Note those are single-quotes around the character. Keyboard.write('z') - This will send a single 'z' character to your computer.

The character passed can be any standard, printable, ASCII-defined character: 0-9, a-z, A-Z, space, symbols, etc.

Keyboard.write(char) - This function will send a single character over USB.

And that's a great thing! There are essentially two functions you'll need to turn your Pro Micro into a USB keyboard: I stress 'simple' at the header of this section, because that's what it is. So lets pop the hood! Part 1: A simple HID Keyboard The goal of this tutorial is to explain how to use the Arduino HID library, so you can get the most out of your Pro Micro. Unfortunately, that HID stuff is somewhat tucked (or even locked) away from us. That USB support includes HID functionality.

Fortunately for us, Arduino exists, and with the release of 1.0 it comes with built-in support for the ATmega32U4 and its USB core. So the ATmega32U4 takes care of the USB-hardware hurdle, but we've still got to clear the firmware one. You might be holding an HID device in one hand right now, while your other hand is hovering over another. There are loads of classes - printers, hubs, speakers and webcams to mention a few - but this tutorial is going to specialize in HID: Human Interface Device. Every USB device is assigned a device class, which defines what exactly its general purpose is. What is HID you might ask? It's one of the many defined USB device classes. You can even program it to act just like a mouse, keyboard or other HID-class USB device. The Pro Micro can be programmed to emulate any USB device you could imagine. So now, when you connect the Arduino to your computer, they're connected directly over the USB bus (excuse my RAS). The Pro Micro is a tiny, Arduino-compatible microcontroller centered around an ATmega32U4 - an 8-bit AVR very similar to the ATmega328 with one huge caveat: The 32U4 comes equipped with a full-speed USB transceiver. What if you could do it with one line of code, inside the comfy confines of Arduino? Interested? Well, say hello to my micro friend: the SparkFun Pro Micro! How cool would it be if you could turn any button, joystick, sensor or other electronic gizmo into a USB keyboard and/or mouse? You could make just about any input device you want into a controller for your computer programs or games. Turn your ProMicro into a USB Keyboard/Mouse

Auto clicker programmable keyboard