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TXS0108 with I2C isolator ISO1540

tj
Prodigy 190 points
Other Parts Discussed in Thread: ISO1540, ISO1541, TXS0108E, TXB0108, LSF0108

I'm working on bidirectional isolated I2C, from 1V8 to 3V3 or 5V voltage level.

I'm using TXS0108 with ISO1540.  My schematic looks like:

I have tested this circuit, I2C lines have many self-oscilators. I2C transmission has a lot of distortions. 

I tried to add capacitor to filtering it, but withou succes.

Will TXS0108 work together with ISO1540 isolator ? What will be proper configuration for this devices ?

@TI - Do hou have I2C isolator for 1V8 ?

  • 0
    TI__Expert 6335 points

    tj,

    TI does not have a 1.8V I2C isolator at the moment. We have the ISO1540 and ISO1541 that supports voltages down to 3.0V.

    The Output Load Considerations Section of the TXS0108E datasheet discusses signal oscillation. Specifically, the recommendation is to reduce line length and line capacitance to avoid false triggering of the one-shots that can yield signal oscillation.

    I would appreciate any oscillscope screen shots that you can send me for debug. Can you capture SCL and SDA at each point in the signal chain?

    1. SCL and SDA on 1.8V side of TXS0108E

    2. SCL and SDA on 3.3V/VDD1 side of TXS0108E/ISO1540

    3. SCL and SDA on VDD2 side of ISO1540

    Best Regards,
    Nirav

  • 0 in reply to Nirav Patel
    Prodigy 190 points

    Hello Nirav

    I have short PCB lengths.

    My test environment is:

    Rasberry as I2C master 3V3 <-> 3V3 TXS0108E 5V <-> 5V ISO1540 3V3

    SDA signal from Rasberry without TXS0108E  connected:

    SDA signal with connected TXS0108E:

    SDA signal on second side TXS0108E 

    SDA signal after ISO1540

    SDA/SCL signals from Raspberry without TXS0108E 

    SDA/SCL signals from Raspberry with TXS0108E connected

  • 0 in reply to tj
    Prodigy 190 points
    Any help ?
  • 0 in reply to tj
    TI__Expert 6335 points
    tj,

    My initial observation is that based on the last two screenshots, it looks to be a crosstalk problem, since noise is induced on the SDA channel whenever there is a transition on the SCL channel. I will have to look into this a bit more.

    Best Regards,
    Nirav
  • 0 in reply to Nirav Patel
    Prodigy 190 points

    Ok, I'm waiting for information.

    Is there alternative for TXS0108 level converter ?

  • 0 in reply to tj
    TI__Expert 6335 points
    tj,

    I don't think so. Normally I would suggest TXB0108, but since these signals are I2C, I can't suggest TXB0108 because it does not support an open-drain interface.

    Best Regards,
    Nirav
  • 0 in reply to Nirav Patel
    TI__Guru 62240 points
    Hi Tj ,

    LSF0108 is another passive level translator which could be considered for open drain applications and push pull applications as well .
    They are suited for I2C signals with pullup resistances on the input lines and the translation happens using the output pull up resistances .