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ISO7041-Q1: Clarification on Series Resistor Values for SPI with ISO7041-Q1

Kalpak Gondane
Prodigy 140 points

Part Number: ISO7041-Q1
Other Parts Discussed in Thread: ISO7041

Hi,

We are currently using the ISO7041-Q1 as an interface between the microcontroller and the UIR IC for SPI communication. As per the datasheet recommendation, we have implemented 300R ohm series resistors on both the input and output sides of the isolator.

However, we are observing that SPI communication gets interrupted when these series resistors are in place. When the resistors on the MOSI and CLK lines are replaced with 0R ohm (shorted), the SPI communication is restored and operates as expected.

We would like to understand the possible reasons for this behavior. Specifically:

  1. Could the series resistance be impacting signal integrity (rise/fall times or edge distortion) at our operating SPI frequency?
  2. Are there recommended resistor values or design guidelines for SPI lines when using this isolator?
  3. Should the resistor values be adjusted based on trace length, load capacitance, or SPI clock speed?

It would be helpful if you could provide guidance on selecting appropriate series resistor values for reliable SPI communication with this isolator.

  • 0
    TI__Guru 57445 points

    Hi Kalpak,

    Thank you for reaching out. Please note that ISO7041 doesn't require any series resistors for it to operate normal and communicate in SPI. The resistors are typically added to slow down the transitions and avoid possible emissions which could make the system fail system level emissions testing like CISPR 32. If you don't have such strict emissions requirement, you don't need to use the resistor.

    If you do have such requirements, then you could have the provision for resistors and test for emissions with 0Ω to see if it passes. If it doesn't pass then you could increase the resistance and test what value works best for your application. The results can be very different from one design to another, therefore, you will need to test your boards with given resistor values to find out what works best for your application.

    For the remaining questions, please see my inputs below.

    1. Yes, the series resistor in conjunction with the parasitic trace and pin capacitance forms an RC low pass filter and slows down the signal by reducing rise/fall times and increasing system-level propagation delay. These resistors can also limit the overall SPI speed you can achieve.
    2. No resistors are needed for the SPI application, if these resistor don't affect your application performance then you could choose some of the common values like 300Ω, 470Ω or 1kΩ to minimize rise/fall time spikes and get best emissions test results. 
    3. Yes, it should be set to the highest value that doesn't affect your system performance but this isn't mandatory rather a general recommendation.


    Regards,
    Koteshwar Rao