27.3.3 TWI Bus State Logic

The bus state logic continuously monitors the activity on the TWI bus lines when the master is enabled. It continues to operate in all Sleep modes, including power-down.

The bus state logic includes Start and Stop condition detectors, collision detection, inactive bus time-out detection, and a bit counter. These are used to determine the bus state. The software can get the current bus state by reading the Bus State bits in the master STATUS register. The bus state can be unknown, idle, busy, or owner, and is determined according to the state diagram shown in Figure 27-12. The values of the Bus State bits according to state, are shown in binary in the figure below.

After a system Reset and/or TWI master enable, the bus state is unknown. The bus state machine can be forced to enter idle by writing to the Bus State bits accordingly. If no state is set by the application software, the bus state will become idle when the first Stop condition is detected. If the master inactive bus timeout is enabled, the bus state will change to idle on the occurrence of a timeout. After a known bus state is established, only a system Reset or disabling of the TWI master will set the state to unknown.

When the bus is idle, it is ready for a new transaction. If a Start condition generated externally is detected, the bus becomes busy until a Stop condition is detected. The Stop condition will change the bus state to idle. If the master inactive bus timeout is enabled, the bus state will change from busy to idle on the occurrence of a timeout.

If a Start condition is generated internally while in an Idle state, the owner state is entered. If the complete transaction was performed without interference (i.e., no collisions are detected), the master will issue a Stop condition and the bus state will change back to idle. If a collision is detected, the arbitration is assumed lost and the bus state becomes busy until a Stop condition is detected. A repeated Start condition will only change the bus state if arbitration is lost during the issuing of the repeated Start. Arbitration during repeated Start can be lost only if the arbitration has been ongoing since the first Start condition. This happens if two masters send the exact same ADDRESS+DATA before one of the masters' issues a repeated Start (Sr).