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Understanding stellaries USB stack .

Hi,

I am newbie to USB protocol.I have read about basic working of USB, how it works, descriptors,
endpoints and other related things.
I have now taken a sample code for understanding how USB works. I am using the stellaries USB stack and the MSC example.
In the stack there is a interrupt handler USB0HostIntHandler which is called continuously which in turn calls other handlers according to ulStatus.
According to my reading USB is a polling mechanism.I am not understanding how the above mentioned handler is called continuously, even if there is no change on D+ and D- lines.
This is really important for me to understand as it seems everything starts from here in the USB stack.

Regards
Rajat