Hello Hong Yan,
For the issue you encoutered, see below, please!
(1) Usage of USB OTG on i.MX6UL
Although there are 2 USB OTG ports on i.MX6UL, only one of them can be configured to be OTG(Dual Role device), the other must be configured to HOST. They can't work at OTG mode simultaneously.
(2) Resistance for USB differential line.
According to your description, it is also possible to cause the issue because of unmatched resistance of USB differential line. So suggest you check PCB layout. Generally, hardware engineer should use "MCZ1210AH900T" on USB differential line to match resistance .
Have a great day,
TIC weidong sun
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