A brief skim of the ISP1761 data sheet suggests that you will need some sort of processor, and - for a Xilinx FPGA - MicroBlaze would be a good choice.
JFDI!
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Here's a data point. We bought an IP USB function for running on an Altera Cyclone III from SLS in India for $5K. It interfaces to a ULPI device. It's a USB high speed device function (virtual COM port). It includes embedded software library to run with NIOS.
As I recall, there aren't a whole lot of choices for complete USB functions. Among the choices, all are expensive (depending on your definition of expensive) and none are free. I'm surprised FPGA vendors aren't supplying that free.
JJS
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