Kontron COMe Starterkit Eval T2 - Manual (Page 76)

COM Express Interfaces
2.8. USB Ports
A COM Express Module must support a minimum of 4 USB Ports and can support up to 8 USB
Ports. All of the USB Ports must be USB2.0 compliant. There are 4 over-current signals shared
by the 8 USB Ports. A Carrier must current limit the USB power source to minimize disruption of
the Carrier in the event that a short or over-current condition exists on one of the USB Ports. A
Module must fill the USB Ports starting at Port 0. The USB SuperSpeed ports 0, 1, 2 and 3, if
used, are to be paired with USB 2.0 ports 0, 1, 2 and 3 in the same order. The USB SuperSpeed
ports use the same over current signaling mechanism as the USB 2.0 ports, but USB 3.0 allows
up to 1A current per port instead of 500mA allowed in USB 2.0. Although USB 2.0 signals use
differential signaling, the USB specification also encodes single ended state information in the
differential pair, making EMI filtering somewhat challenging. Ports that are internal to the Carrier
do not need EMI filters. A USB Port can be powered from the Carrier Main Power or from the
Carrier Suspend Power. Main Power is used for USB devices that are accessed when the
system is powered on. Suspend Power (VCC_5V_SBY) is used for devices that need to be
powered when the Module is in Sleep-State S5. This would typically be for USB devices that
support Wake-on-USB. The amount of current available on VCC_5V_SBY is limited so it should
be used sparingly.
2.8.1. Signal Definitions
All USB Ports appear on the COM Express A-B connector as shown in Table 18 below.
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#, USB_4_5_OC# and USB_6_7_OC# unconnected.
Simple resettable PolySwitch devices are capable of fulfilling the requirements of USB over-
current 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.
2.8.1.2. 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.
PICMG ® COM Express ® Carrier Board Design Guide Rev. 2.0 / December 6, 2013
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