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TCAN1146-Q1: Different CAN FD device supporting differnet speed on bus will be same case like Classical CAN device on CAN FD network

Manoj Agarwal
Prodigy 250 points
Part Number: TCAN1146-Q1

Hi  TI Team,

We have CAN bus network where we are using TCAN1146s with 5MBps data transfer mode, while other device connected on device is having 2Mbps CAN FD support. What issue we may have on the bus. Device supporting 2Mbps will ignore data transfer at speed of 5mbps. It would be same like classical CAN device connected on CAN FD network.

It is like for CAN FD bus network, all devices(transceiver) should support same data speed or different speed CAN FD device connection is ok.

This is required to inform other supplier should provide particular speed support for CAN FD ECUs.

Regards

Manoj A 

  • TI__Guru 53555 points

    Hi Manoj,

    Thanks for bringing your question to E2E. 

    All CAN nodes that share the same bus connections must be configured for the same data rate. When a HS CAN node exists on a CAN FD network, all nodes must share the same Arbitration (nominal) data rate. All CAN FD nodes must be configured for the same FD (data phase) data rate. 

    Let me know if this is clear and if you have any more questions. 

    Regards, 
    Eric Schott 

  • in reply to Eric Schott1
    Prodigy 250 points

    Hi Eric,

    Thanks for reply.

    As per above reply bus speed would be limited by minimum data rate supported by node in can netwrok.?

    How do we select data speed in transceiver. Can we set it in transceiver from 2 to 5 Mbps range for CAN FD device using register.

    Regards

    Manoj A.

  • in reply to Manoj Agarwal
    TI__Guru 53555 points

    Hi Manoj,

    CAN transceivers only need to support the maximum data rate used in a system. They do not need to be programmed or configured to receive a certain data rate while in Normal mode. This only needs to be done in the CAN controller of the system. 

    The partial networking feature of TCAN1146 can detect HS CAN frames while the device is in Sleep mode. The transceiver can be configured to select an expected data rate during Sleep mode using the SW_CONFIG_1 register. This setting does not impact device behavior while in Normal mode. Selective wake can only be done using HS CAN frames, so there is no FD data rate to configure unless the device is programmed to ignore CAN FD frames (SW_FD_PASSIVE = 1).

    Regards, 
    Eric Schott 

  • in reply to Eric Schott1
    Prodigy 250 points

    Hi Eric,

    So if one CAN FD transceiver device(A) is supporting 2Mbps and Other CAN FD transceiver device (B) is supporting is 5Mbps then maximum data rate of that bus would be limited to 2Mbps. Any data coming with 5Mbps data would be ignored or indicated by error by CAN FD device A?. Also this 2Mbps device limitation to be configured to CAN controller Device (B) so that it can communicate with CAN FD Controller/Transceiver Device (A)?

    It is necessary to configure CAN Transceiver for expected data rate for wake-up frame. 50Kbps to 1Mbps).? Or it is managed by bus itself. 

    What do you mean by belwo statement in previous reply.If we chose (SW_FD_PASSIVE = 0) how can we configure CAN FD data rate

    " there is no FD data rate to configure unless the device is programmed to ignore CAN FD frames (SW_FD_PASSIVE = 1).""

  • in reply to Manoj Agarwal
    TI__Guru 53555 points

    Hi Manoj,

    Not necessarily. When a CAN controller is on a CAN bus, it expects all communication on that bus to appear at the data rate data it is configured for. So if node A was configured for 2Mbps, it would throw an error if it sees a 5Mbps CAN frame. Similarly, a node configured for 5Mbps would not be able to recognize a 2Mbps frame and would cause it to throw an error. In general, FD data rates are not changed after the system is initially set up, so all nodes should know before runtime what the agreed upon data rate will be.

    CAN  transceivers (as opposed to the protocol aware controllers) do not care what data rate they are used for provided they are capable of driving the maximum data rate that appears on the bus. So a transceiver that supports 5Mbps CAN rates could be used in a system that is operating at 2Mbps. 

    CAN transceivers that support partial networking are a bit different in this regard because they become protocol aware when the selective wake feature is active. This means that the transceiver needs to be configured to know the CAN data rate so it can interpret data while in sleep mode. 

    Regards, 
    Eric Schott 

  • in reply to Eric Schott1
    Prodigy 250 points

    Hi Eric,

    Thanks for detailed explanation. Many things are clear now. You can close this thread. 

    As per my understanding above explanation is same for the bus where CAN HS device (CAN controller +CAN TxRX) is interface with CAN FD supported device? 

    Regards

    Manoj A

  • in reply to Manoj Agarwal
    TI__Guru 53555 points

    Hi Manoj,

    Correct. A CAN FD transceiver can support HS CAN, but not vice versa. 

    Regards,
    Eric Schott