[U-Boot] USB virtual serial(usbtty) not working through USB hub

Hi,

I have worked on adding usb virtual serial (usbtty)  support on Atmel AT91SAM9G20 based board. It works very well (tested on Windows XP). However when I connect the board through USB hub, device enumeration goes Ok, PC drivers installation goes Ok and a virtual COM port apprears in windows device manager and I am able to connect to that COM port. But it doesn't work after that. I dont see any u-boot prompt on hyperterm and cannot interact with u-boot anymore.

There is no problem if I do not use USB hub and connect the board directly to PC usb port.

I would appreciate if anyone can help me on rectifying this as its very critical for our project.

Thanks a lot in advance NV