How to Use I2C PC8574: Examples, Pinouts, and Specs

The I2C PC8574 is an 8-bit I/O expander that communicates via the I2C protocol, enabling microcontrollers to control additional input/output pins. This component is particularly useful in applications where the number of available GPIO pins on a microcontroller is insufficient. By using the PC8574, you can expand the number of digital I/O pins without requiring additional microcontroller resources.

• Expanding GPIO pins for microcontrollers like Arduino, ESP32, or Raspberry Pi.
• Interfacing with multiple sensors or actuators.
• Controlling LEDs, relays, or other digital devices.
• Building keypad interfaces or matrix displays.
• Applications requiring multiple I/O lines with minimal wiring.

The PC8574 is a versatile and efficient I/O expander. Below are its key technical details:

The PC8574 has 16 pins, as described in the table below:

The PC8574 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 Configuration: Set the A0, A1, and A2 pins to either HIGH or LOW to configure the I2C address. The base address is 0x20, and the address can be adjusted from 0x20 to 0x27.
4. I/O Pins: Connect the P0–P7 pins to the devices you want to control or monitor. These pins can be configured as inputs or outputs in your code.

Below is an example of how to use the PC8574 with an Arduino UNO to toggle an LED connected to one of its pins:

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

#define PC8574_ADDRESS 0x20 // I2C address of the PC8574 (adjust as needed)

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

  // Set all pins on the PC8574 as outputs and turn them off
  Wire.beginTransmission(PC8574_ADDRESS);
  Wire.write(0xFF); // Write 0xFF to set all pins HIGH (off for active-low devices)
  Wire.endTransmission();
}

void loop() {
  // Turn on the LED connected to P0
  Wire.beginTransmission(PC8574_ADDRESS);
  Wire.write(0xFE); // Set P0 LOW (active) and others HIGH
  Wire.endTransmission();
  delay(1000); // Wait for 1 second

  // Turn off the LED connected to P0
  Wire.beginTransmission(PC8574_ADDRESS);
  Wire.write(0xFF); // Set all pins HIGH (inactive)
  Wire.endTransmission();
  delay(1000); // Wait for 1 second
}

• Pull-up Resistors: Ensure pull-up resistors are present on the SDA and SCL lines for proper I2C communication.
• Current Limitations: The PC8574 can sink up to 25mA per pin but can only source 300µA. Use external transistors or drivers for higher current requirements.
• Interrupt Pin: The INT pin can be used to detect changes on input pins, reducing the need for constant polling.

1. I2C Communication Failure

   • Cause: Incorrect wiring or missing pull-up resistors.
   • Solution: Verify SDA and SCL connections and ensure pull-up resistors (4.7kΩ) are in place.

2. Incorrect I2C Address

   • Cause: A0, A1, and A2 pins are not configured correctly.
   • Solution: Check the state of the address pins and calculate the correct I2C address.

3. Pins Not Responding

   • Cause: Pins may not be configured correctly in the code.
   • Solution: Ensure the correct data is being sent to the PC8574 to configure pins as inputs or outputs.

4. Low Current Output

   • Cause: The PC8574 has limited current sourcing capability.
   • Solution: Use external transistors or drivers for devices requiring higher current.

Q: Can the PC8574 be used with 3.3V microcontrollers?
A: Yes, the PC8574 operates at 2.5V to 6V, making it compatible with both 3.3V and 5V systems.

Q: How many PC8574 devices can be connected 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: Can the PC8574 handle analog signals?
A: No, the PC8574 is designed for digital I/O only. Use an ADC for analog signals.

Q: What happens if the INT pin is not used?
A: The INT pin is optional. If not used, it can be left unconnected.