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Original question:

SN65HVD233: How I use the loopback for it ?

SN65HVD233: LBK Signal

Tony Cline
Prodigy 50 points

Part Number: SN65HVD233

Good morning,

I'm using the SN65HVD233 part in a new design.  I do not want diagnostic loopback mode to be enabled.  Per the data sheet pin #5 can be left open and is pulled low internally.

This is the exact configuration in my design.  However, when transmitting data into pin #1 (D signal) I see the data looped back to pin #4 (R signal).  I have confirmed pin #5 is open.

Debugging, I've pulled the LBK signal directly to gnd and diagnostic loopback remains enabled.

Further, the data sheet indicates if LBK is enabled, the output driver is disabled.  However, I'm seeing a signal on both the CANH and CANL pins.

The Vcc rail (3.3V) looks clean.

Any insight would be appreciated.

Best Regards.

  • 0
    TI__Guru 74872 points

    Tony,

    With a CAN transceiver, the data will always loopback to the RXD/R pin. This is because whether the transceiver is transmitting the data itself, or reading from the CAN bus, the controller always needs to check the CAN bus messaging to acknowledge the message, and to verify that what it is transmitting is being correctly transmitted to the bus by immediately reading back on the RXD/R pin.

    As you've stated, the LBK function is meant to read back from D/TXD to R/RXD without sending messages to the bus. That being said, the LBK function is enabled by setting pin 5 to a logic-high state. Grounding it will disable it and transmit the CAN messages to the CAN bus along with reflecting them back onto the R/RXD pin.

    There's no way to disable the data from D/TXD to R/RXD unless the device is in a low-power state, which can be done by placing pin 8 in a logic-high state. 

    Please let me know if you have any other questions.

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    Prodigy 50 points

    Thank you Eric.  After giving this more thought I agree with your explanation.  The transmitting node always monitors each bit of its own transmissions. Hence, the D-outputs are directly tied to the R-inputs internally.  The LBK signal ONLY serves to place the bus outputs/inputs in a high-impedance state thereby shielding the bus if/when internal diagnostics are performed by the controller.  The output/input drivers remain active either way.  Due to internal pullups, when LBK is asserted high, the signals maintain their recessive state.  By my way of thinking this is misleading/unfortunate nomenclature on the part of TI.  Perhaps a better signal description is "HiZ", for example, as loopback is always present.