Additional information and documentation on my site: pcf8574 Article.

If you need more pins here you can find the pcf8575 16bit version of the IC.

Version 2.2

Library to use I2C analog IC with arduino and esp8266. Can read and write digital value with only 2 wires (perfect for ESP-01).

Tutorial:

To download. click the DOWNLOADS button in the top right corner, rename the uncompressed folder PCF8574. Check that the PCF8574 folder contains PCF8574\\.cpp and PCF8574.h. Place the DHT library folder your <arduinosketchfolder>/libraries/ folder. You may need to create the libraries subfolder if its your first library. Restart the IDE.

Changelog

10/08/2022: v2.3.4 Add support for custom SERCOM interface of Arduino SAMD devices. Force SDA SCL to use GPIO numeration for STM32 bug (https://www.mischianti.org/forums/topic/compatible-with-stm32duino/). 28/07/2022: v2.3.3 Force SDA SCL to use GPIO numeration (https://www.mischianti.org/forums/topic/cannot-set-sda-clk-on-esp8266/). 28/07/2022: v2.3.2 Fix the SDA SCL type #58 and add basic support for SAMD device. 26/04/2022: v2.3.1 Fix example for esp32 and double begin issue #56. 06/04/2022: v2.3.0 Fix package size 30/12/2021: v2.2.4 Minor fix and remove deprecated declaration 23/11/2020: v2.2.2 Add multiple implementation for encoder management (you can enable by uncomment relative define)

Reef complete PCF8574 PCF8574AP digital input and output expander with i2c bus.

I try to simplify the use of this IC, with a minimal set of operations.

PCF8574P address map 0x20-0x27 PCF8574AP address map 0x38-0x3f

Constructor: Pass the address of I2C (to check the address use this guide I2cScanner)

	PCF8574(uint8_t address);

For ESP8266 if you want to specify SDA and SCL pins use this:

	PCF8574(uint8_t address, uint8_t sda, uint8_t scl);

You must set input/output mode:

	pcf8574.pinMode(P0, OUTPUT);
	pcf8574.pinMode(P1, INPUT);
	pcf8574.pinMode(P2, INPUT);

then IC as you can see in the image has 8 digital input/output ports:

To read all analog input in one trasmission you can do (even if I use a 10millis debounce time to prevent too much read from i2c):

	PCF8574::DigitalInput di = PCF8574.digitalReadAll();
	Serial.print(di.p0);
	Serial.print(" - ");
	Serial.print(di.p1);
	Serial.print(" - ");
	Serial.print(di.p2);
	Serial.print(" - ");
	Serial.println(di.p3);

To follow a request (you can see It on issue #5) I create a define variable to work with low memory devices, if you uncomment this line in the .h file of the library:

// #define PCF8574_LOW_MEMORY

Enable low memory props and gain about 7 bytes of memory, and you must use the method to read all like so:

   byte di = pcf8574.digitalReadAll();
   Serial.print("READ VALUE FROM PCF: ");
   Serial.println(di, BIN);

where di is a byte like 1110001, so you must do a bitwise operation to get the data, operation that I already do in the "normal" mode. For example:

   p0 = ((di & bit(0))>0)?HIGH:LOW;
   p1 = ((di & bit(1))>0)?HIGH:LOW;
   p2 = ((di & bit(2))>0)?HIGH:LOW;
   p3 = ((di & bit(3))>0)?HIGH:LOW;
   p4 = ((di & bit(4))>0)?HIGH:LOW;
   p5 = ((di & bit(5))>0)?HIGH:LOW;
   p6 = ((di & bit(6))>0)?HIGH:LOW;
   p7 = ((di & bit(7))>0)?HIGH:LOW;

if you want to read a single input:

	int p1Digital = PCF8574.digitalRead(P1); // read P1

If you want to write a digital value:

	PCF8574.digitalWrite(P1, HIGH);

or:

	PCF8574.digitalWrite(P1, LOW);

You can also use an interrupt pin: You must initialize the pin and the function to call when interrupt raised from PCF8574

// Function interrupt
void keyPressedOnPCF8574();

// Set i2c address
PCF8574 pcf8574(0x39, ARDUINO_UNO_INTERRUPT_PIN, keyPressedOnPCF8574);

Remember you can't use Serial or Wire on an interrupt function.

It's better to only set a variable to read on loop:

void keyPressedOnPCF8574(){
	// Interrupt called (No Serial no read no wire in this function, and DEBUG disabled on PCF library)
	 keyPressed = true;
}

For the examples I use this wire schema on breadboard:

https://downloads.arduino.cc/libraries/logs/github.com/xreef/PCF8574_library/