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TMS320F28076: Multi-station transmission using SCI for converting to 485 signals

Alan Cheng
Prodigy 30 points
Part Number: TMS320F28076


Hi~

We are using the THVD1452DR chip to convert UART signals into 485 signals. We also have a 28076 device functioning as the master station and multiple 28076 devices as slave stations using SCI for communication. When Station A wants to transmit, the other slave stations need to pull down the DE pin of THVD1452DR. However, this action also causes the Rx signal of the master station to go to a low level, triggering the RXERROR flag. Is there any way to solve this issue?

Thanks in advance!

Best wishes,

Alan

  • 0
    TI__Mastermind 19795 points

    Hi Alan,

    Thanks for your question. It sounds from your description that the BRKDT bit is being set, which is typical when the RX line is pulled low. If you would like to clear this, you can toggle the SWRESET bit to reset the SCI module, or you can also system reset. Either of these will clear the BRKDT bit (and also other bits as mentioned in the SCICTL1 register).

    I believe it is standard for a long pull low of RX pin on a device to trigger some type of interrupt in the device, as the BRKDT is used in systems to signal the system needs to be alerted to a state change.

    Regards,

    Vince

  • 0 in reply to Vince
    Prodigy 30 points

    Hi Vince, thank you for your reply

    We have been resetting the system to resolve this issue, but repeatedly resetting it during operation can result in slower data reception or loss. Is there a way to solve this problem?

    Thanks,

    Alan

  • 0 in reply to Alan Cheng
    TI__Mastermind 19795 points

    Hi Alan,

    You can also ignore the RXRDY and BRKDT interrupts (has to be both disabled, can't only disable BRKDT) using the SCICTL2.RXBKINTENA bit.

    Regards,

    Vince

  • 0 in reply to Vince
    Prodigy 30 points

    Hi Vince ,

    I ignore the RXRDY and BRKDT interrupts, it does indeed help reduce the occurrence of some errors, but there will still be some framing errors that occur.

    Now I am currently using a pull-up and a pull-down resistor to provide a potential difference for the 485 during idle, although it doesn't cause errors, it doesn't feel like a good solution. Are there any other solutions available?

    Thanks,

    Alan

  • 0 in reply to Alan Cheng
    TI__Mastermind 19795 points

    Hi Alan,

    Is the 485 interface 5V? That will definitely be the issue if that's the case. As mentioned in the guide, a level shifter is needed for translating from 3.3V to 5V, simple voltage step/down using resistors leads to incorrect mid-rail voltages and floating between values, or just accidental pull-downs in general.

    Regards,

    Vince