CayenneLPP¶

TELEMETRY · WIRE FORMAT · PINNED EXACTLY · CHANNELS

CayenneLPP determines how MeshCore puts sensor values on the wire. It is the only library in [arduino_base] pinned to an exact version, and that is understandable: the library does not encode data for internal use but the format in which two nodes exchange telemetry.

[!NOTE] Source. This page was verified against the firmware itself: MeshCore v1.16.0, commit 03b6ef4, 28 July 2026 — files platformio.ini, src/helpers/SensorManager.h, src/helpers/sensors/EnvironmentSensorManager.cpp and examples/kiss_modem/KissModem.cpp.

What it does¶

Cayenne Low Power Payload is a compact binary format for sensor data, originally devised for LoRaWAN. Every measurement consists of a channel number, a type code and the value on a fixed scale — temperature in steps of 0.1 °C, air pressure in steps of 0.1 hPa, and so on. That way a complete sensor reading fits in a few dozen bytes, without field names and without separators. The implementation MeshCore uses is the one by Electronic Cats, github.com/ElectronicCats/CayenneLPP.

How MeshCore pulls it in¶

platformio.ini r.26

  electroniccats/CayenneLPP @ 1.6.1

No ^, no ~: exactly 1.6.1. The line sits in [arduino_base], so all 507 build targets get it. Through its library.json CayenneLPP brings in bblanchon/ArduinoJson — see ../dependencies.md.

The exact pin stands out because the other six lines in the same section do have a range. No explanation for it appears anywhere in the repo. The suspicion that it has to do with the wire format is an obvious one: change the encoding and two nodes on different versions no longer understand each other. That is a suspicion, not an established reason.

How MeshCore uses it¶

CayenneLPP sits at the heart of the sensor abstraction. SensorManager passes a CayenneLPP object to whoever reads sensors:

src/helpers/SensorManager.h r.3

#include <CayenneLPP.h>

src/helpers/SensorManager.h r.19

  virtual bool querySensors(uint8_t requester_permissions, CayenneLPP& telemetry) { return false; }

Every sensor driver writes straight into that object. The query_* functions in EnvironmentSensorManager.cpp do nothing else:

src/helpers/sensors/EnvironmentSensorManager.cpp r.246-251

static void query_ahtx0(uint8_t ch, uint8_t, CayenneLPP& lpp) {
  sensors_event_t humidity, temp;
  AHTX0.getEvent(&humidity, &temp);
  lpp.addTemperature(ch, temp.temperature);
  lpp.addRelativeHumidity(ch, humidity.relative_humidity);
}

At the other end of the chain the buffer is read out and transmitted:

examples/kiss_modem/KissModem.cpp r.557-560

  uint8_t permissions = data[0];
  CayenneLPP telemetry(255);
  if (_sensors.querySensors(permissions, telemetry)) {
    writeHardwareFrame(HW_RESP(HW_CMD_GET_SENSORS), telemetry.getBuffer(), telemetry.getSize());

The text CayenneLPP occurs in 24 of the 590 source files.

What it means for a node¶

Everything a node sends as sensor data passes through this format. The channel number a measurement appears under is assigned at start-up in the order sensors are detected — channel 1 is reserved for the node itself (TELEM_CHANNEL_SELF), the first sensor found gets channel 2. Two nodes with different sensors therefore do not share a channel layout.

Because the library is pinned exactly, a build made today behaves here the same as a build made last year. That does not hold for the libraries with a ^ range.

Sources¶

Translated from Dutch by Anthropic Claude