29.3.2.2.2 TWI Bus State Logic

The bus state logic continuously monitors the activity on the TWI bus when the host 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 (BUSSTATE) bit field in the Host Status (TWIn.MSTATUS) register.

The bus state can be Unknown, Idle, Busy or Owner, and it is determined according to the state diagram shown below.

Figure 29-4. Bus State Diagram
  1. Unknown: The bus state machine is active when the TWI host is enabled. After enabling the TWI host, performing a system Reset, or disabling the TWI host, the bus state is Unknown.
  2. Idle: The bus state machine can be forced to enter the Idle state by writing 0x1 to the Bus State (BUSSTATE) bit field. The bus state logic cannot be forced into any other state. If no state is set by the application software when the first Stop condition is detected, the bus state will become Idle. If the Inactive Bus Time-Out (TIMEOUT) bit field from the Host Control A (TWIn.MCTRLA) register is configured to a nonzero value, the bus state will change to Idle on the occurrence of a time-out. When the bus is Idle, it is ready for a new transaction.
  3. Busy: If a Start condition, generated externally, is detected when the bus is Idle, the bus state becomes Busy. The bus state changes back to Idle when a Stop condition is detected or when a time-out, if configured, is set.
  4. Owner: If a Start condition is generated internally when the bus is Idle, the bus state becomes Owner. If the complete transaction is performed without interference, the host issues a Stop condition, and the bus state changes back to Idle. If a collision is detected, the arbitration is lost, and the bus state becomes Busy until a Stop condition is detected.