Case S4: Collision

If the client cannot send a high-level data bit or a NACK, the Collision (COLL) bit in the Client Status (TWIn.SSTATUS) register is set to ‘1’. The client will commence ordinary operation, except no low values will be shifted out on the SDA. The data and acknowledge output from the client logic will be disabled. The clock hold is released. A Start or repeated Start condition will be accepted.

The COLL bit is intended for systems where the Address Resolution Protocol (ARP) is employed. A detected collision in non-ARP situations indicates that there has been a protocol violation and must be treated as a bus error.