Exchange of data

Once the start condition has been sent, a byte can be transmitted by the MASTER to the SLAVE.

This first byte after a start condition will identify the slave on the bus (address) and will select the mode of operation. The meaning of all following bytes depends on the slave. A number of addresses have been  reserved for special purposes. One of these addresses is reserved for the "Extended Addressing Mode". As the I2C bus gained popularity, it was soon discovered that the number of available addresses was too small. Therefore, one of the reserved addresses has been allocated to a new task to switch to 10-bit addressing mode. If a standard slave (not able to resolve extended addressing) receives this address, it won't do anything (since it's not its address).


If there are slaves on the bus that can operate in the extended 10-bit addressing mode, they will ALL respond to the ACK cycle issued by the master. The second byte that gets transmitted by the master will then be taken in and evaluated against their address. Note: Even in 10-bit extended addressing mode, Bit 0 of the first byte after the Start condition determines the slave access mode ('1' = read / '0' = write).


Once the slave has been addressed and the  slave has acknowledged this, a byte can be received from the slave if the R/W bit in the address was set to READ (set to '1').

Receiving a Byte From a Slave

The protocol syntax is the same as in transmitting a byte to a slave, except that now the master is not allowed to touch the SDA line. Prior to sending the 8 clock pulses needed to clock in a byte on the SCL line, the master releases the SDA line. The slave will now take control of this line. The line will then go high if it wants to transmit a '1' or, if the slave wants to send a '0', remain low.


All the master has to do is generate a rising edge on the SCL line (2), read the level on SDA (3) and generate a falling edge on the SCL line (4). The slave will not change the data during the time that SCL is high. (Otherwise a Start or Stop condition might inadvertently be generated.)

During (1) and (5), the slave may change the state of the SDA line.

In total, this sequence has to be performed 8 times to complete the data byte. Bytes are always transmitted MSB first.


The meaning of all bytes being read depends on the slave. There is no such thing as a "universal status register". You need to consult the data sheet of the slave being addressed to know the meaning of each bit in any byte transmitted.

© Vincent Himpe 2016