The PCF8574T/3: NXP's Remote 8-Bit I/O Expander for I²C-Bus Applications

In the world of embedded systems and IoT, a common challenge is the scarcity of General-Purpose Input/Output (GPIO) pins on a microcontroller. As projects increase in complexity, the need for more sensors, buttons, LEDs, and other peripherals quickly outstrips the available I/O. This is where I/O expanders become invaluable. The PCF8574T/3 from NXP Semiconductors is a quintessential solution, providing a simple and effective method to add eight vital I/O ports using the ubiquitous I²C-bus.

This device serves as an interface between a standard I²C-bus and a parallel 8-bit I/O port. Its core function is to provide remote I/O expansion for most microcontroller families via the two-wire I²C-bus protocol (serial data SDA and serial clock SCL). This dramatically reduces the number of traces on a PCB and frees up critical microcontroller pins for other functions. Each of the eight quasi-bidirectional I/O pins can be independently configured to function as an input or an output without the need for a data direction register.

A key feature of the PCF8574T/3 is its simple operational methodology. To read data, the microcontroller reads a byte from the device; the logic levels present on the P0-P7 pins at that moment are returned. To write data, the microcontroller sends a byte to the device, which then sets the output latches accordingly. This straightforward read/write approach, combined with very low current consumption, makes it ideal for power-sensitive applications.

Furthermore, the PCF8574T includes an open-drain interrupt output line (INT) which becomes active low whenever an input changes state. This is particularly useful for efficient system design, as it allows the microcontroller to enter a low-power sleep mode and be awakened only when a change on an input (like a button press) occurs, rather than constantly polling the device.

The PCF8574T/3 finds its home in a vast array of applications, from driving LCD displays and reading matrices of keys to controlling LEDs and monitoring status switches. Its ability to operate over a wide voltage range (2.5V to 6V) and its tolerance for higher voltages on its I/O pins further enhance its versatility.

ICGOODFIND: The NXP PCF8574T/3 remains a fundamental component for electronics designers, offering a cost-effective, simple, and efficient solution for I²C-based I/O expansion that has stood the test of time.

Keywords: I²C-bus, GPIO Expander, Quasi-bidirectional I/O, Remote I/O, Interrupt Output.