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TMS320F280049: How the DCAN changes the length of the data when transmitted

Cherry Zhou
Mastermind 22235 points
Part Number: TMS320F280049

Hi team,

Here's an issue from the customer may need your help:

How does the DCAN change the length of the data when it is transmitted? CAN has a maximum frame size of 8 bytes and sometimes the data returned from the slave to the host is not necessarily eight as it might also be dynamic. So the customer would like to know how to modify this length when transmitting it?

If directly using the function can_sendMessage(UINT32_t base, UINT32_t objID, UINT16_t msgLen,const UINT16_t *msgData);  the parameter msgLen does not work and even set the parameter to 6,the actual master receive length is still 8.

If using can_sendMessage_updateDLC(UINT32_t base, UINT32_t objID, UINT16_t msgLen,const UINT16_t.*MsgData; to transmit data, the host will report error with data exception.

Could you help check this case? Thanks.

Best Regards,

Cherry

  • 0
    TI__Guru* 95315 points
    Cherry Zhou said:
    How does the DCAN change the length of the data when it is transmitted?

    The length of data that is to be transmitted is configured via the DLC field.

    Cherry Zhou said:
    sometimes the data returned from the slave to the host is not necessarily eight as it might also be dynamic.

    True, a Receiver will not know the data length of a frame to be received. It can be anything from 0 to 8. More importantly, it need not know. That information is embedded in the DLC field of the message to be received. The DLC is only applicable for data to be transmitted, not data to be received. In other words, DLC field is a "don't care" for a receive message object. Since the DLC field is embedded in the transmitted frame, the receive message object can figure out how many bytes are being transmitted (and hence to be received).

    Cherry Zhou said:
    So the customer would like to know how to modify this length when transmitting it?

    The parameter "msgLen" is used to pass the DLC value.

    Cherry Zhou said:
    the parameter msgLen does not work and even set the parameter to 6,the actual master receive length is still 8.

    Are you saying 8 bytes is always transmitted even if msgLen is set for 6 for a transmit message object? Or are you saying 8 bytes is read from a receive message object even when the DLC value of a received frame is 6?

    Cherry Zhou said:
    the host will report error with data exception.

    What is the error? Please translate it into English.

  • +1
    TI__Expert 6035 points
    Cherry Zhou said:
    If directly using the function can_sendMessage(UINT32_t base, UINT32_t objID, UINT16_t msgLen,const UINT16_t *msgData);  the parameter msgLen does not work and even set the parameter to 6,the actual master receive length is still 8.

    Using this function directly for different lengths will not work as it does not update the transmit message object as Hareesh as mentioned. 

    Cherry Zhou said:
    If using can_sendMessage_updateDLC(UINT32_t base, UINT32_t objID, UINT16_t msgLen,const UINT16_t.*MsgData; to transmit data, the host will report error with data exception.

    That is the reason why this API was developed. However, there is a bug in this API, which has been since fixed which will appear in the next release. For a workaround to this issue please refer to the following thread:

    https://e2e.ti.com/support/microcontrollers/c2000-microcontrollers-group/c2000/f/c2000-microcontrollers-forum/1117880/tms320f280049c-can_sendmessage_updatedlc-and-can_sendmessage-did-not-work-to-set-data-length?tisearch=e2e-sitesearch&keymatch=can_sendMessage_updateDLC#

    Thanks.