I²C stands for “Inter-integrated circuit,” or “inter-IC,” and is a simple, 8-bit, serial communication bus protocol that uses just two bus wires; a serial data wire (SDA) and a serial clock wire (SCL). I²C is integrated into many ICs and allows devices to communicate directly with each other, avoiding CPU cycles. I²C operates on a master-slave basis, and all devices on an I²C bus have a unique address. I²C is used both internally in integrated chips to communicate between areas in the chip-based circuit, as well as externally, from chip to chip.

I²C was created by Philips Semiconductor in 1982. (Philips was bought by NXP Semiconductor in 2006.) According to NXP’s I²C-bus Specification and User Manual v6, published in 2014, I²C is “implemented in over 1000 different ICs manufactured by more than 50 companies.”

Integrated chips that integrate I²C capability cover many different applications and devices. For instance, a majority of sensor ICs incorporate I²C communication into their circuitry, as long as I²C data transfer rates are sufficient for a control loop’s purposes. The same is true for I²C communication to output devices such as displays, LEDs, and data converters. However, other devices such as real time clocks, memory devices, and many others also use I²C to communicate information to other chips, which is often a microcontroller or processor.

Table 1: The Various Data-Transfer Rates for I²C

Mode	Data Transfer Speed	Data Travel	Acronyms
Standard	100 Kbits/sec	Bi-directional
Fast	400 Kbits/sec	Bi-directional
Fast Mode Plus	100 Kbits/sec	Bi-directional	Fm+
High-Speed	3.4 Mbits/sec	Bi-directional	Hs
Ultra-Fast	5 Mbits/sec	Uni-directional	UFm

I²C is a multi-master bus that uses collision detection and arbitration to avoid issues when two masters initiate communication at the same time.  Arbitration is a procedure that prevents message corruption. Without arbitration, the “winning” message would get corrupted with bits of information interjected by the competing message. Nevertheless, I²C allows multiple masters on the same I²C bus line, which means that more than one chip can attempt to control the bus line at a time; arbitration decides who is the master if two or more chips attempt to control the bus at the same time. For I²C, arbitration works in a deterministic manner; that is, it is a known issue as to whether a master has control or not and is based on the SDA and SCL line status. It is important to note that not all devices that support I²C support multi-master mode.

The number of ICs using an I²C bus is limited “only by a maximum bus capacitance,“[i] however theoretically, since an address is either seven bits or nine bits long (followed by a data direction bit), the limit is 128 devices on one I²C bus since a 7 bit number goes from 0 to 127.

I²C has become a de facto standard because it is easy to use and offers a shortcut that designers use to eliminate having to design a communication system, I²C is easy to debug, I²C can be easily added or removed without affecting other devices on the I²C bus, and I²C has very low current consumption, high immunity to noise, and offers a wide range of supply voltages for system compatibility. I²C’s main benefit to designers, however, is that it can reduce the use of I/O pins on a chip (as long as the I²C bus speed is sufficient to not cause a bottleneck in data transfer.)

I²C’s main “competitors” as a simple communication bus are the Serial Peripheral Interface (SPI), plain old serial interface, and for external communication, perhaps USB. The main advantage that I²C provides communication between several devices on a simple 2-wire bus, which significantly reduces traces on a PCB and the pin count needed to implement it. Serial communication is primarily intended for communication between two devices. SPI and USB both require more pins to implement a communication transfer scheme, although both of these can also support communication between many different devices. SPI is used when faster data rates (up to 10 Mbits/sec) and full-duplex (sending and receiving in both directions simultaneously) are desired. However, SPI does not support multiple masters. USB has more pins than I²C but has taken over as the default communication bus (formerly serial communication) in personal computers (PCs) such that serial communication ports are now absent on PCs. USB, a faster communication bus than I²C, was designed for external use and is much more complicated than I²C to implement. USB provides more than is necessary for communication between chips and is definitely “overkill” if used as an I²C replacement.

For complete information on I²C, see the I²C-bus specification and User Manual by NXP Semiconductor

[i] I²C-bus specification and user manual. (2014, April 4). Retrieved January 5, 2017.