1.When FSC-BT909 module is disconnected.
At this status you can send ''TPMODE=0'' to enter ''AT Command''.
2. When FSC-BT909 module establish connection, then it will auto enter throughput mode.
At this status, you can draw pin 9 to high level, then it will work at AT mode.
For more details about FSC-BT909 module: