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2.8.1.1 USB Over-Current Protection (USB_x_y_OC#)
The USB Specification describes power distribution over the USB port, which supplies power for
USB devices that are directly connected to the Carrier Board. Therefore, the host must implement
over-current protection on the ports for safety reasons. Should the aggregate current drawn by the
downstream ports exceed a permitted value, the over-current protection circuit removes power from all
affected downstream ports. The over-current limiting mechanism must be resettable without user
mechanical intervention. For more detailed information about this subject, refer to the 'Universal Serial
Bus Specifications Revision 2.0', which can be found on the website http://www.usb.org.
Over-current protection for USB ports can be implemented by using power distribution switches on the
Carrier Board that monitor the USB port power lines. Power distribution switches usually have a
soft-start circuitry that minimizes inrush current in applications where highly capacitive loads are
employed. Transient faults are internally filtered.
Additionally, they offer a fault status output that is asserted during over-current and thermal shutdown
conditions. These outputs should be connected to the corresponding COM Express Modules USB
over-current sense signals. Fault status signaling is an option at the USB specification. If you don't need
the popup message in your OS you may leave the signals USB_0_1_OC#, USB_2_3_OC#,
unconnected.
Fault status signals are connected by a pullup resistor to VCC_3V3_SBY on COM Express Module.
Please check your tolerance on a USB port with VCC_5V supply.
USB 2.0 port's VCC current limit should be set to 500mA. For USB 3.0 implementations, the VCC
current limit is raised to 1A. A different, USB 3.0 compatible, power switch is used.
2.8.1.2 EMI / ESD Protection
To improve the EMI behavior of the USB interface, a design should include common mode chokes,
which have to be placed as close as possible to the USB connector signal pins.
Common mode chokes can provide required noise attenuation but they also distort the signal quality of
FullSpeed, HighSpeed and SuperSpeed signaling. Therefore, common mode chokes should be chosen
carefully to meet the requirements of the EMI noise filtering while retaining the integrity of the USB
signals on the Carrier Board design.
To protect the USB host interface of the Module from over-voltage caused by electrostatic discharge
(ESD) and electrical fast transients (EFT), low capacitance steering diodes and transient voltage
suppression diodes have to be implemented on the Carrier Board design.
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