Nereid - Kintex 7 PCIe FPGA Development Board

For many of us, learning FPGA was a natural next step from the world of micro-controllers and Embedded System. You learn the basics of Embedded System, do some projects using micro-controllers and microprocessors and one day realize that those devices may not be the best choice for a particular problem that you want to solve. A little research could lead you to FPGAs. But the FPGA world is very different from the micro-controller world you have seen so far. While micro-controller and Embedded System experience is very helpful, it can not help you get started with FPGAs without a somewhat steep learning curve. The tools are different, languages are different and even the programming paradigm is different.

But there is hope. By definition (in loose terms) FPGA is a bunch of logic elements that can be configured to make any logical circuit. If that is true, why not make a microprocessor out of those logic cells and program using traditional Embedded System friendly languages and tools? Yes, it is possible. But there is a question as to why this method of making a micro-processor or micro-controller out of FPGA logic is better compared to off the shelf micro-processors? There are many reasons why making microprocessor/controller in FPGA fabric is better (or worse) compared to an off-the-shelf micro. The most important reason is that you have access to the FPGA fabric from the micro-controller (through buses and peripherals created within the fabric, of course) and you can implement peripherals from a vast array of Xilinx’s freely available peripheral IPs or create your IP (or buy third-party IPs). It is possible to make pretty much any peripheral that you want within the capability of the FPGA that you have chosen to work with. There are a few important downsides to this scheme though. The total cost of hardware will be usually higher compared to an off-the-shelf micro for the same amount of raw processing power. If you can not find the peripheral IP that you are looking for, you will end up having to write one using Verilog or VHDL and you are back to square one (remember, what we wanted to do was not to deal with HDLs or FPGA workflow at all). It is possible to buy third-party IPs but depending on the specific IP, it can be very expensive. Also, it is worth mentioning that the tools required to build an embedded system out of an FPGA could set you back by a few hundred dollars.

Let us take a look at what a processor-based embedded system built into an FPGA would look like in it’s simplest form. As you can see in the diagram below, an embedded system built in FPGA fabric is not really different from any other embedded system. It has a processor, memory, buses, and peripherals etc. everything that you would expect in a normal Embedded System configuration. It is just that the processor, buses, and peripherals are all made by using logic cells that are available within the FPGA.

As you can see here, you could use a variety of peripherals from a variety of sources. The peripherals could be communication peripherals such as I2C, SPI, USART or signal processing cores such as FFT, filters or anything that you could imagine and implement on the FPGA fabric. This flexibility gives FPGA based embedded systems an edge in some situations compared to traditional microcontrollers where peripheral choices are often less flexible.

Embedded Infrastructure by Xilinx

Xilinx is arguably the world leader in FPGA and other programmable logic. They provide tools for building embedded systems around many of their FPGAs. Xilinx offers two kinds of fundamental options when it comes to implementing an Embedded System.

1. Processor instantiated on FPGA fabric or so-called Soft Processors (Microblaze, Picoblaze)

1. Hard processor on the die which has dedicated connectivity to FPGA fabric

As you can imagine, soft processors will use part of FPGA fabric to function and you have to be happy with what is left in...