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It is easier to use an interface chip, usb is not easy, it is a high speed differential analogue interface with clock recovery and bit stuffing going on. You need that serial interface engine! Save a lot of headache.
I am using Philips PDIUSBD11 interface chip, application notes, schematic and sample app can be found on beyond logic website.
If U need a simple device acting as USB slave device U can use some RS232 to USB bridge... U'll found many cheap component... I recommand you FTDI chips (works cool and is simple to interface with any kind of controller...):