View source for I/O Expander Shield with MCP23017

==Description==

I/O Expander shield is a shield used to expand the number of I/Os of an Arduino Uno. It is based on the chipset MCP23017. The chipset MCP23017 communicates with Arduino Uno through I2C interface. It adds GPIOA and GPIOB, a total of 16 I/Os. There are two LEDs and two buttons on the shield. User can decide if they want to use the built-in LED/buttons or not by using the 4-bit DIP switch. 

[[File:IOExpand shield 1.jpg|400px]]
[[File:IOExpand shield 2.jpg|400px]]

==Schematics==
*[http://cnlearn.linksprite.com/wp-content/uploads/2014/04/io-expander-shield-SCH.pdf Schematic]

==Code==
<syntaxhighlight lang="c">
#include <Wire.h>
 
const byte  mcp_address=0x20;      // I2C Address of MCP23017 Chip
const byte  GPIOA=0x12;            // Register Address of Port A
const byte  GPIOB=0x13;            // Register Address of Port B
const byte  IODIRA=0x00;            // IODIRA register
const byte  IODIRB=0x01;            // IODIRB register
 
void setup()
{
  //Send settings to MCP device
  Wire.begin();              // join i2c bus (address optional for master)
  Wire.beginTransmission(mcp_address);
  Wire.write((byte)IODIRA); // IODIRA register
  Wire.write((byte)0x03); // set GPIOA-0/GPIOA-1 to inputs
  Wire.endTransmission();
}
 
void loop()
{
  Wire.beginTransmission(mcp_address);
  Wire.write((byte)GPIOA); // set MCP23017 memory pointer to GPIOB address
  Wire.endTransmission();
  Wire.requestFrom(0x20, 1); // request one byte of data from MCP20317
  int inputs=Wire.read(); // store the incoming byte into "inputs"
 
  if((inputs&0x01)==0)
  {
    Wire.beginTransmission(mcp_address);
    Wire.write(GPIOA);    // address bank A
    Wire.write((byte)0x04);  // value to send GPIOA-2 HIGH
    Wire.endTransmission();
  }
  if((inputs&0x02)==0)
  {
    Wire.beginTransmission(mcp_address);
    Wire.write(GPIOA);    // address bank A
    Wire.write((byte)0x08);  // value to send GPIOA-3 HIGH
    Wire.endTransmission();
  }
  else
  {
    Wire.beginTransmission(mcp_address);
    Wire.write(GPIOA);     // address bank A
    Wire.write((byte)0x00);  // value to send GPIOA LOW
    Wire.endTransmission();
  }
}
[/syntaxhighlight]

==Resources==
*[http://ww1.microchip.com/downloads/en/DeviceDoc/21952b.pdf Datasheet]