PCF8574 PCF8574AP digital input and output expander with i2c bus.

Complete documentation on my site: pcf8574 Article.

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

Changelog

01/02/2024: v2.3.7 Add the possibility to insert address at begin()
10/07/2023: v2.3.6 Support for Arduino UNO R4
08/02/2023: v2.3.5 Fix STM32 support and add support for Raspberry Pi Pico and other rp2040 boards
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)

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

Tested with esp8266, esp32, Arduino, Arduino SAMD (Nano 33 IoT, MKR etc.), STM32 and rp2040 (Raspberry Pi Pico and similar)

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:

PCF8574 schema

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/