SPI and multiple slaves + protection


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  1. #1
    Hans Chr
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    Default SPI and multiple slaves + protection

    Hi,

    If more than 1 slaves are enabled/selected by mistake, both slaves SDO will be driven and then we have a short, high to low.

    Would it be good practice to add a series resistor on each slave's SDO pin to limit current?

    What is common practice to resolve this issue?

    Regards

    hc

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  • #2
    Mark Rodgers
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    What you suggest would work but in practice I have never done this.
    My solution would be to read the other CS lines and switch them off before asserting the one you require.

    In reality the sensible way is to address the devices in turn and not leave the routine that talks to the device until it has switched off its CS line.

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  • #3
    Hans Chr
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    Doing the read check before asserting one of the pins was a good idea!

    The read check should then of course always pass, but good to have it if there is an error!

    the series resistor could be 250R which limits current to 20mA at 5v.

    But to be safe put in 470R. Actually it will then be 2*470R.

    The input impedance on the host is very high so the series resistor will not make a voltage divider of any sence, right?

    And possible ringing will vanish due to the resistor! So maybe include one resistor for clk and sdo on PIC as well?

    hc

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  • #4
    Hans Chr
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    double post.. sorry

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  • #5
    Mark Rodgers
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    Hello Hans,
    I have many designs on a commercial level using multiple SPI devices, none has required any of the safeguards you suggest, so to answer your original question there is no common practice to resolve this issue as it is not in fact an issue at all and forms part of the original concept of the SPI system(in my experience).

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