I2C Communication Protocol

I2C stands for Inter-Integrated Circuit. It is a bus interface connection protocol incorporated into devices for serial communication. It was originally designed by Philips Semiconductor in 1982. Recently, it is a widely used protocol for short-distance communication. It is also known as Two Wired Interface(TWI).

Working of I2C Communication Protocol

It uses only 2 bi-directional open-drain lines for data communication called SDA and SCL. Both these lines are pulled high.

Serial Data (SDA) : Transfer of data takes place through this pin.
Serial Clock (SCL) : It carries the clock signal.

I2C operates in 2 modes

• Master mode
• Slave mode

Each data bit transferred on SDA line is synchronized by a high to the low pulse of each clock on the SCL line.

According to I2C protocols, the data line can not change when the clock line is high, it can change only when the clock line is low. The 2 lines are open drain, hence a pull-up resistor is required so that the lines are high since the devices on the I2C bus are active low. The data is transmitted in the form of packets which comprises 9 bits. The sequence of these bits are -

1. Start Condition: 1 bit
2. Slave Address: 8 bit
3. Acknowledge: 1 bit

Steps of I2C Data Transmission?

Here are the steps of I2C (Inter-Integrated Circuit) data transmission

• Start Condition: The master device sends a start condition by pulling the SDA line low while the SCL line is high. This signals that a transmission is about to begin.

• Addressing the Slave: The master sends the 7-bit address of the slave device it wants to communicate with, followed by a read/write bit. The read/write bit indicates whether it wants to read from or write to the slave.

• Acknowledge Bit (ACK): The addressed slave device responds by pulling the SDA line low during the next clock pulse (SCL). This confirms that the slave is ready to communicate.

• Data Transmission: The master or slave (depending on the read/write operation) sends data in 8-bit chunks. After each byte, an ACK is sent to confirm that the data has been received successfully.

• Stop Condition: When the transmission is complete, the master sends a stop condition by releasing the SDA line to high while the SCL line is high. This signals that the communication session has ended.

What is Start and Stop Conditions ?

START and STOP can be generated by keeping the SCL line high and changing the level of SDA. To generate START condition the SDA is changed from high to low while keeping the SCL high. To generate STOP condition SDA goes from low to high while keeping the SCL high, as shown in the figure below.

What is Repeated Start Condition?

Between each start and stop condition pair, the bus is considered as busy and no master can take control of the bus. If the master tries to initiate a new transfer and does not want to release the bus before starting the new transfer, it issues a new START condition. It is called a REPEATED START condition.

Read/Write Bit

A high Read/Write bit indicates that the master is sending the data to the slave, whereas a low Read/Write bit indicates that the master is receiving data from the slave. 

ACK/NACK Bit

After every data frame, follows an ACK/NACK bit. If the data frame is received successfully then ACK bit is sent to the sender by the receiver.  

Addressing

The address frame is the first frame after the start bit. The address of the slave with which the master wants to communicate is sent by the master to every slave connected with it. The slave then compares its own address with this address and sends ACK.

I2C Packet Format

In the I2C communication protocol, the data is transmitted in the form of packets. These packets are 9 bits long, out of which the first 8 bits are put in SDA line and the 9th bit is reserved for ACK/NACK i.e. Acknowledge or Not Acknowledge by the receiver. 

START condition plus address packet plus one more data packet plus STOP condition collectively form a complete Data transfer.

Features of I2C Communication Protocol

• Half-duplex Communication Protocol - 
  Bi-directional communication is possible but not simultaneously.
• Synchronous Communication - 
  The data is transferred in the form of frames or blocks.
• Can be configured in a multi-master configuration.
• Clock Stretching - 
  The clock is stretched when the slave device is not ready to accept more data by holding the SCL line low, hence disabling the master to raise the clock line. Master will not be able to raise the clock line because the wires are AND wired and wait until the slave releases the SCL line to show it is ready to transfer next bit.
• Arbitration - 
  I2C protocol supports multi-master bus system but more than one bus can not be used simultaneously. The SDA and SCL are monitored by the masters. If the SDA is found high when it was supposed to be low it will be inferred that another master is active and hence it stops the transfer of data.
• Serial transmission - 
  I2C uses serial transmission for transmission of data.
• Used for low-speed communication.

Comparison between I2C and SPI Communication Protocols

Advantages of I2C Communication Protocol

• Can be configured in multi-master mode.
• Complexity is reduced because it uses only 2 bi-directional lines (unlike SPI Communication).
• Cost-efficient.
• It uses ACK/NACK feature due to which it has improved error handling capabilities.
• Fewer Wires: Only two wires are needed, making it easier to set up.
• Multiple Devices: You can connect many devices to the same bus.
• Simple Communication: It’s relatively easy to program and use.

Disadvantages of I2C Communication Protocol

• Speed Limitations: I2C is slower compared to some other protocols like SPI.
• Distance: It’s not suitable for long-distance communication.
• Half-duplex communication is used in the I2C communication protocol.

Conclusion

The I2C communication protocol is a simple and effective way for devices to communicate with each other. It allows multiple devices to connect using just two wires, making it easy to add new components to a system. I2C is popular in various applications because it supports multiple devices, is relatively easy to implement, and requires less wiring compared to other protocols. Overall, I2C is a reliable choice for connecting sensors, displays, and other peripherals in electronic projects.