The NXP PCF8575CTS I/O Expander: Design, Application, and Interfacing Guide

In the realm of embedded systems and IoT design, the constant challenge is maximizing functionality while minimizing the use of critical microcontroller resources. The NXP PCF8575CTS stands as a pivotal solution, providing a straightforward and cost-effective method for I/O expansion. This 16-bit remote I/O port is designed for I2C-bus applications, enabling a master microcontroller to control a vast array of inputs and outputs using just two wires.

Design and Internal Architecture

The PCF8575CTS is architecturally simple yet powerful. It is a 16-bit device housed in a compact SSOP24 package, making it suitable for space-constrained applications. Its core function is to add 16 quasi-bidirectional I/O ports via a single I2C bus address. The quasi-bidirectional port design is a key feature; each pin can be used as an input or output without a dedicated control register for data direction. Upon power-up, all ports are configured as inputs (high-impedance state). When written high, they can serve as inputs or as weakly driven high outputs; when written low, they are strongly driven as low outputs.

Communication is handled through the industry-standard I2C serial interface, supporting clock frequencies up to 400 kHz (Fast-mode). The device features three hardware address pins (A0, A1, A2), allowing up to eight identical PCF8575 devices to be connected on the same I2C bus, thereby expanding a system by up to 128 I/O lines with minimal wiring.

A critical design feature is the open-drain interrupt output (/INT). This pin is activated whenever any input port’s state differs from its corresponding register’s state. This allows the PCF8575 to signal the master microcontroller that an input change has occurred, eliminating the need for continuous polling and significantly improving system efficiency.

Key Applications

The versatility of the PCF8575CTS makes it ideal for a wide range of applications:

Driving Actuators: Controlling large sets of LEDs, relays, solenoids, or fans.

Reading Sensors: Monitoring the state of multiple digital switches, buttons, or level detectors.

Industrial Control Panels: Expanding the number of buttons or indicators for human-machine interfaces (HMIs).

System Monitoring: Gathering fault or status signals from various subsystems in a server or telecom hardware.

Interfacing Guide

Interfacing the PCF8575CTS with a microcontroller (e.g., an Arduino, ESP32, or STM32) is remarkably simple.

1. Hardware Connection: Connect the SDA (serial data) and SCL (serial clock) lines to the corresponding pins on your microcontroller. Pull-up resistors (typically 4.7kΩ) are required on both these lines to the positive supply voltage. Connect the /INT pin to a microcontroller interrupt pin if you wish to use the interrupt functionality. Set the address pins (A0-A2) to unique logic levels for each expander on the bus.

2. Software Communication: The I2C protocol sequence is straightforward:

Writing to Outputs: The master sends a start condition, the slave address (with R/W bit low), followed by two data bytes. These bytes set the logic state of all 16 ports (P07-P00 and P17-P10).

Reading from Inputs: The master sends a start condition, the slave address (with R/W bit high), and then reads two data bytes. These bytes represent the logic level present on each of the 16 pins.

A critical consideration is that a read operation actually returns the last value written to the port latch, not the physical pin state, unless an input change has been latched via the interrupt mechanism. For reading inputs, the corresponding port latch bit must first be written high to configure the pin as an input.

ICGOOODFIND

The NXP PCF8575CTS is an indispensable component for system designers. Its elegant simplicity, minimal hardware footprint, and interrupt-driven architecture solve the pervasive problem of I/O limitation efficiently. By leveraging the ubiquitous I2C bus, it allows microcontrollers to effortlessly scale their control and monitoring capabilities, making it a perennial favorite in prototyping and production alike.

Keywords:

I/O Expander

I2C Bus

Quasi-bidirectional

Interrupt Output

PCF8575