The master sends the start condition to every connected slave by switching the SDA line from a high voltage level to a low voltage level before switching the SCL line from high to low:Ģ. The master sends each slave the 7 or 10 bit address of the slave it wants to communicate with, along with the read/write bit:ģ. The stop condition is a voltage transition from low to high on the SDA line after a low to high transition on the SCL line, with the SCL line remaining high.
The ACK bit must be received by either the master or the slave (depending on who is sending the data) before the next data frame can be sent.Īfter all of the data frames have been sent, the master can send a stop condition to the slave to halt the transmission. Each data frame is immediately followed by an ACK/NACK bit to verify that the frame has been received successfully. The data frame is always 8 bits long, and sent with the most significant bit first. The Data FrameĪfter the master detects the ACK bit from the slave, the first data frame is ready to be sent. If the master is requesting data from the slave, the bit is a high voltage level. The address frame includes a single bit at the end that informs the slave whether the master wants to write data to it or receive data from it. If the master wants to send data to the slave, the read/write bit is a low voltage level. If the address doesn’t match, the slave does nothing and the SDA line remains high. If the address matches, it sends a low voltage ACK bit back to the master. Each slave then compares the address sent from the master to its own address. The master sends the address of the slave it wants to communicate with to every slave connected to it. It does this by addressing. The address frame is always the first frame after the start bit in a new message. I2C doesn’t have slave select lines like SPI, so it needs another way to let the slave know that data is being sent to it, and not another slave. If an address frame or data frame was successfully received, an ACK bit is returned to the sender from the receiving device. Read/Write Bit: A single bit specifying whether the master is sending data to the slave (low voltage level) or requesting data from it (high voltage level).ĪCK/NACK Bit: Each frame in a message is followed by an acknowledge/no-acknowledge bit. Stop Condition: The SDA line switches from a low voltage level to a high voltage level after the SCL line switches from low to high.Īddress Frame: A 7 or 10 bit sequence unique to each slave that identifies the slave when the master wants to talk to it. Start Condition: The SDA line switches from a high voltage level to a low voltage level before the SCL line switches from high to low. The message also includes start and stop conditions, read/write bits, and ACK/NACK bits between each data frame: Each message has an address frame that contains the binary address of the slave, and one or more data frames that contain the data being transmitted. Messages are broken up into frames of data. With I2C, data is transferred in messages. The clock signal is always controlled by the master.
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. I2C is a serial communication protocol, so data is transferred bit by bit along a single wire (the SDA line). SCL (Serial Clock) – The line that carries the clock signal.
SDA (Serial Data) – The line for the master and slave to send and receive data. Like UART communication, I2C only uses two wires to transmit data between devices: This is really useful when you want to have more than one microcontroller logging data to a single memory card or displaying text to a single LCD. With I2C, you can connect multiple slaves to a single master (like SPI) and you can have multiple masters controlling single, or multiple slaves. I2C combines the best features of SPI and UARTs. You’ll probably find yourself using I2C if you ever build projects that use OLED displays, barometric pressure sensors, or gyroscope/accelerometer modules.
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