Rust Oven Temperature Monitor

This project came about when my wife noticed that the oven in our appartment didn’t seem to be at the correct temperature, as everything would bake too quickly. Seeing this as a great excuse to do a project, I set off on making an oven temperature monitor. To give myself some constraints, however, I wanted it to be battery powered and last a long time.

My first attempt was written in C using an STM32. you can read more about that here. This is the new and improved version, written in Rust.

Hardware Summary

This project utilizes a Feather M0 board, a quad alphanumeric display, and a thermocouple amplifier to monitor the temperature of dumb ovens that do not have digital temperature readout. It is battery opperated, and written in rust.

Initial Implementation

My first cut at this was simple: port all of the basic functionality over from the C-based version of the project to rust. This was relatively easy given how far along the embedded rust ecosystem is, especially for these simple projects. Since I’m using a feather M0 board, I used the wonderful atsamd-rs HAL crates. This is a great and welcoming codebase tha also had all of the basic features I needed to get this project functional. I also took the oportunity to write my own driver for my alphanumeric display, which I based off of the Adafruit C++ library, with some tweaks for power I’ll get into later on.

Power Optimizations

I had a functional project up and running quickly, but it definitely wasn’t going to work as the final project. The timer APIs as existed at the time had no capability for interrupt-based operation, so sleeping the core for power savings wasn’t easy. Because of this limitation, I opened a PR for this functionality, saving 6 mA of current consumption, and I didn’t have to worry about interrupt memory saftey since rust didn’t allow me to (easily) do anything unsafe.

This change allowed simple sleeping of the core without powering down much else. A lot more could be gained by configuring the power management peripheral to sleep more of the cores busses. I implemented these changes in this PR, which reduced our current consumption (apart from the display) from ~870 uA to ~250 uA.

This was a large win, however we were still consuming about 5 mA of power in idle. The big delta here came from the display, which it turns out has an idle mode that isn’t exposed in the C++ library I was referencing when making my rust driver. With the addition of this change, the displays idle current consumption drops dramatically, bringing us to what I think is the minimum current consumption without making hardware changes, like a more efficient LDO.