This is a medium-ish refactor to attempt to clean up sensor handling logic both for board stability and future potential growth before the code becomes all spaghetti and meatballs.
I'd be curious to see if anyone running sensors out there that knows how to build and flash MeshCore code could give this a try and see how it behaves. It is working fine on my end on multiple nodes.
PR notes are gigantic because it is a fundamental behavior repair for sensors, so I wanted to over-explain. Also, if it hadn't been mentioned previously, push-back is always welcome. I'm just spending my time trying to clean up / fix / enhance this corner of the firmware, and want to contribute my improvements back to the project.
**Problem:**
Current MeshCore code makes no attempt to see what sensors are actually available on the I2C bus at startup and blindly tries to interact with sensors. This has some very bad side-effects, like if a sensor that is unsupported, or has a weird initialization process, the MeshCore node will just hang at boot and never successfully start up and ostensibly looks bricked, or the INA226 and SHT4X both sharing the same address and the code just silently fighting.
The current implementation also gloms sensor readouts from the MCU and environment sensors onto the same telemetry channel, with some arbitrary exceptions for incrementing channels based on certain behavioral situations. The MCU temperature and external temperature sensors would appear on channel 1, and it wouldn't be possible to tell which sensor the temperature value was coming from.
Per [CayenneLPP](https://github.com/myDevicesIoT/CayenneLPP): *Data Channel: Uniquely identifies each sensor in the device across frames, eg. “indoor sensor”* So this channel division implementation falls inline with what CayenneLPP intended. There are up to 256 channels available. So I tried to model this change in that behavioral style.
**Proposed Improvement:**
This implementation scans the I2C bus for what devices are present, sets each sensor to its own CayenneLPP channel, and keeps MCU telemetry on channel 1 only. So Channel 1 is always "self" and no confusion can result.
Details:
- Channel 1 is always the MCU and things about it, so you always know that telemetry is from the board itself. Exception is GPS, GPS stays on channel 1 as well since it is "about the board" even though it's a bit gray-area as GPS can often be a secondary chip.
- Each sensor board is allocated to a dedicated CayenneLPP channel, so if you are reading from that channel, you know the data is from that sensor only. (Sensors emitting more than one of the same type of measurement are exceptions.)
- `scanI2CBus()` probes addresses 0x08–0x77 with raw `beginTransmission`/`endTransmission`. No sensor library is touched until after this completes. This will prevent sensor-based boot hangs, unknown or unresponsive devices never reach a library init call.
- Created `SENSOR_TABLE` a compile-time array that is gated by the existing `ENV_INCLUDE_*` macros. A sentinel `{ 0, nullptr, nullptr, nullptr }` at the end keeps the array non-empty regardless of which sensors are enabled, avoiding zero-length array warnings.
- When `begin()` is called, scan first, then loop: skip if address not detected, skip if `init()` returns 0, otherwise register one ActiveSensor entry per sub-channel.
- `querySensors()` I replaced the entire #ifdef chain with a 3-line loop.
- T1000-E has its own T1000SensorManager, so it should be completely unaffected by this change.
- SHT4X quirky initialization behavior is retained.
- MLX90614 - git commits around this didn't have any notes as to why it is reporting ambient temperature on a separate channel as well as the object temperature, as the ambient temperature is used internally to compute the object temperature and not really needed for the sensor's purpose - just the same, kept the existing behavior of reporting the ambient temperature one channel above the channel assigned to the sensor
- All `bool *_initialized` fields are gone, replaced with `ActiveSensor _active_sensors[16]` (query function pointer and sub-channel index) and `_active_sensor_count. SensorDef` lives entirely in the `.cpp` so the header has no dependency on it.
- Details on the INA226 and SHT4X: both default to address 0x44, the old code had a bug and would have both begin() calls fire and they would just fight each other silently. In the new code, the respective sensor code is only called if the device is actually present, however, if both were present simultaneously, SHT4X comes first in the table and would win, and INA226 would return false and be skipped. The INA226 has 16 possible addresses that are configurable in the hardware itself, so in a potential scenario where both sensors would be present, the person implementing that design could take that into account.
- BME680 gas resistance will now transmit on the same channel as the rest of BME680 telemetry which is inline with CayenneLPP standards. Coupling this PR with https://github.com/meshcore-dev/MeshCore/pull/2146 streamline the whole sensor telemetry, and with https://github.com/meshcore-dev/MeshCore/pull/2149 will overall improve BME680 handling. The gas resistance sensor actually has a binary library to make it more useful, calibration, accounting for age of sensor, and other improvements, but since that adds more flash consumption, I have omitted that in PRs thus far.
- RAK12035 and other current upstream dev branch changes integrated.
Jared Dohrman