General protocol

The I2C protocol is more elaborate than the serial communication protocol because it has to support communication between several devices. Let's see how it works using examples:

Controller → Target

If the Controller wants to send data to the Target:

1. Controller: Broadcast START
2. C: Broadcast target address (7 bits) + the R/W (8th) bit set to WRITE
3. Target: Responds ACK (ACKnowledgement)
4. C: Send one byte
5. T: Responds ACK
6. Repeat steps 4 and 5 zero or more times
7. C: Broadcast STOP OR (broadcast RESTART and go back to (2))

NOTE The target address could have been 10 bits instead of 7 bits long. Nothing else would have changed.

Controller ← Target

If the controller wants to read data from the target:

1. C: Broadcast START
2. C: Broadcast target address (7 bits) + the R/W (8th) bit set to READ
3. T: Responds with ACK
4. T: Send byte
5. C: Responds with ACK
6. Repeat steps 4 and 5 zero or more times
7. C: Broadcast STOP OR (broadcast RESTART and go back to (2))

NOTE The target address could have been 10 bits instead of 7 bits long. Nothing else would have changed.