PCF8575 I2C 16-bit Digital I/O Expander Library

A simple and efficient library to use the PCF8575 I2C 16-bit digital I/O expander with Arduino, ESP8266, ESP32, RP2040 and other platforms.

Author: Renzo Mischianti

Website / Documentation: https://www.mischianti.org/pcf8575-i2c-16-bit-digital-i-o-expander/

GitHub: https://github.com/xreef/PCF8575_library

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📚 Documentation & Articles

Full documentation, tutorials and examples are available on the author site:

• 🌐 PCF8575 main article: https://www.mischianti.org/pcf8575-i2c-16-bit-digital-i-o-expander/
• 🛠️ I2C Scanner: https://playground.arduino.cc/Main/I2cScanner
• 💬 Support forum (English): https://www.mischianti.org/forums/forum/mischiantis-libraries/pcf8575-16bits-i2c-digital-i-o-expander/
• 💬 Forum di supporto (Italiano): https://www.mischianti.org/it/forums/forum/le-librerie-di-mischianti/pcf8575-expander-digitale-i-o-i2c-a-16bits/

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📋 Table of Contents

• Features
• Supported Platforms
• Installation
• API Overview
• Basic Usage
• Interrupts
• Rotary Encoder Support
• Low Memory Mode
• Example: HC-SR04 Ultrasonic Sensor
• Changelog
• License
• Contributing
• Support & Contact

✨ Features

• 16 Digital I/O Pins: Expand your microcontroller's I/O over I2C (two bytes).
• Input & Output Modes: Each pin can be individually configured as INPUT, INPUT_PULLUP, or OUTPUT.
• Interrupt Support: Leverage the PCF8575 interrupt pin to detect input changes without continuous polling.
• Rotary Encoder Support: Helpers for reading rotary encoders (optional compile-time support).
• Flexible I2C configuration:
  • Standard PCF8575 addresses (0x20..0x27) depending on A0/A1 pins.
  • Custom SDA/SCL pins for platforms that support it (ESP8266/ESP32/RP2040).
• Efficient reading: Read all 16 pins in a single I2C transaction with digitalReadAll().
• Low Memory Mode: Optional mode to reduce RAM usage on constrained boards.
• Pulse measurement helpers: pulseIn() and pulseInPoll() to measure pulse widths on expander pins.

🎯 Supported Platforms

• ESP32 — ✅ Full support
• ESP8266 — ✅ Full support
• Raspberry Pi Pico (RP2040) — ✅ Full support
• Arduino AVR (Uno, Mega, ..) — ✅ Full support
• Arduino SAMD (Nano 33 IoT, etc.) — ✅ Full support
• STM32 — ✅ Full support

📦 Installation

Arduino IDE (Library Manager)

1. Open Arduino IDE.
2. Go to Sketch > Include Library > Manage Libraries...
3. Search for "PCF8575_library" by Renzo Mischianti and install.

PlatformIO

Add to platformio.ini:

lib_deps = xreef/PCF8575_library

Manual installation

1. Download the latest release from GitHub: https://github.com/xreef/PCF8575_library/releases
2. Unzip and rename the folder to PCF8575_library.
3. Move it into your Arduino libraries folder.
4. Restart the Arduino IDE.

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🚀 API Overview

Constructors

// Basic constructor with I2C address
PCF8575(uint8_t address);

// For boards with custom TwoWire instance
PCF8575(TwoWire *pWire, uint8_t address);

// For ESP32/ESP8266/RP2040: specify SDA/SCL
PCF8575(uint8_t address, int sda, int scl);

// With interrupt pin and ISR
PCF8575(uint8_t address, uint8_t interruptPin, void (*interruptFunction)());

Main methods

// Initialize the driver (must be called in setup)
bool begin();

// Set a pin mode (INPUT, INPUT_PULLUP, OUTPUT)
void pinMode(uint8_t pin, uint8_t mode, uint8_t output_start = HIGH);

// Read single pin (debounced / buffered)
uint8_t digitalRead(uint8_t pin, bool forceReadNow = false);

// Read all pins at once
#ifndef PCF8575_LOW_MEMORY
  PCF8575::DigitalInput digitalReadAll(); // returns struct with p0..p15 or p00..p17 if NOT_SEQUENTIAL_PINOUT
#else
  uint16_t digitalReadAll(); // returns 16-bit value
#endif

// Write a pin value
bool digitalWrite(uint8_t pin, uint8_t value);

// Pulse measurement helpers
unsigned long pulseIn(uint8_t pin, uint8_t value, unsigned long timeout);
unsigned long pulseInPoll(uint8_t pin, uint8_t value, unsigned long timeout);

See PCF8575.h for full API and compile-time options.

💡 Basic Usage

Simple example: toggle an LED and read a button.

#include <Wire.h>
#include <PCF8575.h>

// PCF8575 address example
PCF8575 pcf8575(0x20);

const uint8_t LED_PIN = P0;       // use P0..P15 (or P00..P17 for NOT_SEQUENTIAL_PINOUT)
const uint8_t BUTTON_PIN = P1;

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

  if (!pcf8575.begin()) {
    Serial.println("PCF8575 not found");
    while (1) delay(1000);
  }

  pcf8575.pinMode(LED_PIN, OUTPUT, LOW);
  pcf8575.pinMode(BUTTON_PIN, INPUT_PULLUP);
}

void loop() {
  uint8_t button = pcf8575.digitalRead(BUTTON_PIN);
  if (button == LOW) {
    pcf8575.digitalWrite(LED_PIN, HIGH);
  } else {
    pcf8575.digitalWrite(LED_PIN, LOW);
  }
  delay(100);
}

⚡ Interrupts

To use interrupts, connect the PCF8575 INT pin to an interrupt-capable MCU pin and provide an ISR. Keep ISR short — set a flag and handle logic in loop().

volatile bool keyPressed = false;
void keyPressedOnPCF8575() { keyPressed = true; }

PCF8575 pcf8575(0x20, 2, keyPressedOnPCF8575); // address, interruptPin, ISR

void setup() {
  pcf8575.begin();
}

void loop() {
  if (keyPressed) {
    keyPressed = false;
    auto vals = pcf8575.digitalReadAll();
    // handle values
  }
}

🔄 Rotary Encoder Support

Optional encoder support can be enabled in PCF8575.h via compile-time defines. See comments in the header for available implementations and usage examples.

🧠 Low Memory Mode

If you enable #define PCF8575_LOW_MEMORY in PCF8575.h, digitalReadAll() returns a single 16-bit value instead of a struct, saving a few bytes of RAM.

Example (low memory):

uint16_t values = pcf8575.digitalReadAll();
bool p0 = (values & bit(0)) > 0;
bool p8 = (values & bit(8)) > 0; // upper byte bit

Example: HC-SR04 Ultrasonic Sensor

Using pulseIn() helper on the expander pin is possible but less accurate than a direct MCU pin due to I2C latency. Prefer connecting the ECHO pin to the MCU for precise timing.

#include <PCF8575.h>
PCF8575 pcf(0x20);
const uint8_t ECHO_PIN = P0; // on expander
const int TRIG_PIN = 9;     // on MCU

void setup() {
  Serial.begin(115200);
  pcf.begin();
  pcf.pinMode(ECHO_PIN, INPUT);
  pinMode(TRIG_PIN, OUTPUT);
}

void loop() {
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  unsigned long duration = pcf.pulseIn(ECHO_PIN, HIGH, 30000UL);
  unsigned long distanceCm = duration / 29 / 2;
  Serial.print("Distance: ");
  Serial.print(distanceCm);
  Serial.println(" cm");
  delay(500);
}

📝 Changelog

• 31/10/2025: v2.0.1

  • Fix: digitalReadAll() in low memory mode returns full 16-bit value (no overflow).
  • Fix: corrected pin mapping for NOT_SEQUENTIAL_PINOUT (PCF8575.cpp).
  • Fix: in PCF8575_LOW_MEMORY, digitalReadAll() now returns bits 8-15 correctly by using a 16-bit return value.

• 30/04/2024: v2.0.0

  • Manage INPUT_PULLUP, LOW latency mode, support Arduino UNO R4 and initial OUTPUT value

• 01/02/2024: v1.1.2

  • Add the possibility to provide the address to begin() and return the status of begin operation

• 10/07/2023: v1.1.1

  • Add support for Arduino UNO R4

• 16/02/2023: v1.1.0

  • Fix STM32 support and add support for Raspberry Pi Pico and other RP2040 boards
  • Add support for custom SERCOM interface of Arduino SAMD devices
  • Force SDA/SCL to use GPIO numeration for STM32 (see: https://www.mischianti.org/forums/topic/compatible-with-stm32duino/)
  • Force SDA/SCL to use GPIO numeration for other boards (see: https://www.mischianti.org/forums/topic/cannot-set-sda-clk-on-esp8266/)
  • Fix the SDA/SCL type issue (#58) and add basic SAMD support

• 06/04/2022: v1.0.3

  • Fix package size

📄 License

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

Copyright (c) 2017-2025 Renzo Mischianti

🤝 Contributing

Contributions are welcome. Please fork the repository, create a feature branch and submit a pull request.

📞 Support & Contact

• Documentation: https://www.mischianti.org/pcf8575-i2c-16-bit-digital-i-o-expander/
• GitHub Issues: https://github.com/xreef/PCF8575_library/issues
• Author: Renzo Mischianti (@xreef)

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