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

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: