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This library allows you to communicate with I2C devices, a feature that is present on all Arduino boards. I2C is a very common protocol, primarily used for reading/sending data to/from external I2C components. To learn more, visit this article for Arduino & I2C.

Due to the hardware design and various architectural differences, the I2C pins are located in different places. The pin map just below highlights the default pins, as well as additional ports available on certain boards.


I2C Default




UNO R3, UNO R3 SMD, UNO Mini Ltd

A4(SDA), A5(SCL)

I2C also available on the SDA / SCL pins (digital header).

UNO R4 Minima, UNO R4 WiFi

A4(SDA), A5(SCL)

Qwiic: D27(SDA), D26(SCL)

I2C also available on the SDA / SCL pins (digital header).

UNO WiFi Rev2, Zero

20(SDA), 21(SCL)

Leonardo, Micro, Yùn Rev2

D2(SDA), D3(SCL)

Nano boards

A4(SDA), A5(SCL)

MKR boards

D11(SDA), D12(SCL)


20(SDA), 21(SCL)

D102(SDA1), D101 (SCL1)

D9(SDA2), D8 (SCL2)

Use Wire1.begin() for I2C1, and Wire2.begin() for I2C2.


20(SDA), 21(SCL)

D70(SDA1), D71(SCL1)

Use Wire1.begin() for I2C1

Mega 2560 Rev3

D20(SDA), D21(SCL)

This library inherits from the Stream functions, making it consistent with other read/write libraries. Because of this, send() and receive() have been replaced with read() and write().

Recent versions of the Wire library can use timeouts to prevent a lockup in the face of certain problems on the bus, but this is not enabled by default (yet) in current versions. It is recommended to always enable these timeouts when using the Wire library. See the Wire.setWireTimeout function for more details.

Note: There are both 7 and 8-bit versions of I2C addresses. 7 bits identify the device, and the eighth bit determines if it’s being written to or read from. The Wire library uses 7 bit addresses throughout. If you have a datasheet or sample code that uses 8-bit address, you’ll want to drop the low bit (i.e. shift the value one bit to the right), yielding an address between 0 and 127. However the addresses from 0 to 7 are not used because are reserved so the first address that can be used is 8. Please note that a pull-up resistor is needed when connecting SDA/SCL pins. Please refer to the examples for more information. MEGA 2560 board has pull-up resistors on pins 20 and 21 onboard.

The Wire library implementation uses a 32 byte buffer, therefore any communication should be within this limit. Exceeding bytes in a single transmission will just be dropped.

To use this library:

#include <Wire.h>