Arduino I2C libraries

Overview of Arduino I2C libraries

The number of software I2C libraries are still increasing. This pages gives an overview of many libraries for I2C for Arduino boards.

The original Wire library for AVR processors is described in https://www.arduino.cc/en/Reference/Wire. Different platforms using different microcontrollers will provide their own versions of the <Wire.h> library and are preinstalled when their "Arduino Core" is installed. These alternative Wire libraries are mostly compatible with API of the original AVR Wire library, but many of them will deviate slightly from the AVR version to implement additional functionality that is available only for the particular microcontroller.

Most 3rd party I2C libraries will also try to follow the same API as the original AVR Wire library. However, many of them don't. In the table below, the "Arduino Wire compatible functions" column indicates whether the given library is mostly compatible with the original Arduino AVR Wire library. In this context, compatibility means source-code API level, instead of runtime level. The default Wire library for each platform will use a class named TwoWire. Some 3rd party I2C libraries will inherit from the TwoWire class, but it is also possible to provide API compatibility without subclassing TwoWire. In fact, since the design of the TwoWire class does not allow polymorphism (see https://github.com/Testato/SoftwareWire/issues/28), it is often better for the 3rd party class to avoid inheriting its implementation from TwoWire or even Stream (see https://github.com/Testato/SoftwareWire/pull/32).

The software I2C libraries are generally master-mode only. There are a few I2C software slave implementations, but they do not follow the I2C requirements and are only for one type of processor.

Library	Hardware / Software	Arduino Wire compatible functions	Multiple I2C buses	Slave mode	RX/TX Buffer Sizes	Notes
General
Steve Marple SoftWire	software	yes, SoftWire inherits from Stream	yes	no	RX:user, TX:user	Supports clock pulse stretching. The RX and TX buffers are supplied by the user, so can be any length.
XantoI2C	software	no, XantoI2C is top-level	yes	no	RX:0, TX:0	Made from scratch, using straighforward Arduino functions. Receive functionality incomplete, no ACK/NACK from master to slave
SWire	software	yes, SoftWire is top-level	yes	no	RX:32, TX:32	Based upon XantoI2C. Implements receive functionality.
felias-fogg SlowSoftWire	software	yes, SlowSoftWire inherits from Stream	yes	no	RX:32, TX:32	Using compatible standard Arduino functions. Using the Stream class, just like the Arduino Wire library.
Seeed-Studio Arduino Software I2C	software	yes, SoftwareI2C is top-level	yes	no	RX:0, TX:0	No delayMicroseconds() between bit transitions. Breaks some devices on some fast 32-bit processors due to exceeding I2C clock maximum. Uses digitalWrite() to set SDA line actively to HIGH instead of relying on passive pullup resistors, which may cause problems on open-drain I2C lines.
AceWire	software	yes, TwoWireInterface, SimpleWireInterface, SimpleWireFastInterface are top-level	yes	no	RX:0, TX:0	Provides an adapter class that delegates to other 3rd party libraries. Also provides 2 simple bigbanging implementations.
AVR
Arduino Wire	hardware	yes, TwoWire inherits from Stream	no	no	RX:32, TX:32	Preinstalled Wire library for AVR.
SoftwareWire	software	yes, SoftwareWire is top-level	yes	no	RX:32, TX:0	Runs at 100kHz clock speed on a Arduino Uno.
felis-fogg SoftI2CMaster	software	yes, SoftWire inherits from Stream	yes ?	no	RX:32, TX:0	using assembly for timing.
todbot SoftI2CMaster	software	yes, SoftI2CMaster inherits from TwoWire	yes	no	RX:0, TX:0	Inherits from TwoWire so SoftI2CMaster inherits the RX/TX buffers used in TwoWire but does not use them
DSSCircuits I2C-Master-Library	hardware	no, I2C is top-level	?	no	RX:32, TX:32	A single 32-byte buffer is shared between RX and TX.
Greiman SoftI2cMaster	software	no, SoftI2cMaster and FastI2cMaster inherit from I2cMasterBase which is top-level	?	no	RX:0, TX:0	Uses its own fastDigitalWrite() functions to write to port registers directly.
HardWire	hardware	yes, TwoWire inherits from Stream	?	no	RX:32, TX:32	Similar to Arduino Wire. Class name clashes with <Wire.h> so cannot be included into the same source code, even indirectly.
jm_Wire	hardware	yes, TwoWire inherits from Stream	yes	no	RX:32, TX:32	Copies most of the <Wire.h> library from AVR and adds time-out and concurrency tweaks.
SoftIIC	software	no, SoftIIC is top-level	yes	yes	RX:0, TX:0	Software I2C for master and slave mode. The software slave mode works with some restrictions.
BitBang_I2C	software	no, provides C functions not C++ classes	no	no	RX:0, TX:0	Supports ATmega, ATtiny, and Raspberry PI. Larry Bank has also a Multi-BitBang version.
Two-Wire Interface	hardware	no	no	no	unlimited	Non-blocking. With timeouts. Supports SMBus.
ATtiny
SpenceKonde Wire	hardware	yes, TwoWire inherits from Stream	no	no	RX:32, TX:32	Preinstalled Wire library for the ATTinyCore
TinyWireM	hardware	no, USI_TWI is top-level	no	no	RX:32, TX:32	For ATtiny microcontrollers. Master mode.
TinyWireS	hardware	yes, USI_TWI_S is top-level	yes (note on request handler)	yes	RX:16, TX:16	For ATtiny microcontrollers. Slave mode.
USIWire	hardware	yes, USIWire is top-level	no	no	RX:16, TX:16	For ATtiny microcontrollers.
TinyI2C	hardware	no, TinyI2CMaster is top-level	no	no	RX:0, TX:0	For ATtiny microcontrollers.
Arduino SAMD
Wire	hardware	yes, TwoWire inherits from Stream	yes	no	RX:256, TX:256	Default preinstalled Wire library for Arduino's SAMD21 boards.
SparkFun SAMD
Wire	hardware	yes, TwoWire inherits from Stream	yes	no	RX:256, TX:256	Default preinstalled Wire library for SparkFun's SAMD21 boards.
STM32
Wire	hardware	yes, TwoWire inherits from Stream	yes	no	RX:32, TX:32	Preinstalled Wire library for STM32duino.
ESP8266
ESP8266 Wire	software	yes, TwoWire inherits from Stream	no	no	RX:128, TX:128	Preinstall Wire library for ESP8266.
brzo_i2c	software	no, provides C functions not C++ classes	no	no	RX:0, TX:0	Written in assembly for the ESP8266. Up to 1 MHz clock speed.
ESP32
Wire	hardware	yes, TwoWire inherits from Stream	yes	no	RX:128, TX:128	Preinstalled Wire library for ESP32. Predefines 2 instances: Wire and Wire1.

Notes:

• There is a I2C library by rinkydinkelectronics.com which is the only library that uses shiftOut() for the 8 databits, and after that it reads or writes the acknowledge bit. This is also one of a very few libraries that sets the SDA signal as a output HIGH and LOW instead of relying on the pullup resistor. When the Slave should respond, the SDA signal is set as input, just before the rising clock of SCL to avoid a shortcut.
• The I2C Anything by Nick Gammon can be used to read and write variables and data that are longer than a byte in a easy way.
• Beside master and slave, there are also I2C sniffers.
• As far as I know, none of the software implementations support multi-master I2C buses.