I2C Protocol
Introduction
\(I^2C\) (Inter-Inter Circuit) is a serial communication protocol that allows multiple devices to communicate with each other using only two wires. It was developed by Philips in the 1980s and is widely used in embedded systems.
There are variants Single Master and Multi-Master. In the Single Master configuration, only one device can be the master and initiate communication. In the Multi-Master configuration, multiple devices can be masters and initiate communication.
SDA (Serial Data) transmits the data and SCL (Serial Clock) is the common clock signal. In the example below, there are 2 master blocks that can initiate communication. They can send data from the buses DataIn_1 and DataIn_2 or receive data from SDA and forward it on the buses DataOut_1 and DataOut_2. The circuit also includes 2 slave blocks. Slave 1 has the \(I^2C\) address of 0x30 and provides the data of an acceleration sensor (SensorDataIn). Slave 2 receives data and controls a motor (ActuatorDataOut), which has the \(I^2C\) address of 0x10.
Like SPI, I2C is synchronous, so the output of bits is synchronized to the sampling of bits by a clock signal shared between the master and the slave. The clock signal is always controlled by the master. and unlimited number of Master and maximum of 1008 slaves.
| Mode | Speed |
|---|---|
| Standard | 100 kbit/s |
| Fast | 400 kbit/s |
| Fast Plus | 1 Mbit/s |
| High Speed | 3.4 Mbit/s |
| Ultra Fast | 5 Mbit/s |
Protocol
The address and data are transmitted on the signal SDA as big endian (MSb first). At the beginning, the master sends a start bit as well as the address and the bit \(Rd/\overline{Wr}\) which indicates whether data is read or written (Read = '1'). The slave must confirm this with an ACK (Ack = '0'). Then the data is transmitted. These can come from a master or slave. The other side must confirm this with a new ACK. At the end, the stop bit indicates the end of the communication.
Advantages
- Only uses two wires
- Supports multiple masters and multiple slaves
- ACK/NACK bit gives confirmation that each frame is transferred successfully
- Hardware is less complicated than with UARTs
- Well known and widely used protocol
Disadvantages
- Slower data transfer rate than SPI
- The size of the data frame is limited to 8 bits
- More complicated hardware needed to implement than SPI