Design Guideline - Advantech SOM-ETX Series Design Manual

5.2.2 Design Guideline

Figure 5.6 shows USB connections for SOM-ETX USB signals. All pull-up resistors
are integrated on the SOM-ETX board. For ESD and EMS protection please integrate
the parts on your baseboard.
SOM-ETX
USB0
USB0#
USB1
USB1#
USB2
USB2#
USB3
USB3#
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 un-powered 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-ETX modules vary in the number of USB ports that are
implemented. Two ports are typical. Some SOM-ETX 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 baseboard and connected to the USB0 or USB1 ports on
the SOM-ETX module. This provides a larger number of USB ports regardless of
which SOM-ETX 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.
Advantech SOM-ETX Design Guide
Figure 5-6 USB Connections
Chapter 5 Carrier Board Design Guidelines
USB Connector
69