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ISO7241C: ISO7241C signal integrity

Victor Grinberg
Prodigy 70 points
Part Number: ISO7241C
Other Parts Discussed in Thread: ISO7741

Hello!

We have a design where the ISO7241 is used to isolate an SPI bus between a microcontroller and an i.MX processor.  This runs at 4Mbps, i.e. the SPI clock is 4MHz.

The bus has bit errors once in a while.  We have a layout with (more or less) controlled impedance and (more or less) no huge stubs, and (more or less) decent location of the bypass cap on the ISO IC.  Yet, bit errors happen.  We added 50 Ohm series resistors near the outputs of the ISO7241C (on MISO, MOSI, SCK and CS lines) and it fixed some of the issues, but heating the IC brings the issues back for some of them, i.e. its marginal.

Has anyone had a similar issue?

Any advice on how to improve this?

  • 0
    TI__Guru 54815 points

    Hi Victor,

    Sorry to hear about the issue. Successful SPI operation not depends on the isolator max datarate but also depends on device max propagation delay. I am not sure whether you are using 3.3V supplies, 5V supplies or the combination based on which the respective propagation delays need to be considered. Assuming worst-case, the max propagation delay is going to be 56ns from the datasheet.

    The max SPI operation speed can be calculated as, SPI-max = 1/(4*tprop-max) where tprop-max is the maximum propagation delay which in our case is 56ns. Hence, SPI-max is going to be 4.46MHz. This is the max SPI speed one can achieve using ISO7241C with a max load cap of 15pF.

    If the load cap is higher or if the MCUs add further delays then the actual SPI speed can be further lower. Hence, it is possible that your solution is marginally supporting 4MHz operation. I would recommend using ISO7741 which supports 100Mbps datarate and 16ns max propagation delay for 5V operation. Under these test conditions, you can achieve 15MHz of SPI operation. Thanks.

    Regards,
    Koteshwar Rao