MCP23008 Button input Board

This board has like.. a bunch of buttons on it. It is controlled by the MCP23008E/P I2C I/O Expander.

It supports configurable 3 bit i2c addresses and allows upto 8 devices on the i2c bus.

The datasheet for the IC can be found on the Microchip website

Pictures

The IC

The Schematic

Some Crappy pictures of the assembled board

nb: Adding the Resistor Network was a small oversight and is not present in this pic

I/O

The board has 2 JST-XH4 headers on it to allow further devices to be daisy-chained.

Either header can be used for input and output.

The board is designed to take 5V however it should support 1.8V to 5.5V, but this has not been tested.

The header pinout is as follows:

------
| +5V|
 |SCL|
 |SDA|
| Gnd|
------

Buttons

The button layout is arranged

All 8 buttons can be read at once in a single byte and upto 8 boards can be read sequencially. The buttons are pulled down to ground via a 10k Resistor Network and pull high when pressed, do not configure the internal pull-ups.

I2C addresses

The dip switches are labelled in the opposite order to the address lines because they are.

Switch 3 = lowest settable bit Switch 1 = highest settable bit

0b0100[1][2][3][r/w]

Configuring the address lines while the device is powered is undefined behavour, so probably don't do that.

Some developement boards require 10k pull-up resistors on the i2c data lines as they are not on the board, the Arduino Uno does, the Arduino Mega does not. Check your developement boards documentation to see if you need this.

Interrupts

The interrupt pin has not been wired, but if you want to add it yourself, it is Pin 8 on the IC. Details for how to use it are listed in the datasheet.

The Adafruit MCP23008 Library does not implement the interrupt, so you'll need to create this yourself

Software interface

There is an Adafruit Arduino library but it is pretty bearbones and has not been updated in a number of years. The GitHub repo for the library can be found here.

Some sample code is as follows:

#### `mcp23008.ino`
#include <Wire.h>
#include "Adafruit_MCP23008.h"

Adafruit_MCP23008 mcp1;
Adafruit_MCP23008 mcp2;
  
void setup() {  
  mcp1.begin();      // use default address 0
  mcp2.begin(1);
  Serial.begin(9600);
  Serial.println("Hi");

  for (int i=0; i<8; i++) {
    mcp1.pinMode(i, INPUT);
    mcp2.pinMode(i, INPUT);
  }
}
void loop() {
  Serial.print("Board 1: ");
  for (int i=0; i<8; i++) {
    Serial.print(mcp1.digitalRead(i));
  }
  Serial.println();
  
  Serial.print("Board 2: ");
  for (int i=0; i<8; i++) {
    Serial.print(mcp2.digitalRead(i));
  }
  Serial.println();
}

This will enumerate through each pin alternating between two boards and output the results into the serial console. The boards can be read a byte at a time, but the library does not implement this.

BOM

1x 5x7CM Perfboard with 0.1" pin spacing
1x MCP23008E/P
8x Buttons
2x 100nF Ceramic Capacitors
1x 9 pin common ground 10k Resistor Network
1x 4 pin common ground 10k Resistor Network (3x 10k Metal Film Resistors were used here)
2x JST-XH 4 pin male headers
1x 6 pin SPST DIP Switch
?x bits of wire

Annon201/MCP23008.md