====== I2C pcf8574 8bit port expander ======
I made a test / prototype setup on a breadboard with two pcf8574 ics ; one for output and one for input. Both connected to the same I2C bus.

Adding more digital inputs to an Arduino was my first objective, as I am planning to use this on a project which has buttons that control a menu.

For now this little write-up should suffice for the moment I am going to design a PCB for this.

===== The breadboard built =====

The four resistors are 100Ω

The four LEDs are regular red 3mm leds

The four buttons are momentary on push buttons

==== Fritzing ====
{{ :projects:i2c_pcf8574_8bit_port_expander:pcf8574-arduino-io_bb.png?400|}}
The program called [[http://fritzing.org/welcome/|Fritzing]] does a nice job of making an understandable picture of the board. 
This is not only nice for using in a write-up but in real-live it is faster than pulling everything from your breadboard just to place components a couple of holes further away (because you need the extra space).

Fritzing can also make a schematic, I've included it here but it looks horrible. When I make the final design on a PCB I will use [[http://www.cadsoftusa.com/|Eagle]] for this.
{{ :projects:i2c_pcf8574_8bit_port_expander:pcf8574-arduino-io_schem.png?200 |}}

==== Breadboard ====
I took a photo of the breadboard build, nice to see but difficult to see where what goes:
{{ :projects:i2c_pcf8574_8bit_port_expander:2012-10-21_11-36-11_88.jpg?200 | }}
{{ :projects:i2c_pcf8574_8bit_port_expander:2012-10-21_15-52-19_558.jpg?200 | }}

==== The code ====

The [[http://www.arduino.cc/playground/Learning/Linux|Arduino IDE]] has a library called [[http://arduino.cc/en/Reference/Wire|Wire]] which is used for I2C stuff.

I wrote code that reads the button input and writes this to the LED output:

<file c PCF8574.ino>
// I2C PCF8574 8 bit i/o port expander
// by AEP
//
// testing / prototyping the Wire library in combination with the ic PCF8574
// Reads data from PCF8574 over I2C and sends data to another PCF8574 over the same I2C bus


// Created 21 oct 2012

#include <Wire.h>

byte iInput=0;
byte iOutput=0;

void setup()
{
  Wire.begin();
}

void loop()
{
  Wire.requestFrom(33,1);// Begin transmission to PCF8574 with the buttons
  if(Wire.available())   // If bytes are available to be recieved
  {
    iInput = Wire.read();// Read a byte
  }
  
  if(iInput<255)         //If the value less than 255
  {
    if (iInput==254) // P0
    { 
      iOutput = 1; 
    }; 
    if (iInput==253) // P1
    { 
      iOutput = 2; 
    }; 
    if (iInput==251) // P2
    { 
      iOutput = 4; 
    }; 
    if (iInput==247) // P3
    { 
      iOutput = 8; 
    }; 
  }
  Wire.beginTransmission(32);  //Begin transmission to PCF8574 (with the LEDs)
  Wire.write(iOutput);         //Send data to PCF8574 (with the LEDs)
  Wire.endTransmission();      //End Transmission to PCF8574 (with the LEDs)
}

</file>