An FPGA-based PCI Express peripheral for Windows: It’s easy

To make a long story short…

There is really no need to work hard to make your FPGA talk with a PC.  Xillybus gives you the end-to-end connection interfacing FPGAs with both Linux and Windows computers.

The challenge

At times, FPGA engineers are faced with the need to transfer data to or from a regular PC. Sometimes it’s the purpose of the project (e.g. data acquisition, frame grabbing, firmware loading etc.). Others need data transport with a PC for testing their HDL design on silicon. Or just run whatever automated tests or calibration processes involved in the project. This way or another, the lowest-level piece of logic (the FPGA) now needs to talk with the highest level form of logic (a user space  application running in protected memory mode within a fullblown operation system). That’s quite a gap to bridge.

Since I published a short guide about the basics of PCI Express, I keep receiving questions implying that some FPGA engineers don’t grasp what’s ahead of them when they start such a project. Even for low-bandwidth assignments (where no DMA is necessary) there’s quite some way to go before having something that works in a stable manner. While Linux offers a fairly straightforward API for writing device drivers, developing any kind of driver for Windows is much of a project in itself. And even though my heart is with Linux, it’s pretty clear that “PC” and “Windows” are synonyms to most people today.

Non-PCIe solutions

There are two common approaches today for making a Windows PC and an FPGA talk:

• Using Cypress’ EZ-USB chip, which supplies a generic data interface for USB communication. Windows drivers are available from Cypress, but interfacing the chip with the FPGA requires a substantial piece of logic, as well as some 8051 firmware hacking. From my own experience and others’, those chips have some “personality” once they’re put on a real design’s board. So all in all, this is not a rose garden, and yet for many years this was considered the only sane solution.
• Connecting the FPGA to the PC’s network plug through an Ethernet chip, typically a Marvell Alaska transceiver. This solution is attractive in particular when data goes from the FPGA to the PC, since raw MAC packets are quite easy to assemble. The main problem with this solution is that even though it usually works fine, it’s just because the hardware and software components are more reliable than required, as detailed in another post of mine.

Painless PCIe

As mentioned above, the good news is that Xillybus makes it amazingly easy: On the computer, open a plain file in an user space application, using whatever programming language you want. No creepy API involved. Just read and write to a file, and put a plain FIFO inside the FPGA to hold the data.

Xillybus supports a variety if Xilinx and Altera FPGAs, regardless of the host’s operating system: All Spartan 6, Virtex-5 and Virtex-6 devices with a “T” suffix (those having a built-in PCIe hardware core). As for Altera, all devices having the hard IP form of PCI compiler for PCI Express.