Complete Arduino library for controlling the MCP23009E I/O Expander with GPIO configuration, interrupts, and Pin-compatible API.

• ✅ Full GPIO control: Configure pins as input/output, read/write levels
• ✅ Pull-up resistors: Built-in pull-up support
• ✅ Interrupt support: Hardware interrupts with callback system
• ✅ Pin-compatible API: MCP23009Pin class similar to Arduino's digital pins
• ✅ Active-low support: MCP23009ActiveLowPin for LEDs and similar loads
• ✅ Register access: Low-level register access for advanced usage

1. Download this library as ZIP
2. In Arduino IDE: Sketch → Include Library → Add .ZIP Library
3. Select the downloaded ZIP file

Add to your platformio.ini:

lib_deps =
    https://github.com/steamicc/MCP23009E.git

#include <Wire.h>
#include <MCP23009E.h>

MCP23009E mcp(Wire, MCP23009_I2C_ADDR);

void setup() {
    Serial.begin(115200);
    mcp.begin();  // Initialize I2C and MCP23009E

    // Configure a GPIO as input with pull-up
    mcp.setup(7, MCP23009_DIR_INPUT, MCP23009_PULLUP);

    // Read the level
    uint8_t level = mcp.getLevel(7);
    Serial.println(level ? "HIGH" : "LOW");

    // Configure a GPIO as output
    mcp.setup(0, MCP23009_DIR_OUTPUT);
    mcp.setLevel(0, MCP23009_LOGIC_HIGH);
}

void loop() {
    // Your code here
}

#include <Wire.h>
#include <MCP23009E.h>

MCP23009E mcp(Wire, MCP23009_I2C_ADDR);

// Create Pin objects (mode will be configured after I2C initialization)
MCP23009Pin button(mcp, 7);
MCP23009Pin led(mcp, 0);

void setup() {
    Wire.begin();
    mcp.begin();

    // Configure pin modes after mcp.begin()
    button.pinMode(INPUT_PULLUP);
    led.pinMode(OUTPUT);

    // Use just like Arduino digital pins!
    led.high();
    if (button.digitalRead() == LOW) {
        led.low();
    }
}

void loop() {
    led.toggle();
    delay(500);
}

For LEDs connected between VCC and GPIO (recommended configuration):

3.3V → [LED] → [Resistor 220-330Ω] → GPIO

#include <Wire.h>
#include <MCP23009E.h>

MCP23009E mcp(Wire, MCP23009_I2C_ADDR);

// Create active-low LED pin (logic is automatically inverted)
MCP23009ActiveLowPin led(mcp, 0);

void setup() {
    Wire.begin();
    mcp.begin();

    // Configure pin mode after mcp.begin()
    led.pinMode(OUTPUT);
}

void loop() {
    led.high();   // LED turns ON (GPIO goes LOW)
    delay(500);
    led.low();    // LED turns OFF (GPIO goes HIGH)
    delay(500);
}

Why active-low? The MCP23009E can sink 25mA per pin but can only source ~1mA. Active-low configuration allows proper LED brightness.

All I2C operations automatically track errors. Check after critical operations:

#include <Wire.h>
#include <MCP23009E.h>

MCP23009E mcp(Wire, MCP23009_I2C_ADDR);

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

    // Check if device is connected
    if (!mcp.isConnected()) {
        Serial.println("MCP23009E not found!");
        Serial.print("Error code: ");
        Serial.println(mcp.getLastError());
        while(1);  // Halt
    }

    // Configure GPIO and check for errors
    mcp.setup(0, MCP23009_DIR_OUTPUT);

    if (mcp.getLastError() != MCP23009_OK) {
        Serial.println("Configuration failed!");
    }
}

void loop() {
    // Read and check for errors
    uint8_t value = mcp.getLevel(7);

    if (mcp.getLastError() == MCP23009_OK) {
        Serial.println(value ? "HIGH" : "LOW");
    } else {
        Serial.println("Read error!");
    }

    delay(1000);
}

MCP23009E(TwoWire& wire = Wire, uint8_t address = MCP23009_I2C_ADDR)

• void begin(int resetPin = -1, int interruptPin = -1) - Initialize the device
• void reset() - Hardware reset (requires reset pin)
• void setup(uint8_t gpx, uint8_t direction, uint8_t pullup, uint8_t polarity) - Configure GPIO
• void setLevel(uint8_t gpx, uint8_t level) - Set output level
• uint8_t getLevel(uint8_t gpx) - Read input level

• MCP23009Error getLastError() - Get last I2C error code
• void clearError() - Clear error status
• bool isConnected() - Check if device is responding on I2C bus

Error codes:

• MCP23009_OK (0) - Success
• MCP23009_ERROR_I2C_WRITE (1) - I2C write failed
• MCP23009_ERROR_I2C_READ (2) - I2C read failed
• MCP23009_ERROR_INVALID_PIN (3) - Invalid pin number
• MCP23009_ERROR_TIMEOUT (4) - I2C timeout

• void interruptOnChange(uint8_t gpx, InterruptCallback callback) - Register change callback
• void interruptOnFalling(uint8_t gpx, SimpleCallback callback) - Register falling edge callback
• void interruptOnRaising(uint8_t gpx, SimpleCallback callback) - Register rising edge callback
• void disableInterrupt(uint8_t gpx) - Disable interrupts on GPIO
• bool processPendingInterrupts() - Call this in loop() to process pending interrupts (returns true if processed)
• void handleInterrupt() - Low-level interrupt handler (usually not called directly)

• setIODIR(), getIODIR() - I/O Direction Register
• setGPPU(), getGPPU() - Pull-up Register
• setGPIO(), getGPIO() - GPIO Register
• And many more...

MCP23009Pin(MCP23009E& mcp, uint8_t pinNumber, uint8_t mode = INPUT)

• void pinMode(uint8_t mode) - Set pin mode (INPUT, OUTPUT, INPUT_PULLUP)
• void digitalWrite(uint8_t value) - Write HIGH or LOW
• uint8_t digitalRead() - Read pin state
• void high() - Set pin HIGH
• void low() - Set pin LOW
• void toggle() - Toggle pin state
• void attachInterrupt(void (*callback)(), int mode) - Attach interrupt
• void detachInterrupt() - Detach interrupt

Inherits from MCP23009Pin with inverted logic for active-low loads.

MCP23009ActiveLowPin(MCP23009E& mcp, uint8_t pinNumber, uint8_t mode = OUTPUT)

All methods are identical to MCP23009Pin, but logic is automatically inverted.

The library includes several examples:

• BasicUsage - Read D-PAD buttons and print states
• LED_ActiveLow - Control LEDs in active-low configuration with effects
• ErrorHandling - Demonstrates I2C error detection and handling

• InterruptBasic - Simple interrupt handling with one button (FALLING/RISING)
• InterruptMultipleButtons - Handle interrupts from multiple buttons (CHANGE mode)
• InterruptWithPin - Use Pin API with interrupts and LED control

Default address is 0x20. Can be changed via hardware pins A0-A2.

• Sink current (IOL): 25 mA max per pin
• Source current (IOH): ~1 mA per pin

Recommendation: Use active-low configuration for LEDs (connect between VCC and GPIO).

The MCP23009E has built-in 100kΩ pull-up resistors that can be enabled per pin.

• Arduino: All boards with I2C support
• ESP32: Tested and working
• ESP8266: Tested and working
• STM32: Tested and working
• Others: Should work on any platform with Wire library

This library is released under the MIT License. See LICENSE file for details.

Based on the MicroPython driver for MCP23009E.

Contributions are welcome! Please open an issue or submit a pull request on GitHub.