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ISO7141CC: SPI comm issue

May Varza
Prodigy 10 points
Part Number: ISO7141CC
Other Parts Discussed in Thread: ISO7741, ISO6741, ISO7841

Due to low stock and long lead times for P/N ISO7741FDBQR, we decided to replace them with P/N ISO7141CCDBQ. These chips are used to isolate an AFE from the rest of our circuit. Output of the circuit is a biphasic pulse at the frequency of 20Hz. Once we replace the 7741 chips with 7141 chips, output frequency changes from 20Hz to 26Hz and AFE tests fail. AFE communicates over SPI to the micro on board and SPI signals pass through this ISO 7141 chip.

So far we have tried slowing down the SPI communication and was able to read back setting registrations back from the AFE but still when trying to read a converted value from the AFE, it fails.

I would appreciate if you could help us solve this issue.

  • 0
    TI__Expert 3482 points

    Hi May,

    Thank you for reaching out to us!

    ISO7141CCDBQ is just a digital isolator that propagates the input signal to the output.

    Therefore, it should not change the frequency. Have you compared the input signal and the output signal of the ISO7141CC?

    Could you please share your schematic or system block diagram so that we have a better understanding of how the ISO7141CC is used in your system?

    The only thing that stands out so far is the default out is low for ISO7741FDBQR, and the default output is high for ISO7141CCDBQ. Will this have an impact in your system?

    Regards,

    Alfred

  • 0
    Prodigy 10 points

    Hi Alfred,

    Thanks for your response. We have actually resolved the issue of the default high output. The second main difference between the two part numbers is the propagation delay which we tried to mitigate by slowing down the SPI bus by a factor of 4, from 4MHz down to 1MHz. Now we can write to AFE's internal registers and also read them back correctly, but still cannot read back converted data from AFE. This very same system was working flawlessly with the 7741 isolators.

    Here is the part of the circuit which pertains to isolators and AFE.

  • 0 in reply to May Varza
    Prodigy 10 points

    Would also like to add that, as you can see two of these ISO chips are being used. We tried using 7741 for U2 which carried all SPI signals and use 7141 for U6 which carries less time sensitive signals and that worked. But we only have a few 7741's left... What is the best alternative for 7741?

  • +1 in reply to May Varza
    TI__Genius 17100 points

    Hi May,

    I am Manuel Chavez, a colleague of Alfred's supporting ISO devices.

    The propagation delay difference between ISO7741 and ISO7141CC is most likely the reason why SPI communication is successful with ISO7741 but not with ISO7141CC. Unlike normal digital signal datarates, the maximum frequency for SPI communication relies on the propagation delay of different components in the system being low enough for signals to be transmitted and sampled by the master and slave (or controller and peripheral) in the same clock cycle.

    Components of an SPI system, like controller and peripheral devices, cables, and isolators, all add delays that reduce the maximum SPI frequency that can be effectively used. As calculated in the E2E post linked below, ISO7741 devices with the max prop delay of 16ns can transmit SPI at ~15.6MHz max, which does not include delays in the system from cabling, etc.



    Since ISO7141CC has a propagation delay >2x of ISO7741, the effective SPI datarate it can isolate is ~7.1MHz max, so reducing the communication rate enough will allow for successful SPI communication in your system.

    If the SPI datarate can be reduced, ISO7141CC can continue being used in your system, however, we do recommend our newest digital isolator devices, like ISO6741 and ISO7841, are used instead since they are made with our newest isolation technology and have similar propagation delay specs as ISO7741.

    I hope this answers your question. Please let us know if there is any additional assistance we can provide.


    Thank you,
    Manuel Chavez