SLUSDF9 June 2020 BQ25790
The ACK signaling takes place after byte. The ACK bit allows the receiver to signal the transmitter that the byte was successfully received and another byte may be sent. All clock pulses, including the acknowledge 9th clock pulse, are generated by the master.
The transmitter releases the SDA line during the acknowledge clock pulse so the receiver can pull the SDA line LOW and it remains stable LOW during the HIGH period of this 9th clock pulse.
A NACK is signaled when the SDA line remains HIGH during the 9th clock pulse. The master can then generate either a STOP to abort the transfer or a repeated START to start a new transfer.