When the TWI slave is properly configured, it will wait for a START condition to be detected. When this happens, the successive address byte will be received and checked by the address match logic, and the slave will ACK a correct address and store the address in the DATA register. If the received address is not a match, the slave will not acknowledge and store address, and will wait for a new START condition.
The slave address/stop interrupt flag is set when a START condition succeeded by a valid address byte is detected. A general call address will also set the interrupt flag.
A START condition immediately followed by a STOP condition is an illegal operation, and the bus error flag is set.
The R/W direction flag reflects the direction bit received with the address. This can be read by software to determine the type of operation currently in progress.
Depending on the R/W direction bit and bus condition, one of four distinct cases (S1 to S4) arises following the address packet. The different cases must be handled in software.