I2C Expansion

Kinematrix provides three I2C-related modules for device scanning, bus expansion, and I/O expansion capabilities.

Available Modules

I2C Scanner

Detect and identify I2C devices on the bus

I2C Expander

TCA9548A multiplexer for multiple I2C buses

I/O Expander

PCF8574 chip for GPIO expansion

I2C Scanner

The I2C Scanner module detects all connected I2C devices and reports their addresses.

Module Definition

lib/modules/communication/wired/i2c/i2c-scanner.h

class I2CScanner {
public:
    I2CScanner();
    virtual ~I2CScanner();

    void beginTransmission();
    void scanI2CAddress();
    void endTransmission();
private:
    int n_address_[8];
    int n_devices_;
};

Basic Usage

#define ENABLE_MODULE_I2C_SCANNER
#include "Kinematrix.h"

I2CScanner scanner;

void setup() {
  Serial.begin(115200);
  
  // Start scanning process
  scanner.beginTransmission();
  scanner.scanI2CAddress();
  scanner.endTransmission();
}

void loop() {
  // Scanning is typically done once in setup
}

Output Example

==============================
| START SCANNING I2C
==============================
| WIRE BEGIN
| Scanning...
| Done Scanning
| I2C devices found at address : 
| 0x0027
| 0x003C
| Total devices found          : | 2
| WIRE END

Common I2C Addresses

Device Type	Typical Address(es)	Notes
PCF8574 I/O Expander	0x20-0x27, 0x38-0x3F	Configurable via A0-A2 pins
LCD with I2C	0x27, 0x3F	Common LCD backpack addresses
OLED SSD1306	0x3C, 0x3D	128x64 displays
BMP280 Pressure	0x76, 0x77	Barometric pressure sensor
BME280	0x76, 0x77	Temperature/humidity/pressure
MPU6050 IMU	0x68, 0x69	Accelerometer/gyroscope
ADS1115 ADC	0x48-0x4B	16-bit analog-to-digital
TCA9548A Multiplexer	0x70-0x77	I2C bus expander
DS3231 RTC	0x68	Real-time clock
AT24C EEPROM	0x50-0x57	External EEPROM

Error Detection

The scanner detects and reports unknown errors:

void I2CScanner::scanI2CAddress() {
    byte error;
    for (address = 1; address < 127; address++) {
        Wire.beginTransmission(address);
        error = Wire.endTransmission();
        
        if (error == 0) {
            // Device found
            n_address_[n_devices_] = address;
            n_devices_++;
        } else if (error == 4) {
            Serial.print("Unknown error at address 0x");
            Serial.println(address, HEX);
        }
    }
}

I2C Expander (TCA9548A)

The I2C Expander module provides access to the TCA9548A I2C multiplexer, allowing up to 8 separate I2C buses from a single master.

Module Definition

lib/modules/communication/wired/i2c/i2c-expander.h

#include "TCA9548A.h"

class I2CExpander : public TCA9548A {
private:
    uint8_t addr;
public:
    using TCA9548A::TCA9548A;
};

Why Use an I2C Expander?

Address Conflicts

Multiple devices with the same I2C address can coexist on different channels:

// Two OLED displays at 0x3C
expander.selectChannel(0);
oled1.display();  // First OLED

expander.selectChannel(1);
oled2.display();  // Second OLED

Bus Capacitance

Reduce total bus capacitance by splitting devices across multiple channels, improving signal integrity and allowing longer cable runs.

Device Isolation

Isolate problematic devices that may hang the bus, allowing the rest of the system to continue operating.

Basic Usage

#define ENABLE_MODULE_I2C_EXPANDER
#include "Kinematrix.h"

I2CExpander expander(0x70);  // TCA9548A default address

void setup() {
  Wire.begin();
  
  // Select channel 0
  expander.selectChannel(0);
  
  // Initialize device on channel 0
  sensor1.begin();
  
  // Select channel 1
  expander.selectChannel(1);
  
  // Initialize device on channel 1
  sensor2.begin();
}

void loop() {
  // Read from channel 0
  expander.selectChannel(0);
  float value1 = sensor1.read();
  
  // Read from channel 1
  expander.selectChannel(1);
  float value2 = sensor2.read();
  
  delay(1000);
}

Advanced Channel Management

// Enable multiple channels simultaneously (bitwise OR)
expander.enableChannels(0b00000011);  // Channels 0 and 1

// Disable all channels
expander.disableAllChannels();

// Check which channels are active
uint8_t activeChannels = expander.getActiveChannels();

TCA9548A Specifications

Parameter	Value	Notes
Supply Voltage	1.65V - 5.5V	Compatible with 3.3V and 5V systems
Number of Channels	8	SC0-SC7
I2C Address Range	0x70-0x77	Configurable via A0-A2 pins
Max Frequency	400 kHz	Fast mode I2C
Channel Selection	Register-based	Write to control register

I/O Expander (PCF8574)

The I/O Expander module provides access to the PCF8574 8-bit I/O expander chip.

Module Definition

lib/modules/communication/wired/i2c/io-expander.h

#include "PCF8574.h"

class IOExpander : public PCF8574 {
private:
    uint8_t addr;
public:
    using PCF8574::PCF8574;
};

Basic GPIO Control

#define ENABLE_MODULE_IO_EXPANDER
#include "Kinematrix.h"

IOExpander expander(0x20);  // PCF8574 address

void setup() {
  expander.begin();
  
  // Set pins 0-3 as outputs, 4-7 as inputs
  expander.pinMode(0, OUTPUT);
  expander.pinMode(1, OUTPUT);
  expander.pinMode(2, OUTPUT);
  expander.pinMode(3, OUTPUT);
  expander.pinMode(4, INPUT);
  expander.pinMode(5, INPUT);
  expander.pinMode(6, INPUT);
  expander.pinMode(7, INPUT);
}

void loop() {
  // Control outputs
  expander.digitalWrite(0, HIGH);
  expander.digitalWrite(1, LOW);
  
  // Read inputs
  bool button1 = expander.digitalRead(4);
  bool button2 = expander.digitalRead(5);
  
  delay(100);
}

LED Control Example

IOExpander leds(0x20);

void setup() {
  leds.begin();
  
  // Set all pins as outputs
  for (int i = 0; i < 8; i++) {
    leds.pinMode(i, OUTPUT);
  }
}

void loop() {
  // LED chaser effect
  for (int i = 0; i < 8; i++) {
    leds.digitalWrite(i, HIGH);
    delay(100);
    leds.digitalWrite(i, LOW);
  }
}

Button Matrix Example

IOExpander buttons(0x21);

void setup() {
  Serial.begin(115200);
  buttons.begin();
  
  // Set all pins as inputs with pull-ups
  for (int i = 0; i < 8; i++) {
    buttons.pinMode(i, INPUT_PULLUP);
  }
}

void loop() {
  // Read all button states
  uint8_t buttonStates = buttons.read8();
  
  // Check individual buttons (active LOW)
  for (int i = 0; i < 8; i++) {
    if (!(buttonStates & (1 << i))) {
      Serial.print("Button ");
      Serial.print(i);
      Serial.println(" pressed");
    }
  }
  
  delay(50);  // Debounce delay
}

PCF8574 Specifications

Parameter	Value	Notes
Supply Voltage	2.5V - 6V	Typically 5V
I/O Pins	8	P0-P7
Current per Pin	25 mA	Maximum sink/source
I2C Address (PCF8574)	0x20-0x27	A0-A2 configurable
I2C Address (PCF8574A)	0x38-0x3F	Different variant
Interrupt Support	Yes	INT pin goes LOW on input change

Multiple Expander Configuration

Combine multiple I/O expanders for large systems:

#define ENABLE_MODULE_IO_EXPANDER
#include "Kinematrix.h"

IOExpander outputs1(0x20);  // 8 outputs
IOExpander outputs2(0x21);  // 8 more outputs
IOExpander inputs(0x22);    // 8 inputs

void setup() {
  outputs1.begin();
  outputs2.begin();
  inputs.begin();
  
  // Configure first 16 pins as outputs
  for (int i = 0; i < 8; i++) {
    outputs1.pinMode(i, OUTPUT);
    outputs2.pinMode(i, OUTPUT);
  }
  
  // Configure 8 pins as inputs
  for (int i = 0; i < 8; i++) {
    inputs.pinMode(i, INPUT_PULLUP);
  }
}

void loop() {
  // Control 16 LEDs
  uint8_t inputState = inputs.read8();
  outputs1.write8(inputState);
  outputs2.write8(~inputState);
  delay(10);
}

I2C Best Practices

Pull-up Resistors: I2C requires pull-up resistors on SDA and SCL lines. Typical values are 4.7kΩ for short distances or 2.2kΩ for longer cables.

Bus Speed: Start with 100kHz (standard mode) for debugging, then increase to 400kHz (fast mode) for better performance once the system is stable.

Address Conflicts: Always scan the I2C bus before adding new devices to avoid address conflicts.

Common I2C Issues

No Devices Found

Possible causes:

Missing or incorrect pull-up resistors
Incorrect wiring (SDA/SCL swapped)
Power supply issues
Wrong I2C pins on ESP32 (use Wire.begin(SDA, SCL))

Solution:

// ESP32: Specify I2C pins
Wire.begin(21, 22);  // SDA=21, SCL=22

// Check pull-ups with multimeter
// SDA and SCL should read VCC when idle

Intermittent Communication

Possible causes:

Bus capacitance too high (long wires)
Insufficient pull-up current
Electrical noise

Solution:

Use stronger pull-ups (2.2kΩ instead of 4.7kΩ)
Reduce wire length
Add capacitors near devices (100nF)
Shield cables for long runs

Bus Lockup

Possible causes:

Device holding SDA low
Power glitch during communication

Solution:

// Software I2C reset
Wire.end();
delay(100);
Wire.begin();

// Or clock out stuck device
pinMode(SDA_PIN, OUTPUT);
for (int i = 0; i < 9; i++) {
  digitalWrite(SCL_PIN, HIGH);
  delayMicroseconds(5);
  digitalWrite(SCL_PIN, LOW);
  delayMicroseconds(5);
}
Wire.begin();

Platform-Specific Notes

ESP32
ESP8266
Arduino

// Default pins: SDA=21, SCL=22
Wire.begin();

// Custom pins
Wire.begin(SDA_PIN, SCL_PIN);

// Clock speed
Wire.setClock(400000);  // 400kHz

// Default pins: SDA=4(D2), SCL=5(D1)
Wire.begin();

// Custom pins
Wire.begin(SDA_PIN, SCL_PIN);

// Clock speed
Wire.setClock(100000);  // 100kHz recommended

// Fixed pins: SDA=A4, SCL=A5 (Uno/Nano)
// Fixed pins: SDA=20, SCL=21 (Mega)
Wire.begin();

// Clock speed
Wire.setClock(100000);  // 100kHz

Serial Communication

Alternative communication protocols

SPI Interface

Higher-speed serial communication

Getting Started

Core Concepts

Sensor Framework

Communication

Control Algorithms

WiFi & Cloud Services

AutoLight System

Guides

Available Modules

I2C Scanner

I2C Expander

I/O Expander

I2C Scanner

Module Definition

Basic Usage

Output Example

Common I2C Addresses

Error Detection

I2C Expander (TCA9548A)

Module Definition

Why Use an I2C Expander?

Basic Usage

Advanced Channel Management

TCA9548A Specifications

I/O Expander (PCF8574)

Module Definition

Basic GPIO Control

LED Control Example

Button Matrix Example

PCF8574 Specifications

Multiple Expander Configuration

I2C Best Practices

Common I2C Issues

Platform-Specific Notes

Serial Communication

SPI Interface

Build docs developers (and LLMs) love

Getting Started

Core Concepts

Sensor Framework

Communication

Control Algorithms

WiFi & Cloud Services

AutoLight System

Guides

​Available Modules

I2C Scanner

I2C Expander

I/O Expander

​I2C Scanner

​Module Definition

​Basic Usage

​Output Example

​Common I2C Addresses

​Error Detection

​I2C Expander (TCA9548A)

​Module Definition

​Why Use an I2C Expander?

​Basic Usage

​Advanced Channel Management

​TCA9548A Specifications

​I/O Expander (PCF8574)

​Module Definition

​Basic GPIO Control

​LED Control Example

​Button Matrix Example

​PCF8574 Specifications

​Multiple Expander Configuration

​I2C Best Practices

​Common I2C Issues

​Platform-Specific Notes

​Related Modules

Serial Communication

SPI Interface

Build docs developers (and LLMs) love

Available Modules

I2C Scanner

Module Definition

Basic Usage

Output Example

Common I2C Addresses

Error Detection

I2C Expander (TCA9548A)

Module Definition

Why Use an I2C Expander?

Basic Usage

Advanced Channel Management

TCA9548A Specifications

I/O Expander (PCF8574)

Module Definition

Basic GPIO Control

LED Control Example

Button Matrix Example

PCF8574 Specifications

Multiple Expander Configuration

I2C Best Practices

Common I2C Issues

Platform-Specific Notes

Related Modules