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 "Arduino Wire compatible functions" means that the functions are named the same as the functions of the Arduino Wire library, and it is a drop-in replacement for the Arduino Wire library.

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
Default
Arduino Wire	hardware	yes, TwoWire inherits from Stream	If the hardware supports it	no	RX:32, TX:32	There are a number of different official Wire libraries for the different boards.
General
Steve Marple SoftWire	software	yes, SoftWire inherits from Stream	yes	no	RX:user, TX:user	Supports clock pulse stretching.
XantoI2C	software	no, XantoI2C is top-level	yes	no	RX:none, TX:none	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.
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.
ATtiny
SpenceKonde Wire	hardware	yes, TwoWire inherits from Stream	no	no	Default <Wire.h> library in 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.
ESP8266
ESP8266 Wire	software	yes, TwoWire inherits from Stream	no	no	RX:128, TX:128	The official Wire compatible library of esp8266.com is a software implementation.
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.

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 the only library 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.