The AVR TWI module can operate in both Master and Slave mode. The mode of operation is distinguished by the TWI status codes in the TWI Status Register (TWSR) and by the use of certain bits in the TWI Control Register (TWCR).
A set of predefined status codes covers the various states the TWI can be present in when a TWI event occurs. The status codes are divided into Master and Slave codes and further in receive and transmit related codes. Status codes for Bus Error and Idle also exist.
The TWI module operates as a state machine and is event driven. If a START CONDITION is followed by a TWI address that matches the address in the Slave’s TWI Address Register (TWAR) the TWINT flag is set. This results in the execution of the corresponding interrupt (if Global Interrupt and TWI interrupts are enabled). The firmware of the Slave responds by reading the status code in TWSR and will respond accordingly. All TWI events will set the TWINT flag, and the firmware must respond based on the status in TWSR.
As long as the TWINT Flag is set, the SCL line is held low. This allows the application software to complete its tasks before allowing the TWI transmission to continue.