General protocol

The I2C protocol is more elaborate than the serial communication protocol because it supports structured communication between several devices. Let’s see how it works:

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 begin a new read transaction

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 begin a new write transaction

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

“Device Registers”

Many I2C targets are organized internally as having “device registers”, each with an 8-bit address and 8-bit contents. Typically, device registers are written with a two-byte write: the first byte is the register address and the second the new register value.

A so-called “combined” or “split” transaction might consist of a write to the target followed by an immediate read back from the target, as shown in the diagram above. Typically, device registers are read in this way: the device register address is written and then the current device register value is immediately read back.

Some I2C targets can read and write multiple device registers with adjacent addresses through some form of “address auto-increment”, which permits sending just the first device register address and then relying on the device to increment the address for subsequent reads or writes.

I2C is a complex protocol, and there are many variations and special features out there. Read the manual for your target carefully to see what needs to be done to talk to it.