27.3.2.3.1.4 Case S4: Collision

If the slave is not able to send a high-level data bit or a NACK, the Collision (COLL) bit in the Slave Status (TWIn.SSTATUS) register is set to ‘1’. The slave will commence its operation as normal, except no low values will be shifted out on the SDA. The data and acknowledge output from the slave 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.