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.
It’s good to know what time it is, and once you work out how to use a RealTime Clock module, making a clock display seems like a natural next step. 7-segment LED displays used to be the go-to way to display time digitally before LCD and OLED screens got so cheap, and I thought it would be fun to make a more modern take on them with diffused multicolor LEDs for the actual lighting. I would like to write a post about drawing to traditional 7-segment digits using shift registers, but this project uses the easy-to-wire ‘Neopixels’ that we all know and love.
You’ll need a few materials for this project if you want to follow along:
- Access to a laser cutter and a 3D printer.
- About 6 x 12″ of 1/8″-thick ‘frosted’ acrylic, to diffuse the LEDs. (About 150 x 300mm, 3mm thick)
- Enough plywood to make a clock face for the front and back; I wound up using about 500 x 200mm each.
- 58 individually-addressable RGB LEDs. (28 sets of 2 for each segment of the 4 digits, and 2 more for the colon dots.)
- A DS3231 or similar realtime clock module.
- A microcontroller to run everything; I used an ATMega328 on an “Arduino” nano board to keep the code simple.
- Craft supplies: solder, soldering iron, glue, hot glue gun, wire, etc.
For the LEDs, I cut lengths off of a reel of LED strip that I ordered off Amazon; I think it used SK6812 LEDs and had 30 LEDs / meter. For the two dots of the colon, you can also buy small single-LED boards that are about 10mm in diameter and usually have WS2812B LEDs. I didn’t run into any problems stringing SK6812 and WS2812B LEDs together with Adafruit’s “Neopixel” library, but caveat emptor.
Some Cars are Old
Recently, I bought a very used car in the hopes of modifying it to use a parallel-hybrid drivetrain. I’d like to cut the driveshaft and slot an electric motor inline with it, and that will probably be the subject of future posts if I can find a garage willing to do most of the heavy lifting and cutting.
But in the meantime, what sort of car do you get if you’re thinking of cutting its driveshaft in half? A cheap one, of course – and my choices were further limited by a few basic requirements:
- The car should actually be a small pickup truck; the bed is an ideal place to put the heavy batteries, they tend to have a lot of ground clearance for fitting a bulky motor underneath the chassis which means little or no changes to the bodywork, and passenger space is not a consideration in this build.
- It should be rear-wheel drive. Since I am planning to add a motor between the transmission and the differential, there needs to be enough space along the driveshaft to fit the motor. That means either rear-wheel drive with the engine in the front, or front-wheel drive with the engine in the back. And I don’t know of any rear-engine pickup trucks.
- It should have an automatic transmission. This kind of simple conversion leaves you with two acceleration pedals, and I don’t have enough feet to use four pedals at once.
So I wound up with a cheap-and-cheerful 1994 Toyota Pickup Truck. It doesn’t actually have a model name; in North America, the Hilux brand was renamed to “Truck” in the 1970s before eventually being replaced with the Tacoma. So this is not a luxury vehicle. It has roll-up windows, it does not have ABS, it came with a broken speedometer, and more importantly its speakers had disintegrated in the decades since 1994.
That’s no good, and it sounded like the buzzing of a thousand wasps. Plus, replacing your speakers seems like a pretty easy first project if you want to learn about a car. You only have to remove a few interior panels, and your sound system is a safe thing to break compared to say, your brakes. So let’s get started! These specific instructions may be for a 1994 Toyota pickup, but I’ll bet the process looks similar for the front speakers of other cars.
Over the past month or two, I’ve been learning about plastic recycling with a friend at a local makerspace. We decided to start out with HDPE as a nice beginner’s material, because it melts at less than 200C and tends to be fairly nontoxic. We’ve tried a few different techniques now, and we’ve learned a lot about what doesn’t work.
This is an exciting field and we hope to make a decent press for forming 3mm and 6mm-thick HDPE sheets soon, but our early designs have shown us some key difficulties to be aware of if you are planning to start recycling plastics on your own. And I’ll mention this later, but please research what you plan to recycle ahead of time and avoid plastics which might release hazardous fumes. As you will see, I am not an expert.
Our first instinct was to imitate an injection-molding press, and we started by scavenging some large heating elements from a Goodwill-sourced griddle. It consisted of four ~13-Ohm coils arranged as two parallel banks, each consisting of two elements in series. That meant an effective resistance of 13 Ohms, or a bit over 9 amps at the mains 120V which it was more or less directly connected to.
We wound up clipping the heating coils out and scavenging some of the crimped and heat-insulated wiring, strapping them to a smooth-bore metal pipe, and then soldering or spot-welding the four coils back together in their original 2×2 arrangement. Temperature monitoring is all well and good, but this was a first attempt; we just wired the coils across a 2-prong plug, and flipped a surge suppressor on and off to control the heat. A handheld infrared thermometer gave us a rough indication of the tube’s temperature, and we aimed for 350-400F to melt the HDPE.
Among the cheap gadgetry that constantly spews forth from the spawning pits of consumer electronics, sometimes you can find a gem. The W1209 is an interesting board which is designed to act as a thermostat which can also switch a relay when certain temperatures are reached. It is used in everything from rice cookers to yogurt machines to people who want their A/C to only turn on when it’s hot out. Originally they shipped with an aging but fairly capable STM8S003F3 microcontroller, and they cost less than $1.50 each.
Even so, their popularity has inspired the usual imperfect cloning process, so chances are that a board you buy from ebay, Aliexpress, Taobao, or Amazon today will need some touching up before you will be able to reprogram it. In this tutorial, we will replace the microcontroller and add a few missing capacitor/resistors. Then we’ll upload a simple test program to test that the microcontroller and board both work. So here is an example of what is probably the worst possible knock-off that you could fear to get, with missing or faulty parts highlighted:
Well, that’s life. You try your best, and then someone comes along and kicks you in the teeth. But let’s make some lemonade and take the opportunity to learn about fixing cheap-o knock-off circuit boards!
In previous tutorials, I covered how to use the STM32 line of microcontrollers to draw to small displays using the SPI communication standard. First with software functions and small ‘SSD1331’ OLED displays, and then with the faster SPI hardware peripheral and slightly larger ‘ILI9341’ TFT LCD displays. Both of those displays are great for cheaply displaying data or multimedia content, because they can show 16 bits of color per pixel and have enough space to present a moderate amount of information. But if you want to design a very low-power application, you might want a display which does not need to constantly drain energy to maintain an image.
Enter ‘E-Ink’ displays, sometimes called “Electrophoretic Displays“. As the name implies, they use the same basic operating principle as techniques like Gel Electrophoresis, which separates polarized molecules such as DNA based on their electric charge. Each pixel in one of these displays is a tiny hollow sphere filled with oppositely-charged ink molecules, and they are separated between the top and bottom of their capsules to make the pixel light or dark. The ink remains in place even after power is removed; I think that they are suspended in a solid gel or something. Modern E-Ink modules sometimes have a third color such as red or yellow, but this post will only cover a humble monochrome display.