USB direct connection and implemention on FPGA

Question

A few months ago I did some research on the web for implementing USB on an FPGA, I came across several articles that hinted that it was not possible for complete USB implementation on an FPGA citing the following concerns

1. The voltage levels of USB data lines are in 3.3V range while FPGA runs on 3.3 or 1.2 logic levels. (i was a bit skeptical regarding this since PIC24F which work on 3.3V supply have implemented USB hardware on chip)
2. USB requires embedded software that needs to be implemented on a MCU that will work in tandem with the hardware part of USB.

Today I came across an FPGA dev board, The Orange Crab where the USB pins were directly to the FPGA. Its an open source board with schematics available to be checked out.

Citing from that page

Direct USB connection to the FPGA (Operate as a DFU, MSC, CDC, or composite device!)

I began to wonder how would they be implementing the USB inside the FPGA and the resources that would be necessary for that. Would they for instance instantiate a soft CPU core inside the FPGA and then handle the necessary USB code on it using a C library for the USB framework. Wouldn't that be a terrible waste of FPGA resources, instead of just using an external FTDI chip to offload the USB part?

It would be interesting to know if my hunch is correct or is it really possible to do USB completely in hardware and how?

Answer 1

It would be interesting to know if my hunch is correct or is it really possible to do USB completely in hardware and how?

It's possible, but the FPGA would need differential transceivers that support 480MHz for USB 2.0, and I suppose that a clock running at 240MHz or more. Because this is difficult for many FPGA's most designs use UMTI which parallelizes the incoming serial stream.

For USB 1.2 I would think that cores are available and speed isn't that much of an issue since it runs at 12MHz

The figure below shows what is involved in implementing UMTI, if you could do this (I never have implemented on directly only a UMTI core) on an FPGA, then yes, you could run the data pins right into the FPGA. I am not aware of any cores that do this but that doesn't mean they aren't out there.

Source: https://www.intel.com/content/dam/www/public/us/en/documents/technical-specifications/usb2-transceiver-macrocell-interface-specifications.pdf

This is an open cores USB 2.0 diagram, the external phy implements the UMTI and the opencores usb 2.0 core processes the UMTI stream. In many cases it's better to have the trancievers outside of the FPGA or microprocessor even though it uses more GPIO.

Source: https://opencores.org/projects/usb

The orange crab uses a 12Mbit USB device which isn't hard to implement since it runs at 12MHz. Full-speed (12Mbit) USB with a direct connection to the FPGA

It probably wouldn't be worth it to do USB on chip because it takes a lot of resources. If you spent 30$ or more for an FPGA and implementing USB uses 5 or 10% of resources, then it doesn't justify the cost of adding an external phy or USB to UART (which can be found for 1-2$). Unless you really need to save on space, I'd use an external phy or USB to Uart.