USB routing

AN3317

16/22

Doc ID 18249 Rev 2

6

USB routing

Ensure that USB signal trace routing follows good high speed PCB rules, and meets the

specifications for differential impedance and maximum trace delay between the connector

and the SPEAr device.

Route USB data traces with the shortest, most direct path possible to their connectors.

USB data traces should have no resistors.

Route USB data traces only over ground planes.

Never route USB data traces across gaps or breaks in the return plane.

Never route USB data traces under other devices or between the pins of other devices.

Widely separate USB data vias from other signal vias.

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

return current very close to the signal vias.

Figure 4.

PCB cross section showing minimum spacing dimensions

Table 2.

USB signal routing constraints

Parameter

Description

Minimum

Typical

Maximum

Units

1.

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

81

90

99

Ω

Td Dev

Trace delay of device port(1)

1.0

ns

Td Host

Trace delay of host port(1)

3.0

ns

Td-match

Trace length mismatch

0.15

3.8

inch

mm

—

Number or length of stubs

0

—

Number of via transitions

1

—

2.

Includes other USB data trace pairs.

3.

The same as the units used for h, the thickness of the dielectric separating the trace from the nearest

plane (see Figure 4), which can vary from board-to-board.

—

4.

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

3h(3)

—

Space to edge of return plane

20

h(3)