10.3.2 Autobaud Loopback (SN65HVD235)

The autobaud loopback mode of the SN65HVD235 is enabled by placing a high level input on pin 5, AB. In autobaud mode, the driver output is disabled, thus blocking the D pin to bus path and the bus transmit function of the transceiver. The bus pins remain biased to recessive. The receiver to R pin path or the bus receive function of the device remains operational, allowing bus activity to be monitored. In addition, the autobaud mode adds an internal logic loopback path from the D pin to R pin so the local node may transmit to itself in sync with bus traffic while not disturbing messages on the bus. Thus if the local node’s CAN controller generates an error frame, it is not transmitted to the bus, but is detected only by the local CAN controller. This is especially helpful to determine if the local node is set to the same baud rate as the network, and if not adjust it to the network baud rate (autobaud detection).

Autobaud detection is best suited to applications that have a known selection of baud rates. For example, a popular industrial application has optional settings of 125 kbps, 250 kbps, or 500 kbps. Once the SN65HVD235 is placed into autobaud loopback mode the application software could assume the first baud rate of 125 kbps. It then waits for a message to be transmitted by another node on the bus. If the wrong baud rate has been selected, an error message is generated by the local CAN controller because the sample times will not be at the correct time. However, because the bus-transmit function of the device has been disabled, no other nodes receive the error frame generated by this node's local CAN controller.

The application would then make use of the status register indications of the local CAN controller for message received and error warning status to determine if the set baud rate is correct or not. The warning status indicates that the CAN controller error counters have been incremented. A message received status indicates that a good message has been received. If an error is generated, the application would then set the CAN controller with the next possibly valid baud rate, and wait to receive another message. This pattern is repeated until an error free message has been received, thus the correct baud rate has been selected. At this point the application would place the SN65HVD235 in a normal transmitting mode by setting pin 5 to a low-level, thus enabling bus-transmit and bus-receive functions to normal operating states for the transceiver.

If the AB pin is not used it may be tied to ground (GND). However, it is pulled low internally (defaults to a low-level input) and may be left open if not in use.