where to get info to write basic usb driver for own device?

[idownes at slingshot.co.nz]

Hi all,
As part of my uni honours project I need to write a basic USB driver for a 
device I am building.  I need to talk to a Texas Instruments DSP (C5509) 
which has on chip USB (2.0 compliant, full speed).  TI supply a library 
for the DSP side of the USB but I need to write a basic program to 
retrieve data from it, nothing fancy at all.  I have full control over the 
software on the DSP.
There is a basic generic Windows driver I could use (theyscon.de) but I 
don't really want to do it under Windows - instead I'd much rather do it 
with FreeBSD.  

So what I guess what I am asking is how hard would this be? (I have a 
reasonable knowledge of C and Java and have been using FreeBSD for a 
couple of years but have never written a device driver (for any OS))  Do I 
actually need to write a driver or can I use something already existing?  
Where would I get information on how to do this?  So far I've been looking 
at the existing drivers; ugen, ufm etc., the programmers handbook and am 
starting to look at libusb...?
Sorry for the long message.  Any pointers etc. would be greatly 
appreciated.
Thanks
Ian