keybrd Tutorial 4 - split keyboard with I/O Expander
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When you finish this tutorial you will be able to be able to modify a 2-matrix keybrd sketch to suite your own split keyboard design.

## Overview of split keyboard with I/O Expander
The breadboard in this picture models a split keyboard.
![breadboard keyboard with 2 rows and 4 columns of keys](images/breadboard_keyboard_2x5_labeled.jpg "2x5 breadboard keyboard")

The breadboard's four bus strips are used as rows.
Two rows (blue bus strips) are connected to the microcontroller.
Two rows (red bus strips) are connected to the shift registers.

The breadboard's four bus strips are used as rows.
Two rows connect to a microcontroller, and two rows connected to a I/O expander.

The I/O expander has a small notch on one end, which identifies pin 1.
In the picture, pin 1 is on the right end.

The microcontroller and I/O expander are connected by 4 jumper wires:
* ground
* power
* Serial CLock signal (SCL)
* Serial DAta signal (SDA)

A decoupling capacitor on the power pin dampens noise coming in through the power and ground wires.

The microcontroller and I/O expander communicate via [I2C](http://en.wikipedia.org/wiki/I%C2%B2C) bus, which consists of two signals: SCL and SDA.
Two resistors pull-up voltage on the SCL and SDA.

I/O expander I2C address is configured by hardware pins.
The MCP23018 with all address pins grounded has an I2C address of ?? todo.

The I/O expander has two ports.  Each port has eight pins.
One port is connected to the matrix's rows.  The other port is connected to the matrix's columns.

## Building a split keyboard with I/O Expander
We will build a split keyboard adding parts to the basic breadboard keyboard described in [tutorial_1_breadboard_keyboard.md](tutorial_1_breadboard_keyboard.md)

todo add schematic

<!-- schematic with IOE power decoupling capacitor
This schematic was written by consulting the I/O expander's datasheet and using the ?? tool. -->

Continuing from the basic breadboard keyboard instructions:

<!--  At some point in the future, Markdown may support starting ordered lists at an arbitrary number. -->

4. Insert the I/O expander

5. Install I/O expander power
 * ground
 * power
 * capacitor

6. Install I2C bus
 * SCL
 * SDA
 * pull-up resistors on SCL and SDA

7. configure I2C address

8. Assemble key matrix as shown in the picture.

9. Connect I/O expander ports to matrix rows and columns

## Sketch for split keyboard with I/O Expander
The [keybrd_4_split_with_IOE_annotated.ino](keybrd_4_split_with_IOE_annotated/keybrd_4_split_with_IOE_annotated.ino)
 sketch explains how the I/O Expander works on a keyboard.

<a rel="license" href="http://creativecommons.org/licenses/by/4.0/"><img alt="Creative Commons License" style="border-width:0" src="https://i.creativecommons.org/l/by/4.0/88x31.png" /></a><br /><span xmlns:dct="http://purl.org/dc/terms/" property="dct:title">keybrd tutorial</span> by <a xmlns:cc="http://creativecommons.org/ns#" href="https://github.com/wolfv6/keybrd" property="cc:attributionName" rel="cc:attributionURL">Wolfram Volpi</a> is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by/4.0/">Creative Commons Attribution 4.0 International License</a>.<br />Permissions beyond the scope of this license may be available at <a xmlns:cc="http://creativecommons.org/ns#" href="https://github.com/wolfv6/keybrd/issues/new" rel="cc:morePermissions">https://github.com/wolfv6/keybrd/issues/new</a>.