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TCAN1146-Q1: frame detection question

Sherry Liang
Genius 10255 points
Part Number: TCAN1146-Q1

Dear team,

There is one description for frame detection in our datasheet as below. In my understanding, in some modes, the bus bias is GND. We can't enable the frame detection until the frame detection is stabilized, otherwise some frames will be missed. How do we detect the frame detection is stabilized?

" When frame detection is enabled transitioning from a mode where the receiver bias is not on up to four CAN frames for 500kbps and slower data rates and up to eight CAN frames for greater than 500kbps may be ignored by the device until the frame detection is stabilized."

Thanks & Best Regards,

Sherry

  • +1
    TI__Guru 74262 points

    Sherry,

    One method I've seen other customers use is to continuously send the wake-up frame to initialize the receiver, and then once it is initialized, the wake-up frame is already being sent so it will wake as soon as it is correctly initialized. Any kind of CAN frame can be sent to initialize the receiver, as long as it's the same data rate as the expected wake-up frame, so it makes sense to just use the wake-up frame to initialize the receiver.

    There isn't any external indication that the frame detection is stabilized, but using the wake-up frame and the device waking up will indicate that it is stabilized.

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    TI__Genius 10255 points

    Hi Eric,

    Thanks for your reply!

    Could you please tell me whether CAN FD mode of TCAN1146-Q1 can support Partial Networking?  

    Thanks & Best Regards,

    Sherry

  • 0 in reply to Sherry Liang
    TI__Guru 53555 points

    Hi Sherry,

    Partial networking only supports wake up frames (WUF) that are formatted as classic CAN frames. Selective wake on CAN FD is not defined an not supported by TCAN1146 or any other device family that I am aware of. TCAN1146 may be configured to ignore CAN FD frames when in frame detection mode so that CAN FD may be used when the device is asleep with selective wake enabled. However, only classic CAN frames may be used as the actual WUF for the transceiver. 

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    TI__Genius 10255 points

    Hi Eric,

    Thanks for your quick reply!

    One more question, Could you please tell me why CAN FD can't support partial networking? Is our device limit or CAN FD standard limit?

    Thanks & Best Regards,

    Sherry

  • +1 in reply to Sherry Liang
    TI__Guru 53555 points

    Hi Sherry,

    The standard does not define how selective wake on a CAN FD frame would be done, so our device does not implement this functionality. I expect that such support will not be added because it would add significant complexity to the CAN transceiver. Higher data rate support means more precise and higher-speed oscillator and ultimately a more complex and costly transceiver. This feature would also not add any practical advantage to the system as the wake-up time is not constrained by the CAN bandwidth, but rather the power-up time of the local node. Power supply rise times and MCU startup routines are much more significant than the time difference between a classic CAN and CAN FD frame.

    The only "advantage" of such a solution would be to allow all CAN frames on a particular bus to be CAN FD (a common misrepresentative requirement we've been seeing lately). This is not advantageous or necessary in any case as all CAN FD controllers are backwards compatible with classic CAN and can easily be configured to use a classic CAN frame for WUF purposes. Such a solution would still allow for all payload frames to use CAN FD (SW transceivers can be configured to ignore). 

    If these questions stem from an OEM requirement that all CAN frames be CAN FD, please push back on this request as this is not possible with the current definition of partial networking. If the OEM is made aware of the reasoning for this, I expect there will be no issue allowing WUF to follow the classic CAN format to maintain the current standard which is nominal for both system performance and IC cost. 

    Regards,
    Eric Schott