How to Use The PCF8574 I2C to 8-Bit Digital IO Port [Bill Ludwig]: Examples, Pinouts, and Specs

The PCF8574 is an I2C interface chip that provides 8 additional digital input/output (GPIO) pins for microcontrollers. It is designed to simplify the expansion of GPIO capabilities in embedded systems by utilizing the I2C communication protocol. This component is particularly useful in applications where the number of GPIO pins on a microcontroller is insufficient for the desired functionality.

• Expanding GPIO capabilities in microcontroller-based projects
• Driving LEDs, relays, or other digital outputs
• Reading digital inputs such as switches or sensors
• Interfacing with LCD displays or keypads
• Home automation and IoT devices

The PCF8574 is a versatile and efficient I2C GPIO expander. Below are its key technical specifications:

The PCF8574 has 16 pins, with the following configuration:

The PCF8574 is straightforward to use in a circuit. Below are the steps and considerations for integrating it into your project:

1. Power Supply: Connect the VCC pin to a 3.3V or 5V power source, and the GND pin to ground.
2. I2C Lines: Connect the SDA and SCL pins to the corresponding I2C pins on your microcontroller. Use pull-up resistors (typically 4.7kΩ) on both lines if not already present.
3. Address Selection: Configure the I2C address by connecting A0, A1, and A2 to either VCC (logic high) or GND (logic low). This allows up to 8 PCF8574 devices on the same I2C bus.
4. GPIO Pins: Connect the P0–P7 pins to your desired input or output devices.

Below is an example of how to use the PCF8574 to control LEDs with an Arduino UNO:

• Connect the PCF8574's SDA and SCL pins to the Arduino's A4 (SDA) and A5 (SCL) pins, respectively.
• Connect LEDs to P0–P3 through 220Ω resistors.

#include <Wire.h> // Include the Wire library for I2C communication

#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
  Serial.println("PCF8574 Example: Controlling LEDs");
}

void loop() {
  Wire.beginTransmission(PCF8574_ADDRESS); // Start communication with PCF8574
  Wire.write(0b00001111); // Turn on LEDs connected to P0–P3
  Wire.endTransmission(); // End communication
  delay(1000); // Wait for 1 second

  Wire.beginTransmission(PCF8574_ADDRESS); // Start communication with PCF8574
  Wire.write(0b00000000); // Turn off all LEDs
  Wire.endTransmission(); // End communication
  delay(1000); // Wait for 1 second
}

• GPIO Configuration: The GPIO pins are open-drain, meaning they can only sink current. Use pull-up resistors or external circuitry for sourcing current.
• Interrupt Pin: The INT pin can be used to detect changes on input pins, reducing the need for constant polling.
• I2C Address Conflicts: Ensure that no two devices on the same I2C bus share the same address.

1. No Response from PCF8574

   • Verify the I2C address configuration (A0, A1, A2 pins).
   • Check the pull-up resistors on the SDA and SCL lines.
   • Ensure proper power supply connections.

2. Incorrect GPIO Behavior

   • Confirm that the GPIO pins are correctly configured for input or output.
   • Check for proper pull-up or pull-down resistors on the GPIO lines.

3. I2C Communication Errors

   • Ensure the I2C clock speed does not exceed 100kHz.
   • Verify the connections between the microcontroller and the PCF8574.

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

Q: How many PCF8574 devices can I connect to a single I2C bus?
A: Up to 8 devices can be connected by configuring unique I2C addresses using the A0, A1, and A2 pins.

Q: What is the maximum current the PCF8574 can sink?
A: Each GPIO pin can sink up to 25mA, but the total current for all pins should not exceed 100mA.

Q: Can I use the PCF8574 with a 3.3V microcontroller?
A: Yes, the PCF8574 supports operating voltages from 2.5V to 6V, making it compatible with both 3.3V and 5V systems.