Introduction: 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.
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)
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(di.p1); Serial.print(" - "); Serial.print(di.p2); Serial.print(" - "); Serial.println(di.p3);
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:
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);