PCF8574 (i2c Digital I/O Expander) Fast Easy Usage

About: Software developer but I love electronics, wood, nature, and everything else as well.

Intro: PCF8574 (i2c Digital I/O Expander) Fast Easy Usage

Library to use i2c pcf8574 IC with arduino and esp8266.

This IC can control (until 8) digital devices like button or led with 2 only pins.

Can read and write digital value with only 2 wire (perfect for ESP-01).

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

Step 1: How I2c Works

I2C works with it's two wires, the SDA(data line) and SCL(clock line).

Both these lines are open-drain, but are pulled-up with resistors.

Usually there is one master and one or multiple slaves on the line, although there can be multiple masters, but we'll talk about that later.

Both masters and slaves can transmit or receive data, therefore, a device can be in one of these four states: master transmit, master receive, slave transmit, slave receive.

Step 2: Library

You can find my library here.

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 PCF8574 library folder your /libraries/ folder.

You may need to create the libraries subfolder if its your first library.

Restart the IDE.

Step 3: Usage

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); 

Step 4: Read Value

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(" - ");
	Serial.print(" - ");

if you want read a single input:

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

Step 5: Write Value

If you want write a digital value you must do:

	PCF8574.digitalWrite(P1, HIGH); 


	PCF8574.digitalWrite(P1, LOW);

Step 6: Interrupt Pin

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);

Step 7: Examples Wiring Diagram

Step 8: Thanks



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    5 Discussions


    9 months ago

    why im still wrong when PCF8574(uint8_t 0x38);
    IDE said "expected primary-expression before '(' token" why?
    thank u

    1 reply

    Reply 9 months ago

    Hi, Send me the sketch file, seems an error on code.


    Reply 11 months ago


    I think the i2c components are very usefully, my first 3 instructable are on i2c components, this, PCF8591 for analog expander and DHT12 temp humidity sensor.

    And I try to create a simple library for all this components as you.


    11 months ago

    Thanks for sharing :)