21.4.9 Collision Detection Exception

The TPI physical layer uses one bi-directional data line for both data reception and transmission. A possible drive contention may occur, if the external programmer and the TPI physical layer drive the TPIDATA line simultaneously. In order to reduce the effect of the drive contention, a collision detection mechanism is supported. The collision detection is based on the way the TPI physical layer drives the TPIDATA line.

The TPIDATA line is driven by a tri-state, push-pull driver with internal pull-up. The output driver is always enabled when a logical zero is sent. When sending successive logical ones, the output is only driven actively during the first clock cycle. After this, the output driver is automatically tri-stated and the TPIDATA line is kept high by the internal pull-up. The output is re-enabled, when the next logical zero is sent.

The collision detection is enabled in transmit mode, when the output driver has been disabled. The data line should now be kept high by the internal pull-up and it is monitored to see, if it is driven low by the external programmer. If the output is read low, a collision has been detected.

There are some potential pit-falls related to the way collision detection is performed. For example, collisions cannot be detected when the TPI physical layer transmits a bit-stream of successive logical zeros, or bit-stream of alternating logical ones and zeros. This is because the output driver is active all the time, preventing polling of the TPIDATA line. However, within a single frame the two stop bits should always be transmitted as logical ones, enabling collision detection at least once per frame (as long as the frame format is not violated regarding the stop bits).

The TPI physical layer will cease transmission when it detects a collision on the TPIDATA line. The collision is signalized to the TPI access layer, which immediately changes the physical layer to receive mode and goes to the error state. The TPI access layer can be recovered from the error state only by sending a BREAK character.