Instructions

board. This can come either from VIN via an on-board regulator, or be supplied by USB or another
regulated 5V supply.
3V3. A 3.3 volt supply generated by the on-board regulator. Maximum current draw is 50 mA.
GND. Ground pins.
Memory
The ATmega328 has 32 KB (with 0.5 KB used for the bootloader). It also has 2 KB of SRAM and 1 KB of
EEPROM (which can be read and written with the EEPROM library
).
Input and Output
Each of the 14 digital pins on the Uno 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:
Serial: 0 (RX) and 1 (TX). Used to receive (RX) and transmit (TX) TTL serial data. These pins are
connected to the corresponding pins of the ATmega8U2 USB-to-TTL Serial chip.
External Interrupts: 2 and 3. These pins can be configured to trigger an interrupt on a low value,
a rising or falling edge, or a change in value. See the attachInterrupt()
function for details.
PWM: 3, 5, 6, 9, 10, and 11. Provide 8-bit PWM output with the analogWrite() function.
SPI: 10 (SS), 11 (MOSI), 12 (MISO), 13 (SCK). These pins support SPI communication using
the SPI library
.
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:
I
2
C: 4 (SDA) and 5 (SCL). Support I
2
C (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?.
Communication
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 ATmega8U2 on the board channels this serial communication over USB
and appears as a virtual com port to software on the computer. The '8U2 firmware uses the standard USB
COM drivers, and no external driver is needed. However, on Windows, a .inf file is required
. The Arduino