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|Arduino Yún Front||Arduino Yún Rear|
The Arduino Yún is a microcontroller board based on the ATmega32u4 (datasheet) and the Atheros AR9331. The Atheros processor supports a Linux distribution based on OpenWrt named OpenWrt-Yun. The board has built-in Ethernet and WiFi support, a USB-A port, micro-SD card slot, 20 digital input/output pins (of which 7 can be used as PWM outputs and 12 as analog inputs), a 16 MHz crystal oscillator, a micro USB connection, an ICSP header, and a 3 reset buttons.
NB : In some countries, it is prohibited to sell WiFi enabled devices without government approval. While waiting for proper certification, some local distributors are disabling WiFi functionality. Check with your dealer before purchasing a Yún if you believe you may live in such a country. If you wish to disable WiFi, run this sketch. For more information, refer to this forum post.
The Yún distinguishes itself from other Arduino boards in that it can communicate with the Linux distribution onboard, offering a powerful networked computer with the ease of Arduino. In addition to Linux commands like cURL, you can write your own shell and python scripts for robust interactions.
The Yún is similar to the Leonardo in that the ATmega32u4 has built-in USB communication, eliminating the need for a secondary processor. This allows the Yún to appear to a connected computer as a mouse and keyboard, in addition to a virtual (CDC) serial / COM port.
The Bridge library facilitates communication between the two processors, giving Arduino sketches the ability to run shell scripts, communicate with network interfaces, and receive information from the AR9331 processor. The USB host, network interfaces and SD card are not connected to the 32U4, but the AR9331, and the Bridge library also enables the Arduino to interface with those peripherals.
Because the Yún has two processors, the summary section shows the characteristics of each one in two separate tables.
|AVR Arduino microcontroller|
|Digital I/O Pins||20|
|Analog Input Channels||12|
|DC Current per I/O Pin||40 mA|
|DC Current for 3.3V Pin||50 mA|
|Flash Memory||32 KB (of which 4 KB used by bootloader)|
|Clock Speed||16 MHz|
|Ethernet||IEEE 802.3 10/100Mbit/s|
|USB Type-A||2.0 Host|
|Card Reader||Micro-SD only|
|RAM||64 MB DDR2|
|Flash Memory||16 MB|
|PoE compatible 802.3af card support (see the note below)|
It is recommended to power the board via the micro-USB connection with 5VDC.
If you are powering the board though the Vin pin, you must supply a regulated 5VDC. There is no on-board voltage regulator for higher voltages, which will damage the board.
The Yún is also compatible with PoE power supply but in order to use this feature you need to mount a PoE module on the board or buy a preassembled board.
Note : early models of the Yún with a PoE adapter were incorrectly providing the board with 12V. An updated version will soon be provided to distributors that provide the expected 5V. If you believe you have the wrong PoE adapter on your board, please contact firstname.lastname@example.org
The power pins are as follows:
The ATmega32u4 has 32 KB (with 4 KB used for the bootloader). It also has 2.5 KB of SRAM and 1 KB of EEPROM (which can be read and written with the EEPROM library).
The memory on the AR9331 is not embedded inside the processor. The RAM and the storage memory are externally connected. The Yún has 64 MB of DDR2 RAM and 16 MB of flash memory. The flash memory is preloaded in factory with a Linux distribution based on OpenWrt called OpenWrt-Yun. You can change the content of the factory image, such as when you install a program or when you change a configuration file. You can return to the factory configuration by pressing the "WLAN RST" button for 30 seconds.
The OpenWrt-Yun installation occupies around 9 MB of the 16 MB available of the internal flash memory. You can use a micro SD card if you need more disk space for installing applications. Follow the procedure described in this tutorial: How to expand the Yún disk space.
It is not possible to access the I/O pins of the Atheros AR9331. All I/O lines are tied to the 32U4.
Each of the 20 digital i/o pins on the Yún can be used as an input or output, using pinMode(), digitalWrite(), and digitalRead() functions. They operate at 5 volts. Each pin can provide or receive a maximum of 40 mA and has an internal pull-up resistor (disconnected by default) of 20-50 kOhms. In addition, some pins have specialized functions:
The SPI pins are also connected to the AR9331 gpio pins, where it has been implemented in software the SPI interface. This means that the ATMega32u4 and the AR9331 can also communicate using the SPI protocol.
There are 3 reset buttons with different functions on the board:
See also the mapping between Arduino pins and ATmega32u4 ports.
The Yún has a number of facilities for communicating with a computer, another Arduino, or other microcontrollers. The ATmega32U4 provides a dedicated UART TTL (5V) serial communication. The 32U4 also allows for serial (CDC) communication over USB and appears as a virtual com port to software on the computer. The chip also acts as a full speed USB 2.0 device, using standard USB COM drivers. The Arduino software includes a serial monitor which allows simple textual data to be sent to and from the Arduino board. The RX and TX LEDs on the board will flash when data is being transmitted via the USB connection to the computer.
Digital pins 0 and 1 are used for serial communication between the 32U4 and the AR9331. Communication between the processors is handled by the Bridge library.
The ATmega32U4 also supports I2C (TWI) and SPI communication. The Arduino software includes a Wire library to simplify use of the I2C bus; see the documentation for details. For SPI communication, use the SPI library.
The Yún appears as a generic keyboard and mouse, and can be programmed to control these input devices using the Keyboard and Mouse classes.
The onboard Ethernet and WiFi interfaces are exposed directly to the AR9331 processor. To send and receive data through them, use the Bridge library. To configure the interfaces, you can access the network control panel as described in the getting started page.
The Yún also has USB host capabilities through OpenWrt-Yun. You can connect peripherals like USB flash devices for additional storage, keyboards, or webcams. You may need to download and install additional software for these devices to work. For information on adding software to the AR9331, refer to the notes on using the package manager.
The Yún can be programmed with the Arduino software (download). Select "Arduino Yún from the Tools > Board menu (according to the microcontroller on your board). For details, see the reference and tutorials.
The ATmega32U4 on the Arduino Yún comes preburned with a bootloader that allows you to upload new code to it without the use of an external hardware programmer. It communicates using the AVR109 protocol.
Rather than requiring a physical press of the reset button before an upload, the Yún is designed in a way that allows it to be reset by software running on a connected computer. The reset is triggered when the Yún's virtual (CDC) serial / COM port is opened at 1200 baud and then closed. When this happens, the processor will reset, breaking the USB connection to the computer (meaning that the virtual serial / COM port will disappear). After the processor resets, the bootloader starts, remaining active for about 8 seconds. The bootloader can also be initiated by pressing the reset button on the Yún. Note that when the board first powers up, it will jump straight to the user sketch, if present, rather than initiating the bootloader.
Because of the way the Yún handles reset it's best to let the Arduino software try to initiate the reset before uploading, especially if you are in the habit of pressing the reset button before uploading on other boards. If the software can't reset the board you can always start the bootloader by pressing the reset button on the board.
The Yún has a resettable polyfuse that protects your computer's USB ports from shorts and overcurrent. Although most computers provide their own internal protection, the fuse provides an extra layer of protection. If more than 500 mA is applied to the USB port, the fuse will automatically break the connection until the short or overload is removed.
The maximum length and width of the Yún PCB are 2.7 and 2.1 inches respectively, with the USB connector extending beyond the former dimension. Four screw holes allow the board to be attached to a surface or case. Note that the distance between digital pins 7 and 8 is 160 mil (0.16"), not an even multiple of the 100 mil spacing of the other pins.
Weight of the board is 32 g.