When single board enters download mode, bootrom will report one BULKIO port. PC will see the port and then MFGTOOL will use the port to communicate with board. But several times PC can not see the port. After switch to another PC or reboot PC, the port will be seen again.
PC setting on USB Root Hub is ‘Allow the computer to turn off this device to save power’ is checked in default.
They found by unchecking and checking ‘Allow the computer to turn off this device to save power’, the device can be recognized.