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TXB0104: Noise is being generated when data is sent

Aditya Teja Damera
Prodigy 185 points
Part Number: TXB0104
Other Parts Discussed in Thread: SN74AVC4T245, 2N7001T, SN74AVC2T245

Hi,

I am using TXB0104 for uart conversion from 1.8V to 3.2V.  Initially the level conversion of constant high signal is happening. But when a data bit is sent, it starts to generate a series of sin waves as noise and settles after some 30 sec or 1 min.  Please help me on this. I have attached the schematic of it for reference.

Note: No pull ups are there on UART lines. Do we need to put them?. On one end of TXB IC there is GSM controller and on other end its open to an external connector.

  • 0
    TI__Genius 16735 points
    Hi Aditya,

    The TXB is a for push pull applications and does not require external pullups.
    Here is some brief information for the different auto bidirectional translator families: e2e.ti.com/.../719537

    The TXB doesn't have too much drive strength and for this reason it might be struggling to transmit the bits. What kind of load is connected to? Can you also provide scope shots for me to see?

    UART is typically unidirectional and not bidirectional so is there a reason you are using the TXB device? May I suggest either the 2N7001T, SN74ACX8T245, SN74AVC4T245?

    Thanks!
    -Karan
  • 0 in reply to Karan Kotadia
    Prodigy 185 points

    Hi Karan,

    We found that it is not getting enough input impedance, so we added series resistor of 470 ohms to TX and RX, it started working fine. Will the TXS series work without the series resistor to inputs, any suggestion on it please?

    Thanks, 

    Aditya Teja Damera

  • 0 in reply to Aditya Teja Damera
    TI__Guru 62240 points

    Hi Aditya,

    Please send the scopeshots if available. The series resistors are added on the input or output lines? Series resistors provide impedance matching as well as suppress ringing/ overshoot/undershoot when interfacing into hi-z loads. What is the loading at the output? TXB devices are sensitive to cap loading and work up to 70pF cap loading as mentioned in the datasheet and the app note here :

    TXS doesnt have series resistors internally and also for UART, we do not recommend using TXS for UART pushpull signals. If drive strength is a concern, then as Karan has suggested, the DIR controlled translation family could be used like the SN74AVC2T245 device(2 DIR pins for 2 channels enables RX and TX transmission) or the SN74AVC4T245( for 4 line UART).

    Refer to the FAQs on common recommended translation devices for std interface.

  • 0 in reply to ShreyasRao
    Prodigy 185 points

    Hi Shreyas,

    The resistors were placed at Tx and Rx of uart lines on 3.3V side

    PFA the scopeshots.

    At 1.8V side

    At 3.3V side

    Please note, initially a when Rx is high, I am able to see the level shift happening, but when ever I am sending a data, thats when this nosie is observed.

    Thanks,

    Aditya Teja

  • 0 in reply to Aditya Teja Damera
    TI__Guru 62240 points
    Aditya,

    Thanks for the scopeshot provided.
    To me, it seems that there are reflections on the lines which is affecting the signal integrity. The reflected wave is overlapping with the incident wave, causing distorted signal every 3rd cycle. Referring to the datasheet section 11.1 on the TXB0104 datasheet, the trace lengths must be kept short.
    section 8.3.5 also mentions that the cap loading must be kept <70pF.
    Would you consider using sn74AVC4T245? there is also an EVM which can be tried to experiment for this device AVCLVCDIRCNTRLEVM.

    Aside from this issue, I also notice that the OE pin is controlled with a voltage divider. What is the OE level? I would advise against using the voltage divider at OE assuming that the OE2 signal is at 1.8V?