How Does an FPGA Work?
When to Use an FPGA
Given all the pros of using FPGAs, you may be asking yourself “Why wouldn’t I use one for everything!” Great question!
In the description of how FPGAs work, you may have noticed that there's a lot of extra "stuff" that has to go on to implement even the simplest circuit dynamically. This isn’t without its cost, and I mean that both in $$$ and in design resources.
FPGAs tend to be expensive. The larger ones easily go for tens of thousands of dollars PER CHIP. This is due to the amount of silicon required to make them, the huge amount of RnD to design the chips and the tools, and the relatively low volume compared to things like tiny processors used in phones.
Another cost is power. There are a lot of transistors used in the LUTs compared to the number needed to implement the circuit directly. All these transistors require power to operate. Because of this, FPGAs tend to be poor candidates for battery operated devices. You can of course design your circuits to be power efficient but even doing absolutely nothing, the FPGA on the Alchitry Au consumes a little over 100mA. You can easily get over 1000mA if you start pushing the chip. As a comparison the ATmega32U4, the chip used on the Arduino Leonardo, uses 27mA when running with 5V at full speed. Granted the Alchitry Au is substantially more capable.
So why use FPGAs at all? Well, you have two major alternatives to creating a custom digital circuit. First, you could build it yourself out of discrete logic. This would take a significant amount of time, likely cost significantly more, and have little flexibility if you needed to change something.
The second, and more realistic, alternative is to create the circuit directly in silicon. This creates a very fast, very efficient circuit, but at the cost of zero flexibility and truck loads of $$$. Custom silicon has huge upfront costs associated with the tooling and setup. The incremental cost per chip would be lower than individual FPGAs though. However, unless you’re making tens of thousands of chips, this will be more expensive overall. Even then, it sometimes doesn’t make sense to lock your design into silicon. With an FPGA you can change it whenever you need to without penalty.
Because of their flexibility and low-cost compared to the alternatives, FPGAs open the doors to adding custom digital circuits to just about any design. But, do you really need a custom circuit?
It is important to keep in mind that FPGAs are just like any other tool. A hammer is great for nailing in nails, but it is terrible for screwing in screws. Just as trying to nail a nail with a screwdriver would be quite futile.
Creating custom circuits can be difficult and you often need to ask yourself if there is a better solution. There are many very capable processors with tons of peripherals that can handle most of the problems you need to solve. Trying to do something like sending and receiving data over WiFi would be a daunting task with an FPGA but is easily accomplished with a few dollar microcontroller like the ESP8266.
I often describe FPGAs as an assembly line. Each station on an assembly line works independent from each other and they are incredibly efficient at what they are designed for. However, it can be difficult to initially set up the line and if you want to make major changes it is often easier to start from scratch.
On the other hand, processors are like people. A single person could do just about any task given enough time and the training. Complex sequential tasks are easy for a person to accomplish.
Do you really want to set up an entire sandwich manufacturing plant just to make you a sandwich for lunch?
FPGAs are amazing and often indispensable for the tasks they excel at but they are just another tool to add to your toolbox. A very powerful and worthwhile tool to invest in, but still just another tool.