MCP23017 I2C expander code help

Projects Programming
1

Hello!

I've been trying to understand I2C lately and I figured a MCP23017 would be a very simple chip to do it with. I've been following the code here

// pins 15~17 to GND, I2C bus address is 0x20

#include "Wire.h"

void setup()
{
  Wire.begin(); // wake up I2C bus

  // setup addressing style
  Wire.beginTransmission(0x20);
  Wire.write(0x12);
  Wire.write(0x20); // use table 1.4 addressing
  Wire.endTransmission();

  // set I/O pins to outputs
  Wire.beginTransmission(0x20);
  Wire.write(0x00); // IODIRA register
  Wire.write(0x00); // set all of bank A to outputs
  Wire.write(0x00); // set all of bank B to outputs
  Wire.endTransmission();
}

void binaryCount()
{
  for (byte a=0; a<256; a++)
  {
    Wire.beginTransmission(0x20);
    Wire.write(0x12); // GPIOA
    Wire.write(a);    // bank A
    Wire.write(a);    // bank B
    Wire.endTransmission();
    delay(100);
  }
}

void loop()
{
  binaryCount();
  delay(500);
}

I found online.

The main thing is that I can't figure out the 0x20 register anywhere on the datasheet. I just don't get this whole thing. Can somebody explain?

2

The 0x20 is the address of the chip when all three address lines are pulled low.

Using the address lines you can select a chip address of anything between 0x20 and 0x27.

This address is the device address of the chip on the I²C bus.

You start an I²C transaction with a device (0x20), send the data, then finish the transaction.

3

It is on page 8 of the data sheet. Note the arduino does not include the read / write as part of the address so that bit pattern is shifted one place to the right.