How to Use PCF8575 I2C Expander: Examples, Pinouts, and Specs

The PCF8575 is a 16-bit I2C I/O port expander designed to extend the number of GPIO pins available to a microcontroller. It communicates via the I2C bus, requiring only two wires (SCL and SDA) for data transmission. This makes it an efficient solution for applications where GPIO pin availability is limited.

Common applications of the PCF8575 include:

• Expanding GPIO pins for microcontrollers in embedded systems.
• Controlling LEDs, relays, or other digital devices.
• Reading inputs from buttons, switches, or sensors.
• Building home automation systems or industrial control panels.

The PCF8575 has the following key technical details:

The PCF8575 is typically available in a 24-pin package. Below is the pinout description:

1. Power Supply: Connect the VCC pin to a 2.5V-5.5V power source and the GND pin to ground.
2. I2C Connections: Connect the SDA and SCL pins to the corresponding I2C pins on the microcontroller. Use pull-up resistors (typically 4.7kΩ) on both lines.
3. Address Configuration: Set the I2C address by connecting A0, A1, and A2 to either VCC (logic HIGH) or GND (logic LOW). This allows up to 8 PCF8575 devices on the same I2C bus.
4. GPIO Pins: Use the P0.x and P1.x pins as inputs or outputs. Note that the pins are open-drain, so external pull-up resistors may be required for output operation.
5. Interrupt Pin: The INT pin can be used to detect changes on input pins. It is active LOW and requires an external pull-up resistor.

• Use decoupling capacitors (e.g., 0.1 µF) near the VCC pin to stabilize the power supply.
• Avoid exceeding the maximum current ratings to prevent damage to the device.
• When using the GPIO pins as inputs, ensure proper pull-up or pull-down resistors are used to avoid floating states.

Below is an example of how to use the PCF8575 with an Arduino UNO to toggle an LED connected to one of the GPIO pins:

#include <Wire.h>
#include <PCF8575.h> // Include the PCF8575 library

PCF8575 expander(0x20); // Initialize PCF8575 with I2C address 0x20

void setup() {
  Wire.begin(); // Start the I2C bus
  expander.begin(); // Initialize the PCF8575
  expander.pinMode(0, OUTPUT); // Set P0.0 as an output pin
}

void loop() {
  expander.digitalWrite(0, HIGH); // Turn on the LED connected to P0.0
  delay(1000); // Wait for 1 second
  expander.digitalWrite(0, LOW); // Turn off the LED
  delay(1000); // Wait for 1 second
}

• The PCF8575.h library can be installed via the Arduino Library Manager.
• Replace 0x20 with the actual I2C address of your PCF8575 if it differs.

1. Device Not Responding on I2C Bus:

   • Ensure the correct I2C address is used.
   • Verify that pull-up resistors are connected to the SDA and SCL lines.
   • Check the wiring for loose or incorrect connections.

2. GPIO Pins Not Functioning as Expected:

   • Confirm the pin mode (input/output) is correctly configured in the code.
   • Check for proper pull-up or pull-down resistors on the pins.

3. Interrupt Pin Not Triggering:

   • Ensure the INT pin is connected to a pull-up resistor.
   • Verify that the input pins are configured correctly and are changing state.

• Use an I2C scanner sketch to detect the PCF8575 on the I2C bus.
• Measure the voltage levels on the SDA and SCL lines to ensure they are within the expected range.
• Test the GPIO pins individually to isolate any faulty connections or components.

By following this documentation, you should be able to successfully integrate the PCF8575 I2C expander into your projects and troubleshoot any issues that arise.