Datasheet

LED: 13. There is a built-in LED connected to digital pin 13. When the pin is HIGH value,
the LED is on, when the pin is LOW, it's off.
The Uno has 6 analog inputs, labeled A0 through A5, each of which provide 10 bits of
resolution (i.e. 1024 different values). By default they measure from ground to 5 volts, though
is it possible to change the upper end of their range using the AREF pin and the
analogReference() function. Additionally, some pins have specialized functionality:
TWI: A4 or SDA pin and A5 or SCL pin. Support TWI communication using the Wire
library.
There are a couple of other pins on the board:
AREF. Reference voltage for the analog inputs. Used with analogReference().
Reset. Bring this line LOW to reset the microcontroller. Typically used to add a reset button
to shields which block the one on the board.
See also the mapping between Arduino pins and ATmega328 ports. The mapping for the
Atmega8, 168, and 328 is identical.
The Arduino Uno has a number of facilities for communicating with a computer, another
Arduino, or other microcontrollers. The ATmega328 provides UART TTL (5V) serial
communication, which is available on digital pins 0 (RX) and 1 (TX). An ATmega16U2 on the
board channels this serial communication over USB and appears as a virtual com port to
software on the computer. The '16U2 firmware uses the standard USB COM drivers, and no
external driver is needed. However, on Windows, a .inf file is required. 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-to-serial chip and USB connection to the computer (but not for serial communication on
pins 0 and 1).
A SoftwareSerial library allows for serial communication on any of the Uno's digital pins.
The ATmega328 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 Arduino Uno can be programmed with the Arduino software (download). Select
"Arduino Uno from the Tools > Board menu (according to the microcontroller on your
board). For details, see the reference and tutorials.
The ATmega328 on the Arduino Uno 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 original STK500 protocol (reference, C header files).
You can also bypass the bootloader and program the microcontroller through the ICSP (In-
Circuit Serial Programming) header; see these instructions for details.
The ATmega16U2 (or 8U2 in the rev1 and rev2 boards) firmware source code is available .
The ATmega16U2/8U2 is loaded with a DFU bootloader, which can be activated by:
On Rev1 boards: connecting the solder jumper on the back of the board (near the map of
Italy) and then resetting the 8U2.
On Rev2 or later boards: there is a resistor that pulling the 8U2/16U2 HWB line to ground,
making it easier to put into DFU mode.