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
📚 Documentation & Articles
Full documentation, tutorials and examples are available on the author site:
📋 Table of Contents
✨ 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.
- 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)
- Open Arduino IDE.
- Go to Sketch > Include Library > Manage Libraries…
- Search for “PCF8575_library” by Renzo Mischianti and install.
Add to platformio.ini:
lib_deps = xreef/PCF8575_library
Manual installation
- Download the latest release from GitHub: https://github.com/xreef/PCF8575_library/releases
- Unzip and rename the folder to
PCF8575_library.
- Move it into your Arduino
libraries folder.
- Restart the Arduino IDE.
🚀 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
-
06/04/2022: v1.0.3
📄 License
This library is released under the MIT License. See the LICENSE file for details.
Copyright © 2017-2025 Renzo Mischianti
🤝 Contributing
Contributions are welcome. Please fork the repository, create a feature branch and submit a pull request.
If this library helped your project, please give it a star on GitHub!
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