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SN65MLVD200A: Rx Glitch after TXEN drops

Brian Cox
Prodigy 10 points
Part Number: SN65MLVD200A

Tool/software:

What a mission to get answers!  How to contact TI directly?

We use this SN65MLVD200ADR multiple times in our designs, and we have found that data is corrupted because the Rx line experiences a glitch each time the TXEN line drops after a Tx event.

Is this a known issue?

Is there a work-around (aside from meddling with the code in the devices around this part)?

Is there an updated part available?

What is its part number?

Is there something that we need to do in our design?

I've inherited this one, and have opportunity to implement improvements to the design, so I'm keen to get this problem sorted at this opportunity.

  • 0
    TI__Expert 3170 points

    Hi Brian,

    It is difficult to diagnose this issue without some more information. One possibility for the oscillation is due to noise if the bus is becoming undriven for a period of time after the driver is disabled. Could you please provide your schematic, how the transceivers on the bus are being controlled, and traces of the receiver inputs?

    Regards,

    Jack

  • 0 in reply to Jack Campbell
    Prodigy 10 points

    Hi Jack

    Thank you for responding.

    It's not oscillation as such; it's a well timed disturbance caused by the TXEN transition from logic high to logic low (i.e. changing the operating mode from transmit to receive).  It's highly repeatable. The period of time that you referred to would be ~20ns as indicated in the 'scope screen image.  By default, the bus sits idle with all transceivers in receive mode listening for communications.  This disturbance is on the MCU side, not the bus side.

    TXEN is the signal on both DE and /RE joined together at the IC, pins 2 and 3.  For the traces, well, this happens on multiple different locations each with their own trace patterns (we use this part is several locations), and the glitches are consistent, so the common factor is the IC and the traces are unlikely to be the problem.

    By inserting the resistor into the Rx signal path, I proved that the disturbance originates in the IC.  A colleague did independent tests and found the same thing.  Without the resistor, the disturbance is closer to logic switching than sinusoid.  The sinusoid appears when the resistor is added, so that is parasitic caused by the disturbance of the inserted resistor.  Note also that there is also a smaller but still readily distinguishable disturbance at the exact instant that the TXEN falls.

    Rx is pulled to logic high with a 12k resistor just mm from the IC pin.  Rx, Tx, and TXEN are directly to / from a microcontroller in most applications, and from a single-board computer device in this case.

  • 0 in reply to Brian Cox
    TI__Expert 3170 points

    Hi Brian,

    Thanks for the updated info. To further help could you please provide your schematic and oscilloscope traces of the A, B signals along with the TXEN and RX signals?

    Regards,

    Jack