PCF8574T 8-Bit I2C GPIO Expander for LCD and Embedded Control

ElecSuppliers Part Number GuideAug 22, 2026

PCF8574T is an 8-bit I2C-bus GPIO expander designed for embedded systems that need additional digital input and output lines. Instead of using a separate microcontroller pin for every switch, LED, control signal or display connection, the device allows multiple GPIO functions to be controlled through a two-wire I2C interface.

This makes PCF8574T particularly useful in designs where the microcontroller has limited GPIO resources but already includes an I2C peripheral.

One of the most common applications is adding an I2C interface to a character LCD such as a 1602 or 2004 display. PCF8574T can also be used for keypad scanning, status indicators, switches, control signals and simple peripheral expansion.

PCF8574T Specifications

PCF8574T provides eight quasi-bidirectional I/O pins.

The device communicates with the host controller through the I2C bus and uses two lines for serial communication: SDA for data and SCL for clock.

The I/O pins can be used for both input and output functions, allowing the same device to support a mixture of switches, LEDs and control signals.

PCF8574T operates from a low-voltage supply and is designed for low-power digital applications.

The device is available in several package configurations, with PCF8574T commonly associated with a 16-pin surface-mount package.

PCF8574T I2C Interface

The main advantage of PCF8574T is its I2C interface.

Only SDA and SCL are required for communication with the microcontroller, in addition to power and ground.

This allows the eight I/O pins to be controlled without consuming eight MCU GPIO pins.

Because I2C is a shared bus, other compatible devices can operate on the same SDA and SCL lines.

The microcontroller selects the required device through its I2C address before reading or writing the GPIO state.

PCF8574T GPIO Expansion

A microcontroller may have enough GPIO pins for its main functions but run out of pins when additional switches, LEDs or display signals are added.

PCF8574T provides a simple way to solve this problem.

For example, eight additional digital signals can be controlled using only the I2C bus.

The I/O lines can be assigned to different functions depending on the application.

A product could use several pins for buttons, several for LEDs and the remaining pins for control signals.

This flexibility is useful in embedded control boards where GPIO requirements change during product development.

PCF8574T Quasi-Bidirectional I/O

PCF8574T does not use conventional push-pull GPIO architecture for every I/O line.

Its pins use a quasi-bidirectional structure that allows them to operate as inputs or outputs with appropriate software control.

When a pin is used as an input, the corresponding latch must be configured appropriately so that the external signal can influence the pin state.

When configured as an output, the device can drive the connected load according to the I/O state.

Designers should understand this architecture before connecting external circuits that require strong push-pull drive.

PCF8574T Pinout

PCF8574T provides eight GPIO pins generally designated P0 through P7.

The device also includes SDA and SCL for I2C communication.

Additional pins provide power, ground, interrupt functionality and address selection.

The address inputs allow the host controller to distinguish between multiple devices on the same I2C bus.

The exact pin numbering depends on the package, so the package-specific datasheet should be checked during PCB design.

PCF8574T Address Pins

Address selection allows multiple PCF8574 devices to share one I2C bus.

The address inputs can be connected to different logic levels to establish different device addresses.

This makes it possible to expand the total number of available GPIO lines beyond the eight provided by one device.

For example, separate expanders can be assigned to an LCD, keypad and front-panel indicators while sharing the same SDA and SCL connections.

The available address range depends on the exact PCF8574 family member being used.

PCF8574T Interrupt Output

PCF8574T includes an interrupt output that can notify the microcontroller when an input state changes.

This can be useful in keypad and switch applications.

Instead of repeatedly polling every input, the MCU can wait for the interrupt signal and then read the GPIO state through I2C.

This can reduce unnecessary processor activity and simplify event-driven input handling.

For battery-powered products, interrupt-based input detection can also help reduce the time the MCU needs to remain active.

PCF8574T for LCD 1602

One of the most recognizable applications for PCF8574T is controlling a 1602 character LCD through I2C.

A traditional parallel 1602 interface can require several MCU GPIO pins.

PCF8574T converts those display-control signals into an I2C-controlled interface.

The eight GPIO pins can be assigned to the LCD's data and control lines.

The MCU then communicates with the PCF8574T through SDA and SCL rather than directly controlling every LCD pin.

This approach significantly reduces the number of MCU pins needed for a character display.

PCF8574T for LCD 2004

The same concept can be used with a 2004 character LCD.

The display requires several parallel data and control signals, while PCF8574T acts as the GPIO bridge between the I2C bus and the LCD.

This is useful in industrial panels, laboratory equipment, controllers and DIY electronics where a text display is required but MCU GPIO resources are limited.

The software typically sends LCD commands and data through the I2C expander.

PCF8574T Keypad Interface

PCF8574T can also be used with matrix keypads.

A keypad matrix requires several row and column signals, which can consume a significant number of MCU GPIO pins.

The expander can provide these signals through the I2C bus.

The controller changes selected output states and reads the corresponding input states to determine which key is pressed.

The interrupt output can further improve the design by notifying the MCU when a keypad event occurs.

PCF8574T for LED Control

PCF8574T can provide additional digital outputs for indicator LEDs.

A control panel might use the eight GPIO lines to drive status indicators representing power, communication, alarms or operating modes.

For low-current indicator applications, the outputs can be used directly when the electrical requirements remain within the device's specifications.

Higher-current LEDs should use external transistor or driver stages.

This keeps the GPIO expander responsible for logic control rather than demanding excessive current from its output pins.

PCF8574T Input Applications

The I/O pins can also be used for digital input signals.

Buttons, switches, limit sensors and other logic-level signals can be connected to the expander.

The microcontroller can read the state of the GPIO port through I2C.

For mechanical buttons, software debounce is normally required because switch contacts can generate multiple transitions during a single press.

The interrupt function can be used to detect a change and then allow firmware to determine the final stable state.

PCF8574T for Arduino

PCF8574T is frequently used with Arduino-compatible microcontrollers.

An Arduino board can communicate with the device using its I2C pins and control eight additional digital signals.

This is particularly useful for projects involving LCD displays, keypads, LEDs and multiple switches.

The I2C connection also leaves the remaining Arduino GPIO pins available for other peripherals.

Because PCF8574 devices are widely supported by embedded software libraries, integration can be relatively straightforward.

PCF8574T for Embedded Systems

The device is useful beyond hobby projects.

Embedded controllers can use PCF8574T to manage front-panel buttons, indicator lights, simple actuators and configuration switches.

It can also provide additional control lines for industrial instruments and compact control panels.

Using an I2C expander can reduce MCU package requirements in some designs because the main processor does not need to expose a large number of dedicated GPIO pins.

PCF8574T Power Supply

PCF8574T operates from a low-voltage supply suitable for common embedded systems.

The supply should be properly decoupled near the device.

Because the device shares the PCB with the MCU and other digital components, local bypassing helps reduce supply disturbances during GPIO switching and I2C activity.

The voltage levels of the I2C bus and external GPIO signals should also be considered when connecting the device to a 3.3V or 5V system.

PCF8574T I2C Pull-Up Resistors

I2C requires pull-up resistors on the SDA and SCL lines.

The appropriate resistance depends on bus voltage, capacitance, communication speed and the number of connected devices.

Too much resistance can result in slow rising edges, while excessively low resistance increases current consumption.

When several I2C devices share the same bus, the total bus capacitance should be considered rather than selecting pull-ups for PCF8574T alone.

PCF8574T PCB Layout

PCF8574T does not require the type of high-speed layout associated with Ethernet or high-speed ADC circuits.

However, clean I2C routing is still useful.

SDA and SCL should be routed away from particularly noisy switching nodes when practical.

The supply bypass capacitor should be located close to the device.

If the GPIO pins connect to long external cables, switches or industrial signals, additional filtering or protection may be necessary.

PCF8574T vs Microcontroller GPIO

The main reason to use PCF8574T is to expand GPIO without changing the main MCU.

If an application needs only one or two additional signals, adding an external GPIO expander may not be worthwhile.

When the design needs eight or more extra lines, however, the I2C approach can significantly reduce MCU pin usage.

The trade-off is that accessing the pins takes place through the I2C bus rather than through direct MCU registers.

This makes PCF8574T more suitable for relatively slow control and input tasks than timing-critical GPIO functions.

PCF8574T vs PCF8574A

PCF8574 and PCF8574A are closely related I2C GPIO expanders.

One of the key differences is the I2C address range.

This distinction matters when designing a system with multiple expanders on one bus.

A PCF8574T should therefore not automatically be replaced with a PCF8574A simply because both provide eight I/O lines.

The address configuration and complete device variant should be checked before making a substitution.

PCF8574T Replacement

When selecting a PCF8574T replacement, designers should compare the number of GPIO pins, I2C address range, operating voltage, I/O characteristics and interrupt behavior.

The quasi-bidirectional architecture is also important.

A conventional push-pull GPIO expander may not behave identically even if it provides the same number of pins.

Package dimensions and pin assignments must also be checked when the goal is a PCB-level replacement.

PCF8574T Alternative

An alternative GPIO expander may be preferable when the application requires higher-speed GPIO access, stronger output drive, more configurable pins or a different communication interface.

SPI GPIO expanders can provide another option when SPI is already available and faster access is important.

Modern I2C GPIO expanders may also provide more configuration features, such as programmable pull-ups, polarity inversion or per-pin direction control.

PCF8574T remains attractive when a simple eight-bit I2C expansion solution is sufficient.

PCF8574T Applications

PCF8574T can be used for LCD interfaces, keypad scanning, LED indicators, switch inputs, front-panel controls, industrial control panels, embedded controllers and general-purpose GPIO expansion.

Its value comes from using a small number of I2C connections to provide access to multiple digital signals.

For designs with relatively slow user-interface and control functions, this can be a simple and cost-effective way to extend an MCU's capabilities.

Choosing PCF8574T

PCF8574T is a practical choice when a microcontroller needs eight additional digital I/O lines through an I2C interface.

Its strongest applications include character LCDs, keypads, buttons, status LEDs and simple embedded control signals.

Before selecting it for a new design, engineers should check the I/O drive requirements, I2C address availability, pull-up configuration and supply-voltage compatibility.

For replacement projects, the exact package, address variant and quasi-bidirectional I/O behavior should be verified before using another GPIO expander.