I2C Functions Library

Version 03.00.01 — Lightweight, zero-allocation Arduino helper library for register-based I2C devices.

I2C_Functions wraps every common I2C transaction — byte, word, buffer, bit, and command — behind a clean single-header API. It works with the standard Wire bus or any custom TwoWire instance and optionally manages a TCA9548-style I2C multiplexer channel automatically before each transaction.

Features

Feature	Details
Zero heap allocation	No `new`, `malloc`, or `String` — safe for constrained MCUs
Single-header	Drop in `src/I2C_Functions.h`, done
Dual API	Status-returning `Try_*` variants for production + simple wrappers for prototyping
Byte operations	Read / write single byte registers
Word operations	Read / write 16-bit registers with configurable endianness (LSB-first or MSB-first)
Buffer operations	Read / write multi-byte payloads with 8-bit or 16-bit register addressing
Command operations	Send raw command bytes without a register address
Bit operations	Set, clear, toggle, read, and write individual register bits (read-modify-write)
Mux support	Automatic TCA9548 channel switching before each transaction
Bus flexibility	Accepts any `TwoWire*` pointer — Wire, Wire1, or a software I2C instance
Platform agnostic	Arduino AVR, megaAVR, ESP32, and any Arduino-compatible target

Installation

Arduino Library Manager

Open Sketch → Include Library → Manage Libraries
Search for I2C_Functions
Click Install

PlatformIO

Add to your platformio.ini:

lib_deps =
    akkoyun/I2C_Functions

Manual

Download or clone this repository
Copy the folder into your Arduino libraries/ directory
Restart the IDE

Quick Start

Basic sensor read

#include <I2C_Functions.h>

// Create an object for a device at 0x40 (e.g. HDC2010)
I2C_Functions sensor(__I2C_Addr_HDC2010__);

void setup() {
    Serial.begin(115200);

    // Start Wire and probe the device
    sensor.Begin();

    if (!sensor.Detect()) {
        Serial.println(F("Device not found"));
        return;
    }

    // Read register 0x00 — explicit status (recommended)
    uint8_t value = 0x00;
    if (sensor.Try_Read_Register(0x00, &value)) {
        Serial.print(F("Register 0x00 = 0x"));
        Serial.println(value, HEX);
    }
}

void loop() {}

Read a 16-bit register (big-endian sensor)

uint16_t raw = 0x0000;

// Pass true for MSB-first (big-endian) — common for most sensors
if (sensor.Try_Read_Register_Word(0x02, &raw, true)) {
    Serial.println(raw);
}

Modify a single bit

// Set bit 3 of register 0x0F
sensor.Set_Register_Bit(0x0F, 3);

// Clear bit 3 of register 0x0F
sensor.Clear_Register_Bit(0x0F, 3);

// Toggle bit 3 of register 0x0F
sensor.Toggle_Register_Bit(0x0F, 3);

// Write a specific value to bit 3 (set or clear depending on bool)
sensor.Write_Register_Bit(0x0F, 3, true);

Device behind a multiplexer

// Channel 3 on a TCA9548 at the default address (0x70)
I2C_Functions mux_sensor(__I2C_Addr_HDC2010__, true, 3);

void setup() {
    // The library switches the mux to channel 3 automatically
    // before every read or write transaction
    mux_sensor.Begin();
}

API Families

The library exposes two parallel families for every operation.

`Try_*` — Status-returning (recommended)

These functions return bool: true on success, false on any failure (device absent, NACK, buffer overflow, bad pointer). Use these in production code.

uint8_t val = 0x00;
if (sensor.Try_Read_Register(0x00, &val)) {
    // val is valid
}

Simple wrappers — Quick prototyping

These functions return the value directly and silently fall back to 0 / false on failure. Useful when you don't need error handling, but hiding failures can mask bus issues.

uint8_t val = sensor.Read_Register(0x00);
// val is 0x00 on failure — indistinguishable from a real 0x00 read
// Use Try_Read_Register for error-aware code

The `_Stop` parameter

Several write and multi-byte read functions expose a stop parameter:

Value	Effect
`true` (default)	Sends a STOP condition — ends the transaction normally
`false`	Sends a repeated START — keeps the bus active for devices that require it

Consult your device's datasheet to know which is needed.

The `_MSB_First` parameter

Word (16-bit) read and write functions support configurable byte order:

Value	Byte order	Use when
`false` (default)	Little-endian — LSB first	Device sends/expects LSB at lower address
`true`	Big-endian — MSB first	Most I2C sensors (MPU, BMP, HDC, etc.)

Constructor and Lifecycle

`I2C_Functions` — Constructor

explicit I2C_Functions(uint8_t address, bool mux_enable = false, uint8_t mux_channel = 0)

Parameter	Type	Description
`address`	`uint8_t`	I2C address of the target device
`mux_enable`	`bool`	`true` to enable automatic mux channel switching
`mux_channel`	`uint8_t`	Channel number 1–8; values outside that range disable the mux

I2C_Functions sensor(0x40);                   // Direct device
I2C_Functions muxed(0x40, true, 3);           // Behind mux channel 3
I2C_Functions wire1_dev(0x52, false, 0);      // Will use Wire1 via Begin(&Wire1)

`Begin`

Starts the TwoWire bus and runs an initial device detection.

void Begin(TwoWire *twi = &Wire)

sensor.Begin();          // Uses Wire (default)
sensor.Begin(&Wire1);    // Uses Wire1

`End`

Stops the active TwoWire bus.

void End(void)

`Detect_Device`

Re-runs device detection and updates the internal state. Call this if the device may appear after startup or following a bus reset.

void Detect_Device(void)

`Detect`

Returns the cached detection result (updated by Begin and Detect_Device).

bool Detect(void)
// Returns: true if the device acknowledged its address, false otherwise

`Address`

Returns the configured device address.

uint8_t Address(void)

`Mux_Enable` / `Mux_Channel`

Query multiplexer configuration.

bool    Mux_Enable(void)    // true if mux is active for this object
uint8_t Mux_Channel(void)  // resolved bitmask (0x01–0x80), or 0x00 if disabled

Read Operations

Read Single Byte

// Status-returning — recommended
bool Try_Read_Register(uint8_t reg, uint8_t *data)

// Simple wrapper — returns 0x00 on failure
uint8_t Read_Register(uint8_t reg)

uint8_t val = 0x00;
if (sensor.Try_Read_Register(0x00, &val)) {
    // val holds the register content
}

Read Single Word (16-bit)

// Status-returning
bool Try_Read_Register_Word(uint8_t reg, uint16_t *data, bool msb_first = false)

// Simple wrapper — returns 0x0000 on failure
uint16_t Read_Register_Word(uint8_t reg, bool msb_first = false)

uint16_t raw = 0x0000;

// Little-endian device (LSB first) — default
sensor.Try_Read_Register_Word(0x04, &raw);

// Big-endian device (MSB first) — most sensors
sensor.Try_Read_Register_Word(0x04, &raw, true);

Multiple Bytes — 8-bit register address

bool Read_Multiple_Register(uint8_t reg, uint8_t *data, uint8_t length, bool stop = true)

uint8_t buf[6];
if (sensor.Read_Multiple_Register(0x3B, buf, 6)) {
    // buf contains 6 bytes starting from register 0x3B
}

Multiple Bytes — 16-bit register address

For devices that use a two-byte register pointer (EEPROMs, some image sensors).

bool Read_Multiple_Register_u16(uint16_t reg, uint8_t *data, uint8_t length, bool stop = true)

The register address is sent MSB-first (high byte then low byte) as required by most devices with 16-bit addressing.

uint8_t page[16];
sensor.Read_Multiple_Register_u16(0x0100, page, 16);

Multiple Bytes — No register address

For devices that stream data immediately after a read request, without needing a register address first.

bool Read_Multiple_Register_NoCMD(uint8_t *data, uint8_t length)

uint8_t frame[4];
sensor.Read_Multiple_Register_NoCMD(frame, 4);

Single Bit

// Status-returning — bit value written to output pointer
bool Try_Read_Register_Bit(uint8_t reg, uint8_t bit_number, bool *bit_value)

// Simple wrapper — returns false on failure
bool Read_Register_Bit(uint8_t reg, uint8_t bit_number)

bool data_ready = false;
if (sensor.Try_Read_Register_Bit(0x00, 7, &data_ready)) {
    if (data_ready) { /* process */ }
}

Write Operations

Write Single Byte

bool Write_Register(uint8_t reg, uint8_t data, bool stop = true)

sensor.Write_Register(0x0F, 0x22);

Write Single Word (16-bit)

bool Write_Register_Word(uint8_t reg, uint16_t data, bool stop = true, bool msb_first = false)

// Little-endian write (default)
sensor.Write_Register_Word(0x10, 0x1234);

// Big-endian write — sends 0x12 then 0x34
sensor.Write_Register_Word(0x10, 0x1234, true, true);

Multiple Bytes

bool Write_Multiple_Register(uint8_t reg, const uint8_t *data, uint8_t length)

uint8_t config[] = {0x01, 0x80, 0x00};
sensor.Write_Multiple_Register(0x10, config, 3);

The Wire internal buffer is 32 bytes on AVR. Writing more bytes than the buffer holds will abort the transaction and return false.

Command (no register address)

For devices that accept command bytes directly without a preceding register field.

bool Write_Command(uint8_t command, bool stop = true)

sensor.Write_Command(0xFE);          // soft reset command, for example

Multiple Commands

bool Write_Multiple_Command(const uint8_t *command, uint8_t length)

uint8_t seq[] = {0xAC, 0x33, 0x00};
sensor.Write_Multiple_Command(seq, 3);

Bit Operations

All bit operations perform a read-modify-write cycle. The bit number must be in the range 0–7 (bit 0 = LSB).

Set a bit

Reads the register, sets the target bit if it is not already set, and writes back. Skips the write if the bit is already 1.

bool Set_Register_Bit(uint8_t reg, uint8_t bit_number, bool stop = true)

sensor.Set_Register_Bit(0x0F, 3);   // set bit 3

Clear a bit

Reads the register, clears the target bit if it is not already clear, and writes back. Skips the write if the bit is already 0.

bool Clear_Register_Bit(uint8_t reg, uint8_t bit_number, bool stop = true)

sensor.Clear_Register_Bit(0x0F, 3);  // clear bit 3

Toggle a bit

Reads the register, inverts the target bit, and writes back unconditionally.

bool Toggle_Register_Bit(uint8_t reg, uint8_t bit_number, bool stop = true)

sensor.Toggle_Register_Bit(0x0F, 3);  // flip bit 3

Write a bit (set or clear by value)

Convenience wrapper that calls Set_Register_Bit or Clear_Register_Bit based on the boolean value.

bool Write_Register_Bit(uint8_t reg, uint8_t bit_number, bool bit_value, bool stop = true)

bool enable = true;
sensor.Write_Register_Bit(0x0F, 3, enable);   // sets bit 3
sensor.Write_Register_Bit(0x0F, 3, !enable);  // clears bit 3

Multiplexer Support

The library integrates directly with a TCA9548A (or compatible) I2C multiplexer at the default address 0x70.

How it works

When mux support is enabled, every read and write transaction automatically:

Reads the current mux channel register
Switches to the configured channel only if it has changed
Runs the actual I2C transaction with the target device

This makes it transparent — you call Try_Read_Register and the mux is handled for you.

Setup

// Channel numbers 1–8 map to TCA9548 bitmasks 0x01–0x80
I2C_Functions temp_sensor(__I2C_Addr_SHT21__, true, 2);   // channel 2
I2C_Functions pres_sensor(__I2C_Addr_MPL3115A2__, true, 5); // channel 5

void setup() {
    temp_sensor.Begin();
    pres_sensor.Begin();
}

void loop() {
    uint8_t t_raw;
    temp_sensor.Try_Read_Register(0x00, &t_raw);   // auto-switches to channel 2

    uint16_t p_raw;
    pres_sensor.Try_Read_Register_Word(0x01, &p_raw, true);  // auto-switches to channel 5
}

Predefined Constants

src/Definition.h provides typed constexpr uint8_t constants for commonly used devices and mux configuration.

Device Addresses

constexpr uint8_t __I2C_Addr_TCA9548__    = 0x70;  // I2C Multiplexer
constexpr uint8_t __I2C_Addr_RV3028C7__   = 0x52;  // RTC
constexpr uint8_t __I2C_Addr_DS28C__      = 0x50;  // EEPROM
constexpr uint8_t __I2C_Addr_HDC2010__    = 0x40;  // Temp / Humidity
constexpr uint8_t __I2C_Addr_MPL3115A2__  = 0x60;  // Pressure / Altitude
constexpr uint8_t __I2C_Addr_TSL2561__    = 0x39;  // Light
constexpr uint8_t __I2C_Addr_SI1145__     = 0x60;  // UV / IR / Visible
constexpr uint8_t __I2C_Addr_MAX17055__   = 0x36;  // Fuel Gauge
constexpr uint8_t __I2C_Addr_BQ24298__    = 0x6B;  // Battery Charger
constexpr uint8_t __I2C_Addr_SHT21__      = 0x40;  // Temp / Humidity
constexpr uint8_t __I2C_Addr_SDP810__     = 0x25;  // Differential Pressure
constexpr uint8_t __I2C_Addr_PCF8574_A__  = 0x3F;  // I/O Expander
constexpr uint8_t __I2C_Addr_PCF8574_B__  = 0x3B;  // I/O Expander
constexpr uint8_t __I2C_Addr_PCF8574_C__  = 0x39;  // I/O Expander
constexpr uint8_t __I2C_Addr_PCF8574_D__  = 0x3E;  // I/O Expander
constexpr uint8_t __I2C_Addr_NA2302__     = 0x3A;  // Humidity

Multiplexer

constexpr uint8_t __Mux_Address__         = 0x70;  // TCA9548 default address

constexpr uint8_t __Mux_Channel_Off__     = 0x00;
constexpr uint8_t __Mux_Channel_1__       = 0x01;
constexpr uint8_t __Mux_Channel_2__       = 0x02;
constexpr uint8_t __Mux_Channel_3__       = 0x04;
constexpr uint8_t __Mux_Channel_4__       = 0x08;
constexpr uint8_t __Mux_Channel_5__       = 0x10;
constexpr uint8_t __Mux_Channel_6__       = 0x20;
constexpr uint8_t __Mux_Channel_7__       = 0x40;
constexpr uint8_t __Mux_Channel_8__       = 0x80;

Using constexpr instead of #define gives you full type safety — the compiler checks that you are passing uint8_t values, and debuggers can display the constant by name.

Project Structure

src/
  Definition.h          constexpr address and mux constants
  I2C_Functions.h       Full single-header library implementation
examples/
  Device_Control/
    Device_Control.ino  Basic read and optional write demonstration
library.properties      Arduino Library Manager metadata
library.json            PlatformIO registry metadata
VERSION                 Single source of truth for the release version
platformio.ini          Local build and validation configuration

Notes and Limitations

Transport only — This library handles the I2C framing. It does not interpret sensor data, apply calibration, or implement device-specific protocols.
Register safety — Writing to a register that is not documented as writable can lock or damage a device. Always consult the datasheet.
Wire buffer limit — The Arduino Wire library buffers up to 32 bytes per transaction on AVR. Write_Multiple_Register and Write_Multiple_Command detect a buffer overflow (Wire's write() returns 0) and abort cleanly, always sending a STOP before returning false.
Mux address — The mux address defaults to 0x70. If your TCA9548 is at a different address, override __Mux_Address__ before including the library.
Simple wrappers hide errors — Read_Register, Read_Register_Word, and Read_Register_Bit return 0 / false on failure. Use the Try_* variants when error handling matters.
Endianness is your responsibility — Pass msb_first = true for big-endian sensors (the majority) and msb_first = false for little-endian ones. The library does not auto-detect byte order.

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