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ISO1540: Asking for the ISO1540 issue's comment

SHH
Mastermind 30301 points
Part Number: ISO1540
Other Parts Discussed in Thread: PCA9306


HI Team,
custome is designing the ISO1540 to do the isolation

The PCA9306 SCL/SDA output was connected to ISO1540.

However, the SDA pin's cannot go to low. ( VIL is too high around 747mV)

Could you please provide your comment and suggestions for this issues?

Here is the schematic, waveform and block diagram for reference in ISO1540 word file. the passport is "ti"

Thanks,
SHH

  • 0
    TI__Expert 6195 points
    Hi SHH,

    I'm getting an error code when I try to open your attachment (Error 0x80004005: Unspecified Error). Could you try re-zipping the file and attaching it again?

    Thanks,
    Dan
  • 0 in reply to Dan Kisling
    TI__Mastermind 30301 points

    Hi Dan,

    Please refer to the document here.

    thanks

    SHH

    Edit: Attachment removed - DK

  • 0 in reply to SHH
    TI__Mastermind 30301 points
    Hi Dan,
    Find a different isolator device, do you have recommendation from the same family of the ISO which has a lower VOL so that the a low value can go through the PCA9306 and directly read by the master?


    Thanks,
    SHH
  • 0 in reply to SHH
    TI__Mastermind 30301 points
    Hi Dan,
    Can you help for this case today?
    thanks,
    SHH
  • 0 in reply to SHH
    TI__Expert 6195 points
    Hi SHH,

    Thanks for bringing us this question.
    The ISO154x is the only bidirectional I2C isolator TI has at the moment.

    Could you elaborate on the problem a bit more? When I look at the scope shots you have supplied me with, it looks like the communication is working correctly - highs are being transmitted as 1's and lows are being seen as 0's. Could you help with my understanding?

    Thanks,
    Dan
  • 0 in reply to Dan Kisling
    TI__Mastermind 30301 points
    Hi Dan,
    I send the info to you. you can see the pin2/pin3 waveform on data pin waveform, it is not at the 0v level. you can use the work to open the original waveform. I also capture the figure on mail to you.
    thanks,
    SHH
  • 0 in reply to SHH
    TI__Expert 6195 points

    Hi SHH,

    We discussed this issue offline so I will summarize our discussion here.

    The customer is seeing that the low value out of the ISO1540 is not 0V, but actually closer to 0.7V See scope shot below: 

    This is coming from the ISO1540. When a low is seen on side 2, Side one will turn on the FET which will allow the diode to conduct. This generates the voltage you are seeing at 747mV (its forward voltage). See the simplified schematic on the front page of the ISO1540 datasheet.

     

    The diode is necessary for the bidirectional channel to function properly. This device was designed keeping I2C I/O thresholds in mind, and so the 0.7V does not violate the I2C specifications. See the I2C specification here: http://www.i2c-bus.org/fileadmin/ftp/i2c_bus_specification_1995.pdf

    On page 19 you can find the following table:

    3.3V is the VDD voltage of interest for this problem, so (3.3V)*(0.3)=990mV.

    As 700mV < 900mV, the I2C specification is not violated.

    So although the low output from the ISO1540 is not zero, it does not violate the I2C specification so it should not cause issues in a system.

    Thanks for the question!

    Best regards,

    Dan