Arduino Uno Board Description and Pinouts

Table of Contents


Below is a description of the Arduino Uno board.

Including tables showing the function of each of the pins in each of the two rows of connectors, along opposite edges of the board.


The USB port and the power connector are very obvious. The power connector is of the dual-concentric type and the hole into which a plug is inserted feels and is circular, although the plastic around it is square with slightly rounded corners.

Hold the board with the USB and power connectors on the right edge of the board.

There are two rows of female IDC connectors, both of which are single, in other words not a double row, like the GPIO pins on a Raspberry Pi.

On both rows of IDC female connectors, pin 0 is at the left edge of the board furthest from the power and USB sockets.

Read the following tables from the left edge of the board furthest from the power and USB sockets. Remember, hold the board with the USB and power sockets to your right.

Top Row of Female Connections

Pin Function
0 ADC5
1 ADC4
2 ADC3
3 ADC2
4 ADC1
5 ADC0
6 V in
9 5V
10 3.3V
11 Reset
12 IO Ref
13 No markings

Bottom Row of Female Connections

Pin Function
0 RX (UART) Input
1 TX (UART) Output
2 GPIO02
4 GPIO04
7 GPIO07
8 GPIO08
12 GPIO12
13 GPIO13
14 GND
15 aref (ADC reference?)
16 No markings
17 No markings

Important note. On all of my Arduino Uno examples, not the entire length of either the top row of pins or the bottom row are populated by a single unbroken run of IDC header female connectors.

This inevitably means that where there is a break between header hardware in each case, there will be a pitch of greater than the usual 2.54mm (one tenth of an inch) between two IDC ‘holes’.

For this reason you should not try to plug in an unbroken row of male headers into either row of connections.

ICSP (in circuit serial programming) Bus Pins

At the left edge of the board, feel the small notch in the end of the Atmel 328P MCU in its socket. Just below this there is a little ‘six-pack’ of pins arranged just like the dots ina single Braille cell.

These pins are the ICSP bus pins which can be used for programming the board with a suitable programmer.

See this blog post for an explanation of the ICSP pin functions.

Other Features

On the bottom right corner of the board, still with it held so that the USB and power sockets are at the left side, there is a tiny reset momentary push button.

Also at the lower right corner of the board, just above the right-hand end of the bottom row of IDC female connections, is another ‘six-pack’ of pins.

I believe these to be the ICSP pins for programming the chip which controls the USB connectivity of the board. It will probably not be necessary to ever worry about these pins.

Additional notes/questions

  • IO ref is presumably GPIO reference value. Not at all sure of this
  • 3.3v Max current 50 milliamps
  • ADC from 0 to 5v
  • ADC is ten bit (0 to 1023)
  • ARef is presumably the ADC reference pin

To connect UART to UART between Raspberry Pi and Arduino, make the connections via two resistors like this:

  • TX Arduino to RX Pi via 4k7 resistor
  • TX Pi to RX Arduino via 1k resistor

It is very important to remember that the GPIO pins of the Raspberry Pi are at a logic level of 3.3 volts and those of the Arduino are 5 volts. Damage can and will occur to the Raspberry Pi if they are connected directly together with no level conversion.

The Arduino can receive enough power from a USB connection during programming, and while connected to send and receive data from the serial port to a PC.

The Arduino Uno has an on-board LED and series resistor connected to GPIO13.

Most people’s initial adventure into programming the Arduino Uno will be to write a program which blinks this LED on and off.

You can also connect a piezo buzzer to this or any other GPIO pin and make an audible ‘blink’ program, with no change to the code necessary.

Such 3.3 Volt piezo buzzers are readily available from component suppliers, including from Ebay.

There is no harm in driving a 3.3 Volt piezo buzzer from a 5 Volt GPIO pin.