Original Arduino Yun
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Differences from the Arduino Leonardo
In addition to the 32U4 chip, the Yún has an additional processor, an Atheros AR9331, which runs a distribution of Linux for embedded systems named OpenWrt-Yun, based on OpenWrt. A full install of Python 2.7 is included as well.
The Yún has a number of different physical characteristics and connectors than the Leonardo. There is an on-board SD slot, Ethernet jack, and a USB-A Host connector. There is no power barrel connector on the Yún; it can be powered from the micro-USB connector.
There is no built in 5V regulator. If you power your Yún with more than 5V, you will likely damage it. If you are not powering the Yún from the micro-USB connection, you can apply power to the VIN and 5V pins on the board. If using the VIN pin, you cannot provide more than 5V, it will damage your board. It is recommended to power the Yún from the USB connection whenever possible.
The SD, Ethernet, and USB-A connectors are not physically connected to the 32U4 processor, they are connected to the AR9331.
The Yún's 32U4 functions in the same way as the Leonardo, except you cannot use Serial1. It is reserved for communication with the AR9331 processor.
The Yún also has a WiFi module built on board, allowing it to connect to a wireless router, or act as an access point.
The 32u4, WiFi, and AR9331 processors all have their own reset buttons.
There are several status LEDs on the Yún, indicating power, WLAN connection, WAN connection and USB. Additionally pin 13 is connected to one of the status lights.
OpenWrt-Yun
The Yún runs a distribution of Linux called OpenWrt-Yun, based on OpenWrt. While it's possible to configure the system from the command line, there is a web page that allows you to configure many of the different options available. The interface (called LuCi) gives you access to most any setting you would need for maintaining the WiFi interface.
Accessing the web interface is described below.
To install additional software on OpenWrt-Yun, you need to use the the Linux package manager opkg. For more information on the manager and some common commands, see the Yún package manager page.
When interfacing with the OpenWrt-Yun system, you have to use a command line, either accessing it through the Arduino's Bridge library, or via SSH. If you are unfamiliar with the command line, you may want to read about some basics about it and some of the programs that are frequently used.
Due to continuous improvements and suggestions coming from both the OpenWrt community and the Arduino community, version of OpenWrt-Yun installed on your Yún may be out of date. Please check the upgrade tutorial and be sure you're running the latest version available.
Python
An installation of Python 2.7 is included with OpenWrt-Yun, with which you can write applications or scripts. For more on Python, visit the Python 2.7 documentation pages.
If you're getting started with Python for the first time, there are a number of excellent resources on-line. "Learn Python the Hard Way" will cover everything you need to know to get up and running with your own scripts.
External storage on the Yún
You're discouraged from using the Yún's built-in non-volatile memory, because it has a limited number of writes.
You can use external memory like a microSD card or thumb drive for saving data, or your own scripts, web pages, etc. For the Yún to access and store information on these drives through the 32U4, they need to have a directory named arduino in the root of the volume.
Web services
OpenWrt-Yun uses REST for clients and servers. REST is an acronym for "Representational State Transfer". It is a software architecture that exposes various parts of the Arduino hardware through URLs.
By default, the REST API access is password protected. It is possible to change this to access the services without a password. To change this setting, enter the Yún configuration panel. At the bottom of the page, you will see toggles for changing the access.
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