Tutorial 10 - writing new IOE Port classes
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Port classes are the keybrd library's interface to I/O expander ports.

Steps to writing a new port class:

1. Get a copy of the I/O expander's datasheet.
2. An I/O expander will use one of two communication protocols: [http://www.byteparadigm.com/applications/introduction-to-i2c-and-spi-protocols/](SPI or I2C).
   Refer to the [Arduino SPI](https://www.arduino.cc/en/Reference/SPI)
   or [Arduino Wire (I2C)](https://www.arduino.cc/en/Reference/Wire) library
3. Get familiar with your I/O expander.
 * Different I/O expanders use different commands (a.k.a. operation codes).
   Refer to your I/O expander's datasheet for read and write commands.
 * Search for Arduino sketch examples containing your I/O expander
   ([sumotoy](https://github.com/sumotoy/gpio_expander) has a large gpio expander library).
4. Study a simple keybrd sketch that uses an I/O expander.
 * [SPI I/O expander example sketch](keybrd_4c_split_with_IOE/keybrd_4c_split_with_IOE.ino)
 * [I2C I/O expander example sketch](../examples/keybrd_PCA9655E/keybrd_PCA9655E.ino)
5. Study other keybrd port classes.
 * SPI I/O expander port classes: Port_MCP23S17
 * I2C I/O expander port classes: Port_PCA9655E 
6. Write the port classes for your I/O expander.
 * [Example](../examples/IOE_PCA9655E_development/) shows how Port_PCA9655E was developed in stages, from fundamental to practical
 * Debugging I/O expander code is hard because
   SPI or I2C protocol, expander configuration, and expander commands.

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