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 with minimal wiring. This breakout is ideal for projects requiring more GPIOs than your microcontroller natively provides, such as controlling LEDs, reading button states, or interfacing with other digital devices.

• Expanding GPIO capabilities for microcontrollers like Arduino, Raspberry Pi, or ESP32.
• Controlling LEDs, relays, or other digital outputs.
• Reading button states or other digital inputs.
• Building I2C-based sensor hubs or control panels.
• Projects requiring multiple I2C GPIO expanders for scalable designs.

The Adafruit PCF8574 I2C GPIO Expander Breakout is based on the PCF8574 chip, which provides 8 configurable GPIO pins. Below are the key technical details:

• Operating Voltage: 2.5V to 6V
• I2C Address Range: 0x20 to 0x27 (configurable via address jumpers)
• GPIO Pins: 8 (individually configurable as input or output)
• Maximum Sink Current (per pin): 25mA
• Maximum Source Current (per pin): 300µA
• I2C Speed: Up to 100kHz
• Dimensions: 25mm x 18mm x 2mm (excluding header pins)

The breakout board has the following pin layout:

1. Connect Power and Ground:

   • Connect the VCC pin to the 3.3V or 5V power supply of your microcontroller.
   • Connect the GND pin to the ground of your microcontroller.

2. Connect I2C Lines:

   • Connect the SDA pin to the microcontroller's SDA pin.
   • Connect the SCL pin to the microcontroller's SCL pin.

3. Set the I2C Address:

   • Use the A0, A1, and A2 jumpers to configure the I2C address. For example:
     • All jumpers open: Address = 0x20 (default).
     • A0 closed: Address = 0x21.
     • A1 closed: Address = 0x22, and so on.

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.

• Pull-Up Resistors: Ensure that the I2C bus has appropriate pull-up resistors (typically 4.7kΩ) on the SDA and SCL lines.
• Current Limitations: Avoid exceeding the maximum sink/source current ratings for the GPIO pins.
• Address Conflicts: If using multiple PCF8574 modules, ensure each has a unique I2C address.
• Debouncing Inputs: For buttons or switches connected to GPIO inputs, implement software debouncing to avoid false triggers.

Below is an example of how to use the Adafruit PCF8574 I2C GPIO Expander Breakout with an Arduino UNO to toggle an LED connected to pin P0:

#include <Wire.h>

// Define the I2C address of the PCF8574 (default is 0x20)
#define PCF8574_ADDRESS 0x20

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

  // Set all GPIO pins to HIGH (default state for outputs)
  Wire.beginTransmission(PCF8574_ADDRESS);
  Wire.write(0xFF); // All pins HIGH
  Wire.endTransmission();

  Serial.println("PCF8574 initialized.");
}

void loop() {
  // Toggle the state of P0 (connected to an LED)
  Wire.beginTransmission(PCF8574_ADDRESS);
  Wire.write(0xFE); // Set P0 LOW, others HIGH
  Wire.endTransmission();
  delay(500); // Wait 500ms

  Wire.beginTransmission(PCF8574_ADDRESS);
  Wire.write(0xFF); // Set all pins HIGH
  Wire.endTransmission();
  delay(500); // Wait 500ms
}

1. I2C Communication Not Working:

   • Solution: Verify the SDA and SCL connections. Ensure pull-up resistors are present on the I2C lines.
   • Tip: Use an I2C scanner sketch to confirm the module's address.

2. GPIO Pins Not Responding:

   • Solution: Check the wiring to the GPIO pins and ensure they are configured correctly in your code.
   • Tip: Use a multimeter to verify the voltage levels on the pins.

3. Address Conflict with Other I2C Devices:

   • Solution: Change the I2C address of the PCF8574 using the A0, A1, and A2 jumpers.

4. LEDs Not Lighting Up:

   • Solution: Ensure the LED is connected with the correct polarity and that the current-limiting resistor is appropriate.

• Can I use multiple PCF8574 modules on the same I2C bus? Yes, up to 8 modules can be used by configuring unique I2C addresses using the A0, A1, and A2 jumpers.

• What is the maximum cable length for I2C communication? The maximum length depends on the pull-up resistor values and the I2C speed, but typically it is limited to a few meters.

• Can the GPIO pins handle analog signals? No, the GPIO pins are digital-only and cannot process analog signals.

• Is the PCF8574 compatible with 3.3V microcontrollers? Yes, the PCF8574 operates at 2.5V to 6V, making it compatible with both 3.3V and 5V systems.