I2C tutorial: All you need to know about I2C Use this 10 minute guide to learn all about the 2 wire I2C serial protocol. Learn how easy it is to use, how it works and when to use it. - JohnHau/mis GitHub Wiki

Easily Learn how to use Microcontrollers in Your Projects Home Arduino Sensors More... HomeArduino Uno SpecsI2C Interface I2C tutorial: All you need to know about I2C Use this 10 minute guide to learn all about the 2 wire I2C serial protocol. Learn how easy it is to use, how it works and when to use it.

The advantages of I2C are Uses only 2 signal wires.

Allows multiple I2C chips on the same I2C bus.

Saves tons of wiring.

Makes projects easier (especially on breadboard e.g. 2 wires instead of 8!). This I2C tutorial shows you how the I2C protocol or more correctly written I2C (sometimes written as IIC) stands for Inter IC Communication and is intended for very short distance communication between ICs on a single PCB. It gives you a fully defined protocol for data transfer between multiple devices over two wires.

TIP: You can save tons of wiring by using the 2 wire I2C bus. In this I2C tutorial you will learn all about the 2 wire I2C serial protocol; How easy it is to use, how it works and when to use it.

The I2C protocol is used in a huge range of chips - just a few examples from this site include the DS1307 (RTC), SSD1306 (OLED Display), MCP23017 (Serial expander). The protocol allows you to connect many devices to a single set of two wires, and then communicate individually with each device.

Warning: The protocol is designed for single board communication it is not a long distance communication system. You can find instances (horror stories) of people designing a multi drop inter office communication system with I2C extenders - just don't do it - it ends in tears! This I2C tutorial shows you how the I2C protocol works at the physical bit level discussing single master mode (a single controlling device) which is the most common use for I2C in a small system.

Note: You can find Master mode soft I2C routines in the DS1307 RTC project.

Note: Some manufacturers avoid paying royalties, or avoid patent problems, by calling it a 2 wire protocol but it's the same I2C protocol (when you examine the timing diagrams).

Speed I2C is a serial protocol that can operate at different speeds 100kHz, 400kHz, and 3.4MHz. Not all chips support all speeds but 100kHz is commonly supported. Speed is important as the data is transmitted serially, so a faster clock allows a quicker update.

Two wires The great strength of the protocol is that it only requires two wires, yet can have many connected devices and all of these can transmit and receive data at high speed. This saves a ton of pcb wiring.

Robust ACK signalling Unlike the SPI protocol, the I2C protocol has an acknowledgement feature that means a sending device knows that a receiver has accepted the data. So I2C is more robust in a noisy environment.

Multi Master Using I2C it is also possible to have multiple master devices making the system more flexible. For SPI there is also no concept of multiple master devices, but SPI is faster.

I2C Tutorial - How I2C works Open drain connections. I2C works by using open drain connections. This simply refers to an N-Channel MOSFET that has connections: Drain, Gate and Source. The top connection is the Drain, the middle connection is the Gate (controller) and the Lower connection is the Source.

When active (Gate voltage > Source voltage) then current flows from drain to source. When inactive (Gate voltage < Source voltage) then no current flows.

The open drain system simply means that multiple MOSFETS can be connected together at the Drain terminal which is then connected to a pull-up resistor. Now any of the MOSFETS can pull the voltage at the drain to ground. Each device has a MOSFET used as the open drain connection. For I2C you need two open drain connections (clock and data).

Each open drain connection (in I2C there are two - SCL and SDA) requires a single pull-up resistor. When all devices are inactive then the "pull-ups" pull the signal wire to the supply voltage.

Warning: You must have one pull-up resistor per signal wire (SCL and SDA) and not more! More means stronger pull-up action (keep above 1k). I2C Start and Address Signalling At any time a master device can start a transaction by pulling SDA low while SCL is high (a unique specific condition that other I2C devices recognise as the start of a master transmission).

The slave device listens to the next 7 serial bits of the address to see if it matches its own address (each I2C must have a unique address). If it does recognise the next 7 bits as an address from the master device then it will consume the R/W bit that follows.

This is the Read/Write bit that tells the slave to accept data or generate data in subsequent transactions.

At the time that the ACK signal is to be generated the master device releases the SDA line and the open drain output is pulled high. This allows the slave device to generate the ACK signal by pulling it low (only for that specific bit period). The master device monitors the I2C bus for this signal from the slave.

The slave generates the acknowledge bit in reply to the master device. This indicates that it understood the address. The diagram further down shows this in graphical form.

Note: Another protocol that uses the open - drain concept is the Dallas 1-wire protocol - but that is far slower. The advantage o the 1-wire protocol is that it allows powering of the device through the signal wire! The 1-wire protocol also allows transmission over very large distances unlike the I2C protocol. On this site this project (DS18B20 ) uses the 1-wire protocol. Master and slave The Phillips I2C protocol defines the concept of master and slave devices. A master device is simply the device that is in charge of the bus at the present time and this device controls the clock and generates START and STOP signals. Slaves simply listen to the bus and act on controls and data that they are sent.

The master can send data to a slave or receive data from a slave - slaves do not transfer data between themselves.

Multi Master Multi master operation is a more complex use of I2C that lets you have different controlling devices on the same bus. You only need to use this mode if you have more than one microcontroller on the bus (and you want either of them to be the bus master).

Multi master operation involves arbitration of the bus (where a master has to fight to get control of the bus) and clock synchronisation (each may a use a different clock e.g. because of separate crystal clocks for each micro).

Note: Multi master is not covered in this I2C tutorial as the more common use of I2C is to use a single bus master to control peripheral devices e.g. serial memory, ADC, RTC etc.

Data and Clock The I2C interface uses two bi-directional lines meaning that any device could drive either line. In a single master system the master device drives the clock most of the time - the master is in charge of the clock but slaves can influence it to slow it down (See Slow Peripherals below).

The two wires must be driven as open collector/drain outputs and must be pulled high using one resistor each (that is one resistor per I2C line i.e.for data and clock) - this implements a 'wired NOR function' - any device pulling the wire low causes all devices to see a low logic value - for high logic value all devices must stop driving the wire.

Note : If you use I2C you can not put any other (non I2C) devices on the bus as both lines are used as clock at some point (generation of START and STOP bits toggles the data line). So you can not do something clever such as keeping the clock line inactive and use the data line as a button press detector (to save pins).

You will often will find devices that you realise are I2C compatible but they are labelled as using a '2 wire interface'. The manufacturer is avoiding paying royalties by not using the words 'I2C'!

There are two wires (three if you include ground!, and four if you also include power!) - but power and ground are taken as given i.e. they are available on a PCB as needed so don't really count.

I2C Tutorial: Signals SDA : Serial Data SCL : Serial Clock I2C Tutorial: Typical signalling transaction I2C Tutorial : Typical SDA and SCL signals

Speed Standard clock speeds are 100kHz and 10kHz but the standard lets you use clock speeds from zero to 100kHz and a fast mode is also available (400kHz - Fast-mode). An even higher speed (3.4MHz - High-speed mode) for more demanding applications - The mid range PIC won't be up this mode yet!

Note: The low-speed mode has been omitted (10kHz) as the standard now specifies the basic system operating from 0 to 100kHz.

Note: Even if you run an I2C peripheral at a high speed the overall data rate depends on how fast you can push data into the internal I2C module and that depends on the processor speed. A slow slave device may need to stop the bus while it gathers data or services an interrupt etc. It can do this while holding the clock line (SCL) low forcing the master into the wait state. The master must then wait until SCL is released before proceeding.

Data transfer sequence A basic Master to slave read or write sequence for I2C follows the following order:

Send the START bit (S).
Send the slave address (ADDR). Usually 7 bits.
Send the Read(R)-1 / Write(W)-0 bit.
Wait for/Send an acknowledge bit (A).
Send/Receive the data byte (8 bits) (DATA).
Expect/Send acknowledge bit (A).
Send the STOP bit (P). Note: You can use 7 bit or 10 bit addresses.

The sequence 5 and 6 can be repeated so that a multibyte block can be read or written.

Device addresses Each device you use on the I2C bus must have a unique address. For some devices e.g. serial memory you can set the lower address bits using input pins on the device others have a fixed internal address setting e.g. a real time clock DS1307. You can put several memory devices on the same IC bus by using a different address for each.

Each device manufacturer is assigned a set of addresses so devices should not conflict with each other.

Note: The maximum number of devices is limited by the number of available addresses (and you need non-conflicting addresses) and by the total bus capacitance (maximum 400pF).

General call The general call address is a reserved address which when output by the bus master should address all devices which should respond with an acknowledge.Its value is 0000000 (7 bits) and written by the master 0000000W. If a device does not need data from the general call it does not need to respond to it.