The Arduino UNO board is one of the most popular options and an excellent choice for beginners.
Over time, the Arduino UNO has undergone several revisions, with the latest being the Arduino UNO R3. While there aren’t many major differences between the versions, the most notable update is in the R3’s USB-to-serial conversion. The R3 uses an ATmega16U2 microcontroller for USB-to-serial conversion, replacing the ATmega8U2 used in the R2 version. Earlier versions of the UNO relied on FTDI chips for this function.
Arduino Uno Pin Diagram
Arduino Uno Pin Diagram
The image above displays the Arduino UNO R3 board.
The Arduino UNO R3 uses the ATmega328P microcontroller as its primary control and processing unit. The ATmega328P features 32kB of in-system programmable flash memory, 1kB of EEPROM, and 2kB of internal SRAM.
The UNO board has 14 digital I/O pins (labeled 0–13 in the image above). Out of these, 6 pins can function as PWM pins (pins 3, 5, 6, 9, 10, and 11), marked with a “~” symbol before the pin number (e.g., ~3).
Digital pins 0 and 1 serve as serial communication pins, designated RX and TX, respectively.
There are also 6 analog input pins (A0–A5), which can be used as digital I/O pins if no analog input is required.
Note: The numbers assigned to the pins on the board are not the same as the ATmega328P microcontroller’s actual pin numbers. These board pin numbers are used for accessing functions in the Arduino libraries.
The table below shows the correspondence between the pin numbers on the UNO board and the pin numbers on the ATmega328P microcontroller.
Table: ATmega328P Pin Mapping with Arduino UNO
| Pin on Arduino UNO | Pin on ATmega328P |
| 0 | 2 |
| 1 | 3 |
| 2 | 4 |
| 3 | 5 |
| 4 | 6 |
| 5 | 11 |
| 6 | 12 |
| 7 | 13 |
| 8 | 14 |
| 9 | 15 |
| 10 | 16 |
| 11 | 17 |
| 12 | 18 |
| 13 | 19 |
| A0 | 23 |
| A1 | 24 |
| A2 | 25 |
| A3 | 26 |
| A4 | 27 |
| A5 | 28 |
| GND | 8,22 |
| 5V | 7 |
| AREF | 21 |
| RESET | 1 |
The Arduino UNO board can be powered in three ways:
- USB Cable: Connect the board to a computer via USB.
- External AC-DC Adapter: Use an adapter with a fixed output voltage between 7V and 12V, plugged into the board’s power jack.
- Battery: Connect a battery with a fixed voltage in the range of 7V to 12V. Connect the battery terminals to the VIN and GND pins on the board.
Note: When uploading a sketch through the Arduino IDE, ensure that no external devices are connected to pins 0 and 1, as these are the ATmega328P’s RX and TX pins, used for serial communication during uploads.