Whenever I talk to someone about FPGAs, the conversation seems to follow a familiar routine. It is almost a catechism to say that ‘FPGAs are very interesting niche products that, sadly, rarely make sense in real-world applications’. I often hear that organizations with Money can afford to develop ASICs, while hobbyists are usually better served by today’s affordable and powerful microcontrollers except in some very specific circumstances like emulating old CPU architectures. I don’t have enough experience to know how accurate this is, but I do have a couple of projects that seem like they could benefit from an FPGA, so I decided to bite the bullet and learn the basics of how to use one.
I chose a popular $25 development board called the ‘Icestick‘ to start with. It uses one of Lattice’s iCE40 chips, which is nice because there is an open-source toolchain called Icestorm available for building Verilog or VHDL code into an iCE40 bitstream. Most FPGA vendors (including Lattice) don’t provide a toolchain that you can build from source, but thanks to the hard work of Clifford Wolf and the other Icestorm contributors, I can’t use “maddeningly proprietary tools” as a reason not to learn about this anymore.
One thing that FPGAs can do much better than microcontrollers is running a lot of similar state machines in parallel. I’d eventually like to make a ‘video wall’ project using individually-addressable LEDs, but the common ‘Neopixel’ variants share a maximum data rate of about 800kbps. That’s probably too slow to send video to a display one pixel at a time, but it might be fast enough to send a few hundred ‘blocks’ of pixel data in parallel. As a small step towards that goal, I decided to try lighting up a single strip of WS2812B or SK6812 LEDs using Verilog. Here, I will try to describe what I learned.
And while this post will walk through a working design, I’m sorry that it will not be a great tutorial on writing Verilog or VHDL; I will try to gloss over what I don’t understand, so I would encourage you to read a more comprehensive tutorial on the subject like Al Williams’ series of Verilog and Icestorm tutorials on Hackaday. Sorry about that, but I’m still learning and I don’t want to present misleading information. This tutorial’s code is available on Github as usual, but caveat emptor.