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01 · Overview

rbAmp signal chain: sensor → module → I²C → master → cloud / Home Assistant

📦 Library source on GitHub: rb-amp/rbamp-arduino — source code, runnable examples and tagged releases. Latest release v1.3.0 (wire-protocol v1.3). Install via the Arduino Library Manager or PlatformIO.

What rbAmp is

rbAmp is a compact hardware module for precision measurement of AC-mains parameters over the I²C interface. It is built on a Cortex-M0+ microcontroller with an integrated isolated analog front-end and factory calibration stored in flash.

From an integrator's point of view the module behaves as a standard I²C slave: connect power, read the registers, and you get ready-made values in physical units. No signal processing on the master side is required.

The library's main purpose — monitoring multiple loads

The RbAmp Arduino library is built to work with a group of modules simultaneously on a single I²C bus, not to drive a single device. The canonical scenario is one main module on the mains feed plus N modules on individual loads, all on one bus, driven by a single Arduino/ESP32. The library provides the RbAmpFleet class — a single object through which you scan the bus, poll every module with one call, obtain total consumed power and the "feed − Σ(loads)" balance, catch errors on each module, and, when needed, synchronize period snapshots with a broadcast command.

Multi-channel modules are also supported (I2, I3 — two or three CTs on one module, each with its own sensor model). This is typical for a sub-panel, where a single installation point monitors several feeders.

Working with a single module is the minimal case of this same library: the RbAmp class is the building block on top of which RbAmpFleet is layered. If you have one module on the bus, you use that same handle directly — the fleet API is not needed.

The 80% scenario — mains + N sub-loads

text
┌──────────────────────────┐
                            │   ESP32/Arduino (master) │
                            │   RbAmpFleet fleet(Wire) │
                            └──┬─────────┬─────────┬───┘
                               │   I²C (shared bus)
        ┌──────────────────────┘         │         │
        │                                │         │
   ┌────▼─────┐                    ┌─────▼─────┐   │
   │ rbAmp #1 │                    │ rbAmp #2  │   │
   │ MAINS    │                    │ Boiler    │   │
   │ 0x50     │                    │ 0x51      │   │
   └──────────┘                    └───────────┘   │
        │ (on the feed)            (load 1)        │
                                                   │
                                             ┌─────▼─────┐
                                             │ rbAmp #3  │
                                             │ A/C       │
                                             │ 0x52      │
                                             └───────────┘
                                            (load 2)
  • 1 module on the mains feed — overall consumption and grid quality.
  • N modules on individual loads — per-channel metering.
  • A single fleet.pollAll() call polls everything; the mains − Σ(sub_loads) balance reveals "the rest" (background consumption, unmetered loads, leakage).
  • One shared master timestamp → correct Wh metering without inter-module drift (see 02_tiers.md and the section on master wall-clock in 09_api_reference.md).

Important architectural detail. The I-variants (I1/I2/I3) measure current only — they have no voltage, no active power, and no PF. Active energy (Wh) and power are computed only on UI-variants (which have a voltage front-end). In the canonical deployment the "mains-meter" role (billing energy) is played by a single UI1 on the mains feed; the sub-meter role (per-load current detail) is played by I2/I3 modules.

For details, see chapter 06 · Examples, scenario 1 ("Mains + N sub-loads — the 80% canon"). For the API, see chapter 09 · API Reference, the "Fleet manager" section.

What rbAmp measures

Quantity Type Range
RMS voltage U_rms float32, V 0…300 V
Peak voltage U_peak float32, V 0…450 V
RMS current I_rms float32, A depends on the selected sensor — see 03_sensor_selection.md
Peak current I_peak float32, A depends on the selected sensor
Active power P (signed in RT) float32, W depends on the selected sensor
Power factor PF float32 −1…+1
Grid frequency uint8, Hz 45…65 Hz
Period-averaged P float32, W as for P

The RT value of active power P is always signed on any module tier (negative means export to the grid). The behavior of the period-averaged value (PERIOD_AVG_P_W) depends on the module tier — see the "Module tiers" section below and 02_tiers.md.

Connection

The module connects to the master with four wires on the LV side. The mains side is already routed inside the enclosure and is galvanically isolated.

Wire Purpose
VCC +5 V (4.5..5.5 V), current ~15 mA, peak ~25 mA. On-board regulator and filtering
GND common with the master (mandatory)
SDA, SCL I²C, 3.3 V logic, 5 V-tolerant. Built-in 4.7 kΩ pull-ups to 3.3 V
DRDY optional, open-drain LOW for ~10 µs every ~200 ms

The default address is 0x50 (7-bit, change range 0x08..0x77), at 100 kHz (Standard) or 400 kHz (Fast). Cold start until the first valid measurement is ~250 ms.

The full rules (bus length, pull-up recommendations on a multi-module bus, wiring schemes with different hosts) are in chapter 04 · Connection.

Module tiers (quick cheat sheet)

The current rbAmp firmware implements only the BASIC tier — one-way (consumption-only) metering following the logic of a classic mechanical meter:

Tier RT power P Per-period accumulator PERIOD_AVG_P_W Library Wh counter
BASIC signed (negative = export to the grid, visible in real time) every 200 ms window of average P is clamped to max(P, 0) before being added to the period accumulator monotonic — meters consumption only
STANDARD planned planned — separate export accumulator planned — bidirectional metering
PRO planned planned + diagnostics planned + additional counters

Details are in chapter 02 · Module tiers. At the library level the same RbAmp class is used regardless of tier; the differences show up in the values the module returns.

Bidirectional metering on BASIC: the library's dev.energy().wh(ch) on the BASIC tier gives consumption only. If you need to account for export separately, sample the RT power dev.readPower(0) at ~5 Hz and split the positive and negative samples into two accumulator variables yourself. An example is in 06_examples.md (the BidirectionalEnergy scenario).

What this library does

The main job is to hide the protocol exchange and relieve the user sketch of the routine of working with registers, byte order, and the settling times after commands. In addition, the library computes energy (Wh) — the module returns only the period-averaged power, and integration is done by the master against its own clock.

Building for ESPHome? Use the external component instead of this library directly — it integrates the rbAmp protocol natively into the ESPHome YAML pipeline.

Without the library (direct register access)

cpp
// Reading U_RMS — in four transactions.
Wire.beginTransmission(0x50);
Wire.write(0x86);
Wire.endTransmission(false);
Wire.requestFrom((uint8_t)0x50, (uint8_t)1);
uint8_t b0 = Wire.read();
// ...repeat for b1, b2, b3...
float u_rms;
uint8_t buf[4] = {b0, b1, b2, b3};
memcpy(&u_rms, buf, 4);

With the library

cpp
float u_rms = dev.readVoltage();

Reading the period — without the library

cpp
Wire.beginTransmission(0x50);
Wire.write(0x01); Wire.write(0x27);   // CMD_LATCH_PERIOD
Wire.endTransmission();
uint32_t t_latch_ms = millis();
delay(50);                            // settling time after the command
Wire.beginTransmission(0x50);
Wire.write(0x07);
Wire.endTransmission(false);
Wire.requestFrom((uint8_t)0x50, (uint8_t)1);
if ((Wire.read() & 0x01) == 0) {
    // snapshot is stale — t_latch_ms must STILL be recorded,
    // otherwise the next snapshot will count Wh over two periods
    t_prev_latch_ms = t_latch_ms;   // <-- this line is easy to forget
    return;
}
// ...read 4 bytes of avg_p, assemble the float, integrate Wh:
//   float dt_s = (millis() - t_prev_latch_ms) / 1000.0f;
//   E_Wh += avg_p × dt_s / 3600.0f;

Reading the period — with the library

cpp
RbAmpPeriodSnapshot snap;
if (dev.readPeriodSnapshot(snap)) {
    // snap.avg_p[0] is filled,
    // dev.energy().wh(0) was incremented by avg_p × master_dt_s / 3600,
    // master_dt_ms is accounted for and saved for the next cycle.
}

Internally the library guarantees:

  • A 50 ms settling time after CMD_LATCH_PERIOD, and a correct read of the snapshot (atomic latch on the module side).
  • A check of the PERIOD_VALID flag before reading.
  • Recording the time against the master's clock even on a stale snapshot (protects against double-counting Wh on the next period).
  • Per-channel Wh accumulation at double precision.
  • Automatic loading of calibration coefficients when a current sensor is selected via setSensorClass(class) + setCTModel(code) — the user need not know about the NF/GAIN registers.

When to use the library, when to use direct register access

Task Library Direct register access
Reading U / I / P / PF / frequency ✅ only for debugging on a logic analyzer
Per-period energy metering ✅ only if Wh are stored outside the library (e.g., in a database on the master side)
Several modules on one bus ✅ —
Bidirectional metering on BASIC ✅ (via RT sampling — see 06_examples.md) alternative — your own loop of RT reads + manual integration
Porting to a non-Arduino platform n/a ✅ — see 09 · API Reference, the "Wire-protocol details" section
Hard RAM limit (AVR Uno) ~42 B per RbAmp instance + 63 B per snapshot; feasible, but tight on multi-module (3 modules × ~42 B + 3 × 63 B ≈ 315 B already takes a noticeable share of the application's RAM budget) ✅ if the library instance does not fit

For all the typical tasks in the first rows the library is the right fit.

Architecture

text
┌─────────────────────────────────────────────────────┐
│  User sketch (.ino)                                 │
│   RbAmp dev(Wire, 0x50);                            │
│   dev.begin();                                      │
│   dev.setSensorClass(RbAmpSensorClass::Sct013);     │
│   dev.setCTModel(RbAmpCTModel::Sct013_030);         │
│   dev.readPeriodSnapshot(snap);                     │
│   dev.energy().wh(0);                               │
└─────────────────────────────────────────────────────┘
                       │
                       ▼
┌─────────────────────────────────────────────────────┐
│  Public API of the RbAmp class (RbAmp.h)            │
│   ┌─────────────────────────────────────────────┐   │
│   │ Lifecycle:      begin / probe / waitReady   │   │
│   │ RT reads:       readVoltage / readPower / … │   │
│   │ Period:         latchPeriod /               │   │
│   │                 readPeriodSnapshot          │   │
│   │ Configuration:  setSensorClass / setCTModel │   │
│   │ Diagnostics:    lastError / errorString /   │   │
│   │                 retryExhaustionCount        │   │
│   └─────────────────────────────────────────────┘   │
│                       │                             │
│                       ▼                             │
│   ┌─────────────────────────────────────────────┐   │
│   │ Internal helpers (RbAmp.cpp):               │   │
│   │  readU8 / readU16LE / readU32LE / readFloatLE│   │
│   │  writeReg / writeCmd                        │   │
│   └─────────────────────────────────────────────┘   │
│                       │                             │
│                       ▼                             │
│   ┌─────────────────────────────────────────────┐   │
│   │ Instance state:                             │   │
│   │  • RbAmpEnergy — Wh accumulator             │   │
│   │  • RbAmpSnapshot / RbAmpPeriodSnapshot      │   │
│   │  • Time of last latch (master_dt_ms)        │   │
│   └─────────────────────────────────────────────┘   │
└─────────────────────────────────────────────────────┘
                       │
                       ▼
┌─────────────────────────────────────────────────────┐
│  Arduino Wire — native I²C on the host platform     │
└─────────────────────────────────────────────────────┘
                       │
                       ▼
┌─────────────────────────────────────────────────────┐
│  rbAmp module at address 0x50                       │
│   • Measurement pipeline U / I / P / PF (~200 ms)   │
│   • Period accumulator (atomic CMD_LATCH_PERIOD)    │
└─────────────────────────────────────────────────────┘

Interaction diagrams

The begin() flow

text
sketch        RbAmp                Wire                rbAmp module
  │             │                    │                       │
  ├─dev.begin()►                     │                       │
  │             ├─readU8(REG_VERSION)►                       │
  │             │                    ├──[0x50, 0x03]────────►│
  │             │                    │◄────────────────ACK───┤
  │             │                    ├──[read 1 byte]───────►│
  │             │                    │◄──────────────[0x03]──┤
  │             │◄─────── version_byte ─────┤                  │
  │             │                    │                       │
  │             ├─readFloatLE(U_RMS)─►│ (4 single-byte reads) │
  │             │  → 226.3 V         │                       │
  │             │  → has_voltage_hw = true │                  │
  │             │                    │                       │
  │             ├─writeCmd(LATCH)───►│                       │
  │             │                    ├──[0x50, 0x01, 0x27]──►│
  │             │  delay(50)         │                       │
  │             │                    │                       │
  │◄────true────┤                    │                       │

The first latch is a primer (the module returns what it has accumulated since power-on, which is unsuitable for tariff metering). The library records on its own that this snapshot must be discarded — the user code never sees this.

The readPeriodSnapshot(snap) flow

text
sketch        RbAmp                Wire                rbAmp module
  │             │                    │                       │
  ├─readPeriodSnapshot(snap)─►       │                       │
  │             ├─writeCmd(LATCH)───►│  master_t_now = now() │
  │             │                    │                       │
  │             │  delay(50)         │                       │
  │             │                    │                       │
  │             ├─readU8(PERIOD_VALID)► │                     │
  │             │  → bit0 = 1        │                       │
  │             │                    │                       │
  │             ├─readFloatLE(AVG_P0)►(4 reads) │            │
  │             ├─readFloatLE(MAX_P0)►(4 reads) │            │
  │             ├─readU32LE(LATCH_MS)►(4 reads) │            │
  │             │                    │                       │
  │             │  dt_s = (now() − last_latch_ms) / 1000 │    │
  │             │  energy_[ch] += avg_p[ch] × dt_s / 3600│    │
  │             │  last_latch_ms = master_t_now          │    │
  │             │                    │                       │
  │◄────true────┤                    │                       │

The atomic latch on the module side guarantees that every ADC micro-sample within the period lands in exactly one snapshot — no duplication, no losses at the period boundary.

Mapping to the direct register API

A table for those migrating from their own work built on direct access:

Direct API Equivalent via the library
Manual 4-byte read + memcpy dev.readVoltage() / dev.readPower(ch)
Wire.write(0x01); Wire.write(0x27); delay(50) dev.latchPeriod(); delay(50) (or simply dev.readPeriodSnapshot())
Checking Wire.read(0x07) & 1 dev.isPeriodValid()
The manual formula E_Wh += avg_p × dt_s / 3600 dev.energy().wh(0) — updated automatically
Manual current-sensor configuration via the direct register API dev.setSensorClass(class) + dev.setCTModel(code) — factory presets are loaded automatically

What's next

What rbAmp is The library's main purpose monitoring multiple loads The 80% scenario mains + N sub-loads What rbAmp measures Connection Module tiers (quick cheat sheet) What this library does When to use the library, when to use direct register access Architecture Interaction diagrams Mapping to the direct register API What's next