ARM running Linux talking easily with a Xilinx FPGA
Unless your embedded application happens to be a router, there’s some application-dependent electronics you need to talk with. If some SoC device covers your needs, that’s always nice, but what about that specific piece of electronics? And what if your application includes a part that needs to be run on an FPGA?
Making a processor talk with an FPGA is pretty doable, as long as there’s no heavy I/O, and the processor doesn’t run a sophisticated operating system. But if you picked Linux (probably to support some USB device, storage and/or network), the task of getting high-bandwidth data running between the processor and the FPGA can turn into a project by its own merits.
Xilinx addresses this issue partly with its Zynq-7000 FPGA-ARM combo, making the ARM’s internal AXI bus directly available to FPGA logic. Whether this new generation of devices is going to have a different fate than the Virtex-2 Pro and Virtex-4 FX FPGAs, which had PowerPC cores built-in and direct PLB bus access, is still to see. It seems like many believe, that putting the processor core inside the FPGA doesn’t necessarily make things easier. Anyhow, with first engineering samples of a completely new architecture to be out on the first half of 2012, it’s not clear when the Zynq-7000 solution will be live and kicking.
As many have found out, running Linux on an embedded processor may be difficult, but not a significant obstacle. Getting the Linux-running processor to access a register or two on the FPGA is not an impossible mission either. But when data needs to be transported fast and efficiently, things start to get tricky: The FPGA needs to be bus master capable, so it can transport the data over DMA. The Linux kernel driver needs to be written correctly to orchestrate DMA accesses at a high rate without using up too much CPU. The asynchronous nature of the data transfer creates corner cases, in particular when the data arrives in anything else than chunks of a constant size. In short, the distance between the “Hello, world” application and the actual working horse is sometimes not as close as it may seem at first.
Xillybus offers a simple solution for systems having a PCI or PCIe bus. As this is not usually the case for embedded processors, this doesn’t necessarily help.
On the other hand, a special port of Xillybus to Freescale’s i.MX51 is already available. Using DMA for transferring data over the external bus lines, data rates of 35 MByte/s and above are possible, with a minimal use of the ARM Cortex A8 processor. The application designer meets the same simple and intuitive interface, as in the PCIe version: The FPGA engineer faces a simple and standard FIFO or RAM interface. The programmer writes simple user space applications which interact with device files, as I/O is usually done in Linux systems.
The demo version is available for the Armadeus APF51 board, which forms, together with its development docking board, a jump start kit for evaluating Xillybus on embedded ARM platform. As the Xillybus evaluation kit is pretty much like the real thing, and the board’s design is straightforward, taking evaluation to real-life implementation is at a hand’s reach.
