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THVD8000: R output low when MODE=1

Ruixin Feng
Intellectual 2405 points
Part Number: THVD8000

Tool/software:

Hi team,

customer found that when mode=1, R will output a low voltage pulse. R is connect to MCU through 51ohm resistor. 

The yellow waveform is MODE, green waveform is R pin. Why R will output low when MODE change to 1? Could you pls help with this case? Thanks!

Rayna

  • 0
    TI__Genius 13872 points

    Hi Rayna,

    Table 8-4 in the datasheet explains the full functionality of the R pin while mode is high. 

    Specifically, we guarantee R to be high impedance ("Z") while mode = high. This is because in mode = high, the device is in transmit mode (receiver is disabled).The reason the R pin might be pulsing in an unusual way is because it should be tied to VCC with a pullup resistor (~5k should work). 

      

    Let me know if this helps or if you have more questions,

    Ethan

  • 0 in reply to Ethan Sempsrott
    TI__Intellectual 2405 points

    Hi Ethan，

    is mandatory to have pullup resistor on R pin? 

    Why high impedance status of R will have this kind of pulse? could you pls help to clarify the root cause here? Thanks!

    Rayna

  • 0 in reply to Ruixin Feng
    TI__Genius 13872 points

    Rayna,

    Could you share a schematic of your system?

    It is not mandatory to include the R pin pullup. It simply gives a known, fixed value when the R output is high impedance rather than floating. 

    Based on the image you provided in the first post, I see a green R pulse but that is when MODE = low (yellow). When MODE = high, I see only a low R value, which is expected since the R output is high impedance. Could you clarify the information about this pulse you are seeing?

    Regards,

    Ethan

  • 0 in reply to Ethan Sempsrott
    TI__Intellectual 2405 points

    Hi Ethan,

    Do you mean it's normal for R to be pulled low when Mode=high?  I thought R should not have any changes when mode switch to high. 

    Rayna

  • 0 in reply to Ruixin Feng
    TI__Genius 13872 points

    Hi Rayna,

    When the transceiver is mode = high, the R pin will have high resistance and effectively be "disconnected" from the line. This is what is shown on the table I shared in my first response. Because it is high impedance in mode = high, the R line will drop to 0V (if nothing else is driving the line). The R line is not technically pulled down, simply left at idle. Depending on the protocol you are using, it might be beneficial to have R pulled high with a resistor. That way when R is high impedance, the R line will stay idle at VCC rather than 0V. 

    Regards,

    Ethan