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RM57L843: CAN bit rate configuration

Marcio Borges
Expert 1820 points
Part Number: RM57L843
Other Parts Discussed in Thread: TMS320F2812, HALCOGEN

A TMS320F2812 has to communicate with a RM57L843 via a CAN. We succeed to implement this communication using 500Kbps. However 1Mbps does working.  The RM57L843 does not recognize (it does not set the ACK bit) the TMS320F2812 frame, which keeps re-transmitting it and vice-versa. Using the same CAN bus (wiring) two  TMS320F2812 communicate at 1MHz.

We suspect the problem is the bit rate configuration. Could you give advice on how the choose the bit rate configuration parameters?  Should both CPUs have the same sample point? 

For the TMS320F2812, the clock is 114688000Hz and the value of CANBTC register was provided by http://www.bittiming.can-wiki.info/ as Type: eCAN, Clock : 114MHz, max brp : 256, SP : 87.5%, min tq : 8, max tq : 25, FD factor : undefined, SJW : 1 (0x00050079)

For the RM57L843 the clock is 75MHz and the configuration parameters was defined with HALCoGen. NOTE: for the 1Mbps the tool ignores the value we choose for "SP Ref" and the Sample Point is always set to 90.909.

Thanks.

  • 0
    TI__Guru**** 186196 points

    Hi,

    The three nodes might not be synchronized and re-synchronized. 

    The oscillator tolerance between the slowest node and the fastest node can be used to determine the minimum SJW. Assuming Node A is the slow node and Node B is the fast node,  you can select the SJW for the fast Node B using this formula:

    SJW for Node B > 10* Node A bit time - 10* Node B bit time

    In your configuration, the actual bit time for RM57 CAN node is 974ns, and the error is 1000-974=2.6%. Can you adjust the prop delay (for example 500ns, I guess the prop delay in your setting is < 700ns) to reduce the difference between the actual bit time and the expected bit time?

  • 0 in reply to QJ Wang
    Expert 1820 points

    Thank you for the response. 

    Where do you want to adjust the SJW? In the 2812? The  HALCoGen does not allow me to change the SWJ the RM57L843.

  • 0 in reply to QJ Wang
    Expert 1820 points

    Hello,

    We have succeed to communicate at 1MHz using the value of "Propagation Delay" you have recommend. The "Nominal Bit Rate" on the RM57 CAN changed to 1MHz. See the picture.

    Our concern now is whether these setting they depend on other factors like distance, number of nodes, etc. Should we worry about that?

    Thanks,

  • 0 in reply to Marcio Borges
    TI__Guru**** 186196 points

    The propagation delay needs to be considered, and the all nodes on the network to receive to transmit messages should use the same baudrate. 

  • 0 in reply to QJ Wang
    Expert 1820 points

    Ok. Thank you.