Additional information and document update here on my site: pcf8574 Article.

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

Version 2.0

Library to use i2c analog IC with arduino and esp8266. Can read and write digital value with only 2 wire (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.

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 operation.

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

Constructor: you must pas the address of i2c (to check the adress use this guide I2cScanner)

	PCF8574(uint8_t address);

for esp8266 if you want specify SDA e SCL pin 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 have 8 digital input/output:

So 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 memori device, if you decomment this line on .h file of the library:

// #define PCF8574_LOW_MEMORY

Enable low memory props and gain about 7byte 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, here an 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 read a single input:

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

If you want write a digital value you must do:

	PCF8574.digitalWrite(P1, HIGH);

or:

	PCF8574.digitalWrite(P1, LOW);

You can also use 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 interrupt function.

The better way is to set only 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: