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ISO1641: Not Getting Pulled Down to Zero

William Mcknight
Intellectual 380 points
Part Number: ISO1641

Hello,

We have implemented an isolated I2C Bus using ISO1641BDR.

Communication works properly. No communication errors are occurring says our FW team.

Not sure why the image below is so hazy; But, the Yellow trace is micro-controller SDA side of isolating device, and the Blue trace is the Bus side. The purple arrows are pointing to areas where SDA isn't being pulled down to GND. Seems to be around 640mV where the purple arrows are pointing on side 2, and on side 1 around 730mV. Is that being caused by the ISO1641? If so, why?

Left side of circuit is micro side, right side of isolating device is I2C bus side. The TVS bus diode is I2C specific, so very low capacitance. The pull-up on the bus side are large because many at least 8 devices can plug into the bus, which end up with a resistance around 2k all together, somewhere around there.

Resistor values and capacitances will either be tuned or removed, it's early days at this point.

  • +1
    Guru 287840 points

    This is OK. The ISO1641 must output a voltage that is higher than the low-level voltage of any other device so that it is possible to detect whether a low level is coming from itself or from another device. This is necessary to prevent a bus lockup (when both SDA1 and SDA2 output low, but no other device on the bus is still pulling low).

  • 0 in reply to Clemens Ladisch
    Intellectual 380 points

    Hi Clemens, thanks for getting back quickly. I have a few follow up questions:

    1. When you say the low level voltage of other devices are you speaking about VIL of other devices? 
    2. So, where the purple arrows are pointing you are saying the low level is coming from the ISO1641?
      1. Does that imply the bus in a bad state, and if the ISO1641 was not there the bus would lockup?
    3. Can you dive deeper into how and why the device outputs these intermediary voltage levels?
  • 0 in reply to William Mcknight
    Guru 287840 points

    1. I am speaking about VOL of other devices.

    2. Yes, this signal comes from the ISO1641. There is nothing bad about this. (A lockup would happen only if the ISO1641 were to output a lower voltage.)

    3. The ISO1641 outputs a low-level voltage that is above 570 mV (see the VOL1 specification). But only when the input signal is below 560 mV will the low level be propagated to the other side (see the VILT1 specification).