

Symbol

Description

Min

Typ

Max

Units

ZODiff

1.

Trace delay between SPEAr and USB connector (Universal Serial Bus Specification, Revision 2.0)

81

90

99

Ω

Trace delay of device port(1)

1.0

ns

Trace delay of host port(1)

3.0

ns

Trace length mismatch

0.15/3.8

inch/mm

Number or length of stubs

0

Number of via transitions

1

2.

This includes other USB data trace pairs.

3.

The unit 'h' is the thickness of the dielectric separating the trace from the nearest plane.

4.

Do not use "guard" traces or ground flood/fill adjacent to high speed signal traces.

3h(3)

Space to edge of return plane

20

h(3)