37.3.1 Byte Format

As previously mentioned, all I²C communication is performed in 9-bit segments. The transmitting device sends a byte to a receiver, and once the byte is processed by the receiver, the receiver returns an Acknowledge bit. There are no limits to the amount of data bytes in a I²C transmission.

After the 8th falling edge of the SCL line, the transmitting device releases control of the SDA line to allow the receiver to respond with either an Acknowledge (ACK) sequence or a Not Acknowledge (NACK) sequence. At this point, if the receiving device is a client, it can hold the SCL line low (clock stretch) to allow itself time to process the incoming byte. Once the byte has been processed, the receiving device releases the SCL line, allowing the host device to provide the 9th clock pulse, within which the client responds with either an ACK or a NACK sequence. If the receiving device is a host, it may also hold the SCL line low until it has processed the received byte. Once the byte has been processed, the host device will generate the 9th clock pulse and transmit the ACK or NACK sequence.

Data are valid to change only while the SCL signal is in a low state and is sampled on the rising edge of SCL. Changes on the SDA line while the SCL line is high indicate either a Start or Stop condition.