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LAUNCHXL-F280049C: About the remote frame in the DCAN module

Green Deng
Expert 4490 points

Part Number: LAUNCHXL-F280049C

Dear team:

My client wants to know:
1. If node A sends a standard format remote frame, then node B must send a standard format data frame before node A can receive it and store it in the mailbox? If node B sends an extended format data frame (that is, the Xtd bit in the mailbox is set to 1), can node A not receive it?

2. Conversely, if node A sends a remote frame in extended format, if node B sends a data frame in standard format (that is, the Xtd bit in the mailbox is set to 0), can node A also not receive it?

Best regards

  • 0
    TI__Mastermind 43810 points

    Hi Green,

    The Xtd bit defines the message ID length.  If Xtd is '0', it uses the 11-bit data in CAN_IFxARB.ID [28:18].  If Xtd is 1, it uses the entire 29-bit data in  CAN_IFxARB.ID [28:0].  Let us take for instance standard ID 0x401.  When configured in CAN_IFxARB register, this will be written as 0x10040000 (see register details of CAN_IFxARB in TRM).  Arb ID will be 0x401 since bits ID[17:0] are ingnored because Xtd is '0'.

    When you switch the Xtd bit to '1' on the existing CAN_IFxARB data from the above example (0x10040000), CAN_IFxARB will have the value 0x50040000.  This changes the Arb ID to 0x10040000 since Xtd being '1' will use the entire 29-bit ID field.

    So to answer to the question on remote frame response:

         - If remote frame was sent, only the node with the corresponding ID (standard or extended) will send back the requested data frame.  Switching the Xtd bit in the mailbox (through the CAN_IFxARB register) will alter the Arb ID/message identifier/node ID of that message object and a remote request to an ID where the Xtd bit was switched in the mailbox will not have a matching ID hence there will not be a data frame response due to ID mismatch.

    Regards,

    Joseph

  • 0 in reply to Joseph Casuga
    TI__Expert 4490 points

    Hi Joseph:

    Thank you for your detailed reply, and my customer has two more questions:

    1. If the ID uses the corresponding value in the CAN_IFxMSK register, the ID can increase the matching range. If it matches, can the node sending the remote frame receive the returned data frame and store it in the mailbox?

    2. If the node sending the remote frame receives the returned data frame, is the ID received in the receiving mailbox the originally configured ID or the ID returned through the data frame?

    Best regards

  • +1 in reply to Green Deng
    TI__Mastermind 43810 points

    Hi Green,

    Please see comments below:

    Green Deng said:
    1. If the ID uses the corresponding value in the CAN_IFxMSK register, the ID can increase the matching range. If it matches, can the node sending the remote frame receive the returned data frame and store it in the mailbox?

    - Yes, using mask values / filtering can selectively allow message IDs that fit the masking bits, but this only works with receive message objects.  A remote frame is a transmit object so the mask bits are useless/ignored when the remote frame is sent.

    Green Deng said:
    2. If the node sending the remote frame receives the returned data frame, is the ID received in the receiving mailbox the originally configured ID or the ID returned through the data frame?

    - It will be the same ID sent in the remote frame

    Regards,

    Joseph

  • 0 in reply to Joseph Casuga
    TI__Expert 4490 points

    Hi Joseph:

    1. If CANA is the sender, the data frame is sent and CANB receives it. When CANB is received and filtered successfully, will the ID sent by CANA replace the ID in the CANB receiving mailbox?

    2. If CANA sends a remote frame, CANB returns the data frame to CANA after receiving it. After CANA receives and filters the data frame successfully, will the ID in the received data frame not replace the ID in the CANA receiving mailbox?

    Best regards

  • +1 in reply to Green Deng
    TI__Mastermind 43810 points

    Hi Green,

    In CAN message filtering, when mask/filtering criteria is met, the whole message (message ID, DLC and up to eight data bytes) is stored into the message object, so yes the message ID will be written over along with data and length information with when filter criteria is matched.

    Regards,

    Joseph