During Normal mode, the only mode where the CAN driver is active, the TXD DTO circuit prevents the local node from blocking network communication in event of a hardware or software failure where TXD is held dominant longer than the time out period tTXD_DTO. The TXD DTO circuit is triggered by a falling edge on TXD. If no rising edge is seen before the time out constant of the circuit, tTXD_DTO, expires, the CAN driver is disabled. This keeps the bus free for communication between other nodes on the network. The CAN driver is re-activated when a recessive signal is seen on the TXD terminal, thus clearing the dominant time out. The receiver and RXD terminal reflects what is on the CAN bus and the bus terminals is biased to recessive level during a TXD DTO. This fault is indicated via the TXDDTO flag shown on the nFAULT terminal.
The minimum dominant TXD time allowed by the TXD DTO circuit limits the minimum possible transmitted data rate of the device. The CAN protocol allows a maximum of eleven successive dominant bits (on TXD) for the worst case, where five successive dominant bits are followed immediately by an error frame. The minimum transmitted data rate may be calculated by: Minimum Data Rate = 11 bits / tTXD_DTO = 11 bits / 1.2 ms = 9.2 kbps.