How to Use Adafruit PCF8574 I2C GPIO Expander Breakout: Examples, Pinouts, and Specs

The Adafruit PCF8574 I2C GPIO Expander Breakout (Part ID: 5545) is a versatile module designed to expand the number of digital input/output (GPIO) pins available to your microcontroller. It communicates via the I2C protocol, allowing you to control up to 8 additional GPIO pins using only two microcontroller pins (SDA and SCL). This makes it an excellent choice for projects requiring multiple digital inputs or outputs, such as controlling LEDs, reading button states, or interfacing with sensors.

• Expanding GPIO capabilities of microcontrollers like Arduino, Raspberry Pi, or ESP32.
• Controlling multiple LEDs, relays, or other digital devices.
• Reading the state of multiple buttons or switches.
• Building I2C-based modular systems with multiple PCF8574 modules.

• I2C Address Range: 0x20 to 0x27 (configurable via address pins A0, A1, A2)
• Operating Voltage: 2.5V to 6V
• Maximum Sink Current per Pin: 25mA
• Maximum Source Current per Pin: 300µA
• Number of GPIO Pins: 8 (P0 to P7)
• Communication Protocol: I2C (400kHz maximum clock speed)
• Dimensions: 25mm x 18mm x 2mm (excluding header pins)

The PCF8574 breakout board has the following pin layout:

1. Connect Power and Ground:

   • Connect the VCC pin to the microcontroller's power supply (e.g., 5V for Arduino).
   • Connect the GND pin to the microcontroller's ground.

2. Connect I2C Lines:

   • Connect the SDA pin to the microcontroller's SDA pin.
   • Connect the SCL pin to the microcontroller's SCL pin.
   • Use pull-up resistors (typically 4.7kΩ) on the SDA and SCL lines if not already present.

3. Set the I2C Address:

   • Configure the address by connecting the A0, A1, and A2 pins to either VCC (logic HIGH) or GND (logic LOW). The default address is 0x20 when all address pins are connected to GND.

4. Connect GPIO Devices:

   • Use the P0 to P7 pins to connect LEDs, buttons, or other digital devices. Configure each pin as input or output in your code.

• Current Limitations: The GPIO pins can sink up to 25mA but can only source 300µA. Use external transistors or drivers if higher current is required.
• Pull-Up Resistors: The GPIO pins are open-drain, so external pull-up resistors may be needed for certain applications.
• I2C Address Conflicts: Ensure that the I2C address does not conflict with other devices on the same bus.

Below is an example of how to use the PCF8574 with an Arduino UNO to toggle an LED connected to pin P0 and read the state of a button connected to pin P1.

#include <Wire.h>
#include "Adafruit_MCP23008.h" // Include the Adafruit PCF8574 library

#define PCF8574_ADDRESS 0x20 // Default I2C address of the PCF8574

void setup() {
  Wire.begin(); // Initialize I2C communication
  Serial.begin(9600); // Start serial communication for debugging

  // Set P0 as output (for LED) and P1 as input (for button)
  pinMode(0, OUTPUT);
  pinMode(1, INPUT_PULLUP); // Use internal pull-up resistor for button
}

void loop() {
  // Toggle LED on P0
  digitalWrite(0, HIGH); // Turn LED on
  delay(500); // Wait for 500ms
  digitalWrite(0, LOW); // Turn LED off
  delay(500); // Wait for 500ms

  // Read button state on P1
  int buttonState = digitalRead(1);
  if (buttonState == LOW) {
    Serial.println("Button Pressed!");
  } else {
    Serial.println("Button Released!");
  }

  delay(100); // Short delay for stability
}

1. I2C Device Not Detected:

   • Ensure the SDA and SCL lines are correctly connected.
   • Check for proper pull-up resistors on the I2C lines.
   • Verify the I2C address configuration (A0, A1, A2 pins).

2. GPIO Pins Not Responding:

   • Confirm that the pins are correctly configured as input or output in the code.
   • Check for proper connections to external devices (e.g., LEDs, buttons).

3. Interference with Other I2C Devices:

   • Ensure no address conflicts by configuring the PCF8574 to a unique I2C address.

4. Low Current Output:

   • Remember that the GPIO pins can only source 300µA. Use external drivers for higher current requirements.

Q: Can I use multiple PCF8574 modules on the same I2C bus?
A: Yes, you can connect up to 8 PCF8574 modules by configuring unique I2C addresses using the A0, A1, and A2 pins.

Q: Do I need external pull-up resistors for the GPIO pins?
A: It depends on your application. For open-drain configurations, external pull-up resistors may be required.

Q: What is the maximum I2C clock speed supported?
A: The PCF8574 supports a maximum I2C clock speed of 400kHz.

Q: Can the PCF8574 handle analog signals?
A: No, the PCF8574 is designed for digital input/output only. Use an ADC for analog signals.

This documentation provides a comprehensive guide to using the Adafruit PCF8574 I2C GPIO Expander Breakout effectively in your projects.