tinyboot - Initial Release for CH32V003
While I was waiting for the CH32V006 dev board to arrive, I ended up going on a side quest: making firmware updates less painful for future OpenServoCore-based servos. Once a servo is built into a system, it is not really practical to pull it back out, open the case, desolder the board, and hook up a debugger just to flash new firmware. Updating over the existing one-wire UART bus is a must-have feature if I wanted to keep both you and me sane. And that is how tinyboot was born.
If you’re new here, OpenServoCore
is my effort to turn cheap MG90S-class servos into networked smart actuators with sensor feedback, cascade control, and a DYNAMIXEL-style TTL bus. tinyboot is the bootloader half of that story: a small Rust bootloader that lives where the WCH factory bootloader normally sits, so a deployed servo can take firmware updates over the same one-wire UART bus it already speaks on.
This first release targets the CH32V003, fits in 1920 bytes of system flash, and ships with CRC validation, a trial-boot counter that rolls back bad updates on its own, a modular transport layer (standard UART plus DXL TTL with TX_EN), and a tinyboot-cli for driving updates from the host. It is, in the author’s words, a little unhinged on the inside. But it works.
Demo
This is a demo of tinyboot running on CH32V003. Here, tinyboot refers to the command-line interface (tinyboot-cli) used to interact with the bootloader.
What is tinyboot
tinyboot is a small but capable bootloader for resource-constrained MCUs. It is written in Rust and currently targets the CH32 family. It should be fairly easy to expand to other microcontroller families in the future, but there are no immediate plans to do that right now.
At the moment, it works on CH32V003, mostly because I happened to have a few lying around. Support for other CH32 families is coming soon, since I have already picked up a few additional boards to cover different chip variants.
One little-known detail about the CH32 family is that these chips include a system flash region containing the WCH factory bootloader. That bootloader is meant to be used with wchisp / WCHISPTool to upload firmware over UART or USB. However, the factory bootloader is not open source, and it is not flexible enough for what I need. In particular, it does not let me customize the UART/USB pin layout or support DXL TTL-style single-wire UART with a TX_EN pin.
So tinyboot is designed primarily as a replacement for the factory bootloader, living directly in the system flash region. That said, flashing it into the user flash region is also supported if you prefer to keep things arranged that way.
Feature Highlights
I wanted something more robust than the factory-provided bootloader so I could minimize the chance of bricking a servo and needing physical access to the board again. Most of the features were designed with that goal in mind, while still keeping resource usage to a minimum.
Trial Boot
The bootloader keeps track of whether the currently flashed application has successfully booted. With help from a small application-side library, the firmware can report back once startup is complete.
After a fresh firmware update, if the application fails to confirm boot after a power reset, the bootloader decrements a trial boot counter. Once that counter is exhausted, it refuses to jump to the application and stays in bootloader mode instead, allowing a new firmware image to be flashed. This gives the bootloader a way to recover from bad updates without needing a debugger.
Configurable and Modular Transport
The bootloader is modular and intended to be extended. Right now it supports UART transport, including both standard UART and DXL TTL / RS485-style UART with a TX_EN pin. There are no current plans for USB support, but it could be added later by implementing the Transport trait.
Tiny Footprint
Right now tinyboot fits in 1920 bytes of system flash on the CH32V003. Getting it this small required a lot of compromises, so this is not exactly a showcase of idiomatic safe Rust. It is a little unhinged, but it works.
Library Over Binary
This is not a single binary crate. Instead, tinyboot is structured as a set of library crates that can be used to build your own bootloader binary. That makes it much easier to adapt the bootloader to different pin layouts and transport configurations just by changing a few settings.
Next Steps
This is still a young project, and right now it only supports CH32V003. The next step is expanding support to other CH32 parts. I plan to maintain this long term, since reliable in-system firmware updates will be a core part of the OpenServoCore project.