Bare-Metal STM32: Please Mind The Interrupt Event

March 26, 2021 by Maya Posch 27 Comments

Interruptions aren’t just a staple of our daily lives. They’re also crucial for making computer systems work as well as they do, as they allow for a system to immediately respond to an event. While on desktop computers these interrupts are less prominent than back when we still had to manually set the IRQ for a new piece of hardware using toggle switches on an ISA card, IRQs along with DMA (direct memory access) transfers are still what makes a system appear zippy to a user if used properly.

On microcontroller systems like the STM32, interrupts are even more important, as this is what allows an MCU to respond in hard real-time to an (external) event. Especially in something like an industrial process or in a modern car, there are many events that simply cannot be processed whenever the processor gets around to polling a register. Beyond this, interrupts along with interrupt handlers provide for a convenient way to respond to both external and internal events.

In this article we will take a look at what it takes to set up interrupt handlers on GPIO inputs, using a practical example involving a rotary incremental encoder.

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USB Comes To The ESP32

March 26, 2021 by Bryan Cockfield 20 Comments

Since the ESP8266 came on the scene a few years ago and revolutionized the way microcontrollers communicate with other devices, incremental progress on this chip has occurred at a relatively even pace. First there was the realization that code could be run on the chip itself. Next the ESP32 was released which built more on that foundation. The next step in that process of improvement may be here now as well, with this project which turns the ESP32 into a USB host.

USB is not a native feature on all microcontrollers or even Arduino-compatible boards. While some do have it built in like those based on the 32u4 for example, most either don’t have it at all or rely on a separate on-board chip to do some form of translating. The ESP32 is lacking this advanced feature so the USB needs to be cobbled together from scratch if you want this specific board to be able to interface directly with peripherals. This project does just that, allowing for four USB 1.1 devices to be connected directly to the ESP32 without a separate dedicated chip.

If you’ve been waiting for USB on this tiny, capable microcontroller this might be your chance to try it out. All of the project’s code is available on the project page. And, while it is limited in scope, it’s easily able to handle a keyboard or mouse. This might be a more cost-effective way of doing something like a KVM switch rather than doing it with three Arduinos.

DIY I2C Tester

March 23, 2021 by Chris Lott 26 Comments

[Dilshan] built a dedicated I2C tester which allows for I2C bus control over USB using simple commands such as init, read, write, etc. The Linux kernel has had I2C driver support for a couple of decades, but you’ll be hard pressed to find a computer or laptop with a I2C connector (excluding Bunnie Huang’s Novena hacker’s laptop, of course). This tester does require a Linux host, and his programs use libusb on the computer side and V-USB on the embedded side.

[Dilshan] put a lot of time into building this project, and it shows in the build quality and thorough documentation. With its single-sided PCB and all thru-hole construction, it makes a great beginner project for someone just getting into the hobby. At the heart of the tester is an ATmega16A in a 40-pin PDIP package (despite the Microchip overview page calling it a 44-pin chip), supported by a handful of resistors and transistors. Schematics are prepared in KiCad, code is compiled using gcc and avr-gcc, and he provides a label for the enclosure top. The only thing missing is information on the enclosure itself, but we suspect you can track that down with a little sleuthing (or asking [Dilshan] himself).

If you use I2C quite a lot, give this project a look. Easy to build, useful in the lab, and it looks nice as well. We have featured [Dilshan]’s work over the years, including this logic pattern generator and his two-transistor-on-a-breadboard superheterodyne receiver.

Cursed USB-C: When Plug Orientation Matters

March 22, 2021 by Maya Posch 71 Comments

One of the selling points of the USB-C plug is that supposedly there is no way to incorrectly insert it. As [Pim de Groot] shows with a ‘Cursed USB-C 2.0 Device‘, reality is a bit more complicated when it comes to USB 2.0 compatibility in USB-C. He made a PCB that elegantly demonstrates the simplicity of the problem, featuring two LEDs. Only one orientation of the USB-C plug will cause one of the LEDs to light up green, with the other orientation leaving both LEDs blinking red.

Sigil on the back of the cursed USB-C 2.0 device, by Pim de Groot.

The reason for this behavior is simple: as [Pim] explains, although the USB-C plug has only a single pair of data lines (D+/-) for USB 2.0 connectivity, the receptor duplicates these on either side of its pins, leading out two pairs of D+/- lines. Normally you would connect the matching lines in these pairs together to ensure consistent behavior no matter the plug orientation, but you don’t have to.

By leading each USB 2.0 data pair to its own SAMD11C MCU, only one of the MCUs would be connected to USB, resulting in the connected MCU blinking the LEDs. With a bit more circuitry it’s possible to detect which way around the plug is inserted and use this information in a single MCU system, altering its behavior. While at first glance this seems little more than a fun party trick, but it also offers insight in a possible failure mode of USB-C 2.0 devices where only one plug orientation works, due to broken traces or pads.

Board view of [Pim]’s Cursed USB-C 2.0 Device.

(Heading image: Cursed USB-C 2.0 Device, by Pim de Groot)

A Handy Reference For Display Drivers And LCD Controllers

March 20, 2021 by Chris Lott 3 Comments

Ever tried to find the data on a mysterious LCD controller that’s kicking around in your parts bin? Well check out this list of various LCD controllers that [Achim] has put together. He summarizes the basic specifications for each controller and includes data sheet links if available (note — the website is in German, although most of the data itself is in English). All in all, he has collected 72 controllers from five different manufacturers, and 46 of them have data sheets. For each controller, he tabulates maximum resolution, color depth, type of interface, and the targeted display technology. For example, here is the entry for the Ilitech ILI9341 TFT controller commonly found in embedded projects:

Furthermore, many of the controllers also have a short video clip showing them in operation posted over on [Achim]’s YouTube channel, where he also has a bunch of quick (less than one minute) videos of all sorts of embedded goodies. We do find this table of controllers to be a little dated — for example, another popular controller used on small color OLED displays, the Solomon Systech SDS1351, is not included. But it is certainly a good resource to bookmark.

We suspect that [Achim] made this table as a result of developing µGUI, a small (only three files) C-language graphics library (see the GitHub repository) he released back in 2015. Do you have any good resources for tracking down unknown LCD controllers? If so, share in the comments below. And thanks to [Dmitry] for sending in this tip.

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Getting Started With FreeRTOS And ChibiOS

March 18, 2021 by Maya Posch 15 Comments

If operating systems weren’t so useful, we would not be running them on every single of our desktop systems. In the same vein, embedded operating systems provide similar functionality as these desktop OSes, while targeting a more specialized market. Some of these are adapted versions of desktop OSes (e.g. Yocto Linux), whereas others are built up from the ground up for embedded applications, like VxWorks and QNX. Few of those OSes can run on a microcontroller (MCU), however. When you need to run an OS on something like an 8-bit AVR or 32-bit Cortex-M MCU, you need something smaller.

Something like ChibiOS (‘Chibi’ meaning ‘small’ in Japanese), or FreeRTOS (here no points for originality). Perhaps more accurately, FreeRTOS could be summarized as a multi-threading framework targeting low-powered systems, whereas ChibiOS is more of a full-featured OS, including a hardware abstraction layer (HAL) and other niceties.

In this article we’ll take a more in-depth look at these two OSes, to see what benefits they bring. Continue reading “Getting Started With FreeRTOS And ChibiOS” →

Your Plants Can Take Care Of Themselves Now

March 14, 2021 by Bryan Cockfield 41 Comments

One of [Sasa]’s life goals is to be able to sit back in his home and watch as robots perform all of his work for him. In order to work towards this goal, he has decided to start with some home automation which will take care of all of his house plants for him. This project is built from the ground up, too, and is the first part of a series of videos which will outline the construction of a complete, open-source plant care machine.

The first video starts with the sensors for the plants. [Sasa] decided to go with a completely custom module based on the STM32 microcontroller since commercial offerings had poor communications designs and other flaws. The small board is designed to be placed in the soil, and has sensors for soil moisture as well as other sensors for amount of light available and the ambient temperature. The improvements over the commercial modules include communication over I2C, allowing a large number of modules to communicate over a minimum of wires and be arranged in any way needed.

For this build everything is open-source and available on [Sasa]’s GitHub page, including PCB layouts and code for the microcontrollers. We’re looking forward to the rest of the videos where he plans to lay out the central unit for handling all of these sensors, and a custom dashboard for controlling them as well. Perhaps there will also be an option for adding a way to physically listen to the plants communicate their needs as well.

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