TOF Sensor

The standard TOF sensor provided for the project was the ST VL53L3CX (specifically it was provided in the format of the ST VL53L3CX-SATEL breakout board). There was no existing Rust driver available for this when starting the project. Rewriting the driver in Rust was not feasible in the timeframe as the original C driver from ST contains thousands and thousands of lines of code with a lot of logic (most of the logic for this sensor is not running on the sensor chip but is instead in the driver).

Design Of C-based ST TOF Drivers

The C implementation of drivers for the TOF sensors provided by ST follow a common pattern:

  • Platform-independent logic is implemented by them
  • You have a platform-specific implementation which instantiates the driver and has a struct (which is then passed to the driver) which can afterwards be used to handle communication
  • They provide a platform-specific header file with APIs which they call, and you have to implement it (e.g. I2C communication; the aforementioned custom, platform-specific struct is used for this)

Using FFI (Foreign Function Include) To Use The C Driver

My first approach was to use FFI to be able to use the ST-provided driver and call the C code from Rust (to interact with it) and call Rust from C (to handle the I2C communication).

While I was able to get the Rust to C and C to Rust calls working, the driver wouldn't start up completely due to wrong data being read from the sensor. Debugging the I2C calls showed that most of them returned the exact same values as when using the Arduino C++ code (it was faster to get that one running & debugged than the C-based driver, but their logic is the same, and they're both from ST themselves). However, certain reads (always the same ones in each run) would just return 0x00 instead of the expected value.

Using An Alternative TOF Sensor

ST manufactures multiple different TOF sensors and offers them on the same breakout board format (i.e. they are pin compatible). Namely, the ST VL53L1X (with the ST VL53L1X-SATEL breakout board) offers a similar ranging distance (most others were too short for our use-case) and already has a Rust driver (which is even fully implemented in Rust - this sensor runs more logic on-chip and thus has a smaller driver).

Decision

Due to the amount of time already invested in trying to get the VL53L3CX driver running and the cheap cost of just getting another TOF I went for the VL53L1X and used the existing driver, which worked well.