User manual

Advantech SOM-Express Design Guide
48 Chapter 5 Carrier Board Design Guidelines
5.2.2.1 Low ESR Capacitor
You can hot plug USB devices. In fact, this is one of the virtues of USB relative to
most other PC interfaces. The design of the USB power-decoupling network must
absorb the momentary current surge from hot plugging an unpowered device.
Reducing these values is not recommended. These capacitors should be low ESR,
low inductance.
5.2.2.2 ESD or EMI suppression components
The following guidelines apply to the selection and placement of common mode
chokes and ESD protection devices. Some USB designs will need additional ESD or
EMI suppression components on the USB data lines. These are most effective when
they are placed near the external USB connector and grounded to a low-impedance
ground plane. SOM-Express modules vary in the number of USB ports that are
implemented. Two ports are typical. Some SOM-Express modules implement three
or four ports. If the application needs more than two USB ports, a low cost USB hub
IC can be integrated onto the carrier board and connected to the USB0 or USB1
ports on the SOM-Express module. This provides a larger number of USB ports
regardless of which SOM-Express module is in use.
A design may include a common mode choke footprint to provide a stuffing option in
the event the choke is needed to pass EMI testing. Figure 5-7 shows the schematic
of a typical common mode choke and ESD suppression components. The choke
should be placed as close as possible to the USB connector signal pins.
Figure 5-7 Common Mode Choke
Note:
ESD protection and common mode chokes are only needed if the design does not
pass EMI or ESD testing. Basically, it is recommended to add them in the USB 2.0
interface. Footprints for common mode chokes and/or ESD suppression components
should be included in the event that a problem occurs (General routing and
placement guidelines should be followed).
5.2.3 Layout Guideline
5.2.3.1 Differential pairs
The USB data pairs (ex. USB [0] and USB [0]-) should be routed on the carrier board
as differential pairs, with a differential impedance of 90 #. PCB layout software