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SPI info

Discussion in 'Electronic Design' started by joble, Aug 27, 2004.

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  1. joble

    joble Guest

    I'm using an ST72264 to drive 44 SPI switches (ST: L9822E).
    Communicating with those devices works fine, but i'm looking for
    practical information about SPI.

    ST only has an app note for communication with one single EEPROM. Also
    other manufacturers i've looked at do not have this information.

    I'm planning to not to daisy chain all 44 of them, but make 4 chains
    with 11 SPI-devices. The MCU sends 4 independent enable signals, one
    for each chain. I do this to prevent all 44 devices to stop working
    when one fails.

    How long can an SPI connection be? Can i bring the SPI to another PCB
    without trouble?

    Can anybody help me find an appnote or give advice.
  2. Leon Heller

    Leon Heller Guest

    SPI should be OK over a few inches of wire. It depends how fast it is.

  3. It can be potentionally very long if you control edge speeds and use low
    clock frequencies. This is not rigorously specified, you have to work
    this out yourself. Going from one PCB to another will probably require
    some passive filtering on the signal lines.
  4. Tim Wescott

    Tim Wescott Guest

    Within a board you should be OK. Going off board you'll have to pay
    attention to signal grounding and EMI -- you may have to slow down your
    data rate to compensate.
  5. joble

    joble Guest

    Thanks for your responses!

    The application permits me to run the spi line "very" slow. When i
    check the signal with a scope on the slave boards, i do not expect
    troubles. I'll do EMI tests either, but i do not think there will be
    any problem her either.

    What is the usual way to connect multiple SPI receivers to an mcu?
    Daisy chaining, in parallel, or a combination? For a prototype i have
    chosen the combination and i seems to work fine.

  6. It depends (you expected this answer, right?).

    Normally you would wire them in parallel, since that allows addressing
    them separately. If you have a bunch of shift registers that are always
    accessed together, you'll daisy chain them. Let the software
    requirements decide.
  7. Unless your environment is extremely benign and shielded you may well
    have problems. Slow on the desired signals doesn't matter, if there
    are brief transients, the slaves may respond. Or the noise may screw
    up your MCU.
    Whichever way works best. There's even a third possibility. If the MCU
    has to do SPI access inside an ISR it may be better to use a separate
    interface (perhaps even bit-banged) to some slaves rather than set up
    semaphores or whatever and perhaps increase interrupt latency. There
    are more timing issues and perhaps ground-bounce issues with
    daisy-chaining, but it saves a port pin for each /CS.

    Best regards,
    Spehro Pefhany
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