when usb loopback works, it will detect which signal? The usb port or the usb control

[anglela]

recently i use passmark USB2.0 loopback to test a usb2.0 port, and the usb2.0 port signal is HSIC and controller is XHCI. USB2.0 loopback test result was fail. then i use passmark USB3.0 loopback to test this usb port, the result was pass. i wondered whether usb2.0 loopback cannot test usb port which controller is XHCI? and when loopback works, it will detect which signal? The usb port or the usb controller?

[Tim (PassMark)]

USB2 plugs should work fine on XHCI controllers, though looking a the HSIC specs that it can run at a lot lower voltage than normal so it may be a power detection issue (only speculation).

What sort of motherboard is this on?

[anglela]

my motherboard information:
CPU: intel atom E3845 (bay trail)
memory: DDR3L 1333 4GB
when i use 2.0 loopback to test the USB port(loopback mode), the error information is: Number of device transcceiver errors: 15
when i test benchmark, it can test and the max, min, ave results are 360mb/s
whether the FW of USB2.0 loopback need to be updated?
and would you please kindly tell me the way of loopback works? thanks

[David (PassMark)]

If it says you have transceiver errors, then I suspect you have transceiver errors.
While we could update the software to hide the errors, this would defeat the purpose of a USB test device.

Transceiver errors are accumulated in the micro-controller in the plug, then periodically they are collected and reported back to the user. These are events at a low level that may lead to the packet being retransmitted. They do not represent data errors at the application level.

Examples of what can cause an error are,

• Bad PID
• CRC error
• Bit stuff error
• Extra bits in a packet
• Full Speed EOP ending on K
• Loss of high-speed valid before EOP (truncated packet)
• Overflow (host clock running too fast or device clock running too slow)
• Token larger than 3 bytes (this may occur when using the USB 2.0 Loopback plug behind a hub, with Low/Full speed devices connected to the hub).

Unfortunately the USB chip isn't sophisticated enough to keep a count of each error type. you really need a full protocol analyser for this. (For example the Teledyne LeCroy Voyager M310).