I recently started learning Embedded Rust. As I mentioned at the top of the last post, there are a couple of toolchain options:
OpenOCD + GDB
probe-rs + cargo-embed
Turns out there is also a third one that I just discovered:
probe-rs + probe-run
This was a little confusing at first, I was unsure what was the better option and how these projects all connect to each other. Embedded Rust seems to be moving fast, so this might get outdated but here is a basic summary if you are just getting into Embedded Rust as well.
The Embedded Rust way
Rust-embedded is a working group in the official Rust organization. Among other things, they maintain The Embedded Rust Book, which you may have come across. In the book, they describe what I would call the “official” toolchain, using OpenOCD and ARM-compatible GDB (gdb-multiarch). This is probably the way to go if you need to do serious development today. OpenOCD and GDB are stable and mature projects and for a Rust programmer, it is the most fully-featured and reliable option right now.
As an alternative to those external (non-Rust) dependencies, a team has formed an ambitious project around replacing it all with software written in Rust – probe-rs. Here is an illustration (borrowed from the video below):
Here is a very informative talk by one of the people behind probe-rs:
My major learning from this talk was that probe-rs is really a library. Other projects, like cargo-embed and probe-run are built on top.
The probe-rs team built cargo-embed to show off the capabilities of probe-rs. As such, it was the first tool I came across when I found the official probe-rs website. One might imagine that being built by the same team, cargo-embed will stay closer to the latest features of probe-rs and have a shorter path to get new features in. But this is just speculation.
To build and upload programs, you simply run cargo embed --release (see my last post about why --release is important for timing). It possible to do logging with rtt, a debugger-based thing that uses an internal buffer that gets read out by the debugger instead of, for example, printing over UART. Debugging is also supported (but not at the same time as rtt currently) from the command line, or visually in something like Visual Studio Code, by hooking into the GDB stubs that probe-rs provides. This is an interface that (to the best of my understanding) mimics GDB’s, but actually goes directly to probe-rs.
Configuration is done in a new file called Embed.toml. There you configure what chip you are using and whether to use rtt or GDB debugging, or set up separate profiles for each.
The vision of probe-rs is to offer a full development environment for embedded Rust, so they are also working on a VSCode plugin. It is still in alpha, and I have not tried it yet.
Ferrous Systems is a company that pops up everywhere in embedded Rust. They are a consultancy specializing in Rust for embedded applications and are also very active in open source development for embedded Rust. They started a project called Knurling dedicated to improving the experience working with embedded Rust.
Knurling has many sub-projects and probe-run is one of them. Built on top of probe-rs, it gives you the same features as cargo-embed, but in a slightly different packaging. The philosophy is that embedded development should work the same way as native development, so instead of introducing a new cargo command, probe-run is a so called Cargo runner. This means you configure the “usual” cargo run command to use probe-run under the hood. And there is no new configuration file to keep track of, just the regular Cargo.toml and .cargo/config.toml. Does it matter? Up to you.
Knurling also has an interesting logging framework; defmt. Instead of doing string formatting on the embedded device, it relies on a tool on the host side and simply generates a list of strings at compile time that is kept on the host. The embedded device then simply sends the index into that list (using rtt), causing much less overhead.
I do like the idea of keeping the main Rust interface unchanged, which speaks in favor of probe-run, but I’m not sure about plans for integrating probe-run with VSCode, or debugging with breakpoints. As I learn more, I hope to find a favorite and maybe also start contributing myself.
TL;DR: When doing timing critical stuff, use the --release flag to get a faster binary! For example: cargo embed --release.
I’m learning Embedded Rust on a STM32 Bluepill board (with a STM32F103 microcontroller). At the time of writing there seems to be two toolchain options:
The “official” Embedded Rust way, using OpenOCD and ARM-compatible GDB.
Up-and-coming probe-rs that is working on having everything in Rust and installable via cargo. Their tool cargo-embed basically replaces OpenOCD and GDB.
OpenOCD + GDB is true and tested, but a lot more work to set up. Probe-rs is litteraly just cargo install cargo-embed, but it is a work in progress and far from feature-complete. I tried both, but this particular thing caught me while using cargo-embed, so that’s the command I will be showing.
The Timer Problem
I wanted to talk to a ws2812 adressable RGB LED (also known as NeoPixel). I found the crate smart-leds that seemed perfect. It comes with several “companion crates” with device drivers that support different LEDs and several options for different ways of driving the ws2812, like the ws2812-spi and ws2812-timer-delay.
The SPI crate unfortunately did not work in my attempts so far. It manages to write to my LED once, then panics with the error “Overrun”. Probably I’m using a newer version of the embedded-hal and/or stm32f1xx-hal than it was written for. Maybe a topic for another day.
The Timer Delay crate also did not work at first. I broke out my Analog Discovery 2 to look at the data signal:
The time between bits was around 200 us. To get a comparison, I fired up a Platformio project for the same STM32 Bluepill board and imported Adafruit’s Neopixel library. Now, the LED of course worked perfectly and the problem was obvious:
The time between the bits was now only around 1,4 us. I will spare you the details of all the things I tried while wrongly thinking either the entire MCU or the timer was running at the wrong frequency.
The solution turns out to be almost silly: Rust binaries can be really slow if you do not compile them in release mode. Just add the --release flag and all is well! 💩
cargo embed --release
There is apparently a way to override this per-dependency in Cargo.toml, that might be worth a try if you need it.
I tried adding the following to Cargo.toml to make all dependencies build with the highest optimization level, but this still was not enough to make the LED work in my case.