SLOA101B August   2002  – May 2016 SN55HVD233-SEP , SN65HVDA1040A-Q1 , SN65HVDA1050A-Q1 , SN65HVDA540-5-Q1 , SN65HVDA540-Q1 , SN65HVDA541-5-Q1 , SN65HVDA541-Q1 , SN65HVDA542-5-Q1 , SN65HVDA542-Q1

 

  1.   Introduction to the Controller Area Network (CAN)
    1.     Trademarks
    2. 1 Introduction
    3. 2 The CAN Standard
    4. 3 Standard CAN or Extended CAN
      1. 3.1 The Bit Fields of Standard CAN and Extended CAN
        1. 3.1.1 Standard CAN
        2. 3.1.2 Extended CAN
    5. 4 A CAN Message
      1. 4.1 Arbitration
      2. 4.2 Message Types
        1. 4.2.1 The Data Frame
        2. 4.2.2 The Remote Frame
        3. 4.2.3 The Error Frame
        4. 4.2.4 The Overload Frame
      3. 4.3 A Valid Frame
      4. 4.4 Error Checking and Fault Confinement
    6. 5 The CAN Bus
      1. 5.1 CAN Transceiver Features
        1. 5.1.1  3.3-V Supply Voltage
        2. 5.1.2  ESD Protection
        3. 5.1.3  Common-Mode Voltage Operating Range
        4. 5.1.4  Common-Mode Noise Rejection
        5. 5.1.5  Controlled Driver Output Transition Times
        6. 5.1.6  Low-Current Bus Monitor, Standby and Sleep Modes
        7. 5.1.7  Bus Pin Short-Circuit Protection
        8. 5.1.8  Thermal Shutdown Protection
        9. 5.1.9  Bus Input Impedance
        10. 5.1.10 Glitch-Free Power Up and Power Down
        11. 5.1.11 Unpowered Node Protection
        12. 5.1.12 Reference Voltage
        13. 5.1.13 V-Split
        14. 5.1.14 Loopback
        15. 5.1.15 Autobaud Loopback
      2. 5.2 CAN Transceiver Selection Guide
    7. 6 Conclusion
    8. 7 Additional Reading
  2.   Revision History

5.1.15 Autobaud Loopback

In autobaud loopback, the “bus-transmit” function of the transceiver is disabled, while the “bus-receive” function and all of the normal operating functions of the device remain intact. With the autobaud function engaged, normal bus activity can be monitored by the device.

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 a logic-high has been applied to pin 5 (AB) of the HVD235, assume a baud rate such as 125 kbps; then wait 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 host CAN controller. However, because the “bus-transmit” function of the device has been disabled, no other nodes receive the error message of the controller.

This procedure makes use of the CAN controller’s status register indications of message received and error warning status to signal if the current baud rate is correct or not. The warning status indicates that the CAN chip error counters have been incremented. A message-received status indicates that a good message has been received.

If an error is generated, reset the CAN controller with another baud rate and wait to receive another message. When an error-free message has been received, the correct baud rate has been detected.
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