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2.7.1.1 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.
Simple resettable PolySwitch devices are capable of fulfilling the requirements of USB overcurrent
protection and therefore can be used as a replacement for power distribution switches.
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.
2.7.1.2 Powering USB devices during S5
The power distribution switches and the ESD protection shown in the schematics can be powered from
Main Power or Suspend Power (VCC_5V_SBY). Ports powered by Suspend Power are powered during
the S3 and S5 system states. This provides the ability for the COM Express Module to generate system
wake-up events over the USB interface.
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