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Family Overview
The Philips family of I2C/SMBus General Purpose parallel Input/Output (GPIO) devices provides a simple solution for application requiring more I/Os. About 20 different devices can be chosen from two sub-families, commonly called ‘quasi bi-directional General Purpose I/Os’ and ‘totem-pole General Purpose I/Os’. Devices can be chosen with 4-bit, 8-bit or 16-bit width. Additional features (not available on all the devices) are active-LOW Interrupt output, active-LOW Reset input, programmable I2C address pins and low power consumption.

Finally, some devices come with additional functions (EEPROM, DIP switch) providing integrated and price attractive combination solutions. Quasi bi-directional GPIOs use a push-pull I/O port with an internal weak current-source pull-up to keep the port HIGH since the upper transistor is on for only 1⁄ 2 clock cycle. Totem-pole GPIOs use a configuration register that programs an I/O port as either an input or output.

Devices with a Reset input pin (RESET) can be set to a known default state by a master device when, for instance, an ‘I2C-bus ...
... hung’ situation occurs, thus allowing the master to take control of the bus without having to cycle power to the device. Devices with an Interrupt output pin (INT) are able to provide an ‘input change’ status to a master device anytime an I/O used as an input changes its logic state. Programmable I2C address pins allow more than one device in the same I2C-bus without any address conflicts.

Low power consumption GPIOs are attractive for portable applications or in general any application where current consumption is a key parameter. The following is a selection guide that provides the designers and engineers with an overview of the similarities and important differences, allowing them to select the best port expander for the job. Even though important technical details are explained in this application note, the reader is encouraged to thoroughly review the data sheets for specific information on the device.

Keypad and switch control
ACPI power switch, relays, timer
LED control
Signal monitoring
Sensors, fan control

PCF8574T Circuit
Interfacing Arduino with I2C LCD circuit:

Example circuit diagram is shown in the image below.

The main component of the I2C LCD display is the PCF8574 I/O expander, with only two pins SDA and SCL we get a maximum of 8 pins from P0 to P7. PCF8574A also can be used but it has a different address.
All LCD data pins are connected to the PCF8574 where: RS, RW, E, D4, D5, D6 and D7 are connected to P0, P1, P2, P4, P5, P6 and P7 respectively.
PCF8574 I/O expander SDA and SCL pins are connected to Arduino pin A4 and A5 respectively (Arduino SDA and SCL pins).
PCF8574 I/O expander A0, A1 and A2 pins are the address pins which decide the I2C address of the chip. In this example each pin is connected to +5V through a 10k ohm resistor (the 10k resistor is optional, each pin can be connected directly to +5V).
The I2C address of the PCF8574 is: 0x20 | A2 A1 A0 ( | means OR)
In our circuit A2, A1 and A0 are connected to +5V (through 10k resistors) which means the I2C address is equal to 0x20 | 7 = 0x27
If the PCF8574A is used instead of the PCF8574 the I2C address is: 0x38 | 7 = 0x3F.

PCF8574T Raspberry Pi
Comimark 3Pcs PCF8574 PCF8574T I2C 8 Bit IO GPIO Expander Module for Arduino & Raspberry Pi

I2C interface, I / O expansion module, two I / O scalability 8 I / O (up to 8 simultaneous use the PCF8574 expanded to 64 I / O)
Typical application: I / O resource shortage MCU I / O expansion
Main resources: the PCF8574 I2C interface, 8-bit parallel
Package Included: 3 x PCF8574 IO Expansion Board I/O Expander I2C-Bus Evaluation Development Module

PCF8574T Chip Datasheet: File:PCF8574T Datasheet.pdf

WiringPi: [ http://wiringpi.com/extensions/i2c-pcf8574/ ]

telecnatron:[ https://telecnatron.com/articles/Utility-To-Control-1602-LCD-On-Raspberry-Pi-Via-A-PCF8574-I2C-Backpack-Module/index.html]

Circuitbasics:[ https://www.circuitbasics.com/raspberry-pi-i2c-lcd-set-up-and-programming/ ]


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