Getting Acknowledge from a Slave Device
When an address or data byte has been transmitted onto the bus then this must be ACKNOWLEDGED by the slave(s). In case of an address: If the address matches its own then that slave and only that slave will respond to the address with an ACK. In case of a byte transmitted to an already addressed slave then that slave will respond with an ACK as well.
The slave that is going to give an ACK pulls the SDA line low immediately after reception of the 8th bit transmitted, or, in case of an address byte, immediately after evaluation of its address. In practical applications this will not be noticeable.
This means that as soon as the master pulls SCL low to complete the transmission of the bit (1), SDA will be pulled low by the slave (2).
The master now issues a clock pulse on the SCL line (3). the slave will release the SDA line upon completion of this clock pulse (4).
The bus is now available again for the master to continue sending data or to generate a stop condition.
In case of data being written to a slave, this cycle must be completed before a stop condition can be generated. The slave will be blocking the bus (SDA kept low by slave) until the master has generated a clock pulse on the SCL line.