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SPI unterminated

Discussion in 'Electronic Design' started by Dave Boland, Mar 3, 2005.

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  1. Dave Boland

    Dave Boland Guest

    I saw some postings about cards that have an SPI interface,
    which made me think about a few things relative to a project
    I'm doing. In this case, the plan is to run the SPI signal
    lines and three chip selects to some header pins. Up to
    three daughter cards can be stacked on the processor card,
    and some or all may have SPI devices.

    My concerns are for unterminated SPI lines and radiated EMI.
    Initial calculations look like reflections from an
    umterminated line shouldn't be a problem because a 12.5 MHz
    signal has a wave length of about 79 feet, which is much
    longer than trace lengths for the SPI bus. This ignores the
    effects of the rise time of the waveform though.

    I haven't attempted to deal with EMI at this stage of
    development, but it is always a concern. This is especially
    true when a clock is passed from card to card and there will
    be a pin radiating the clock (top card on the stack).

    Has anyone done a design similar to this and got it through
    UL/CSA/IEC? If so, I would appreciate any helpful advice.

    Thanks,
    Dave
     
  2. Luhan Monat

    Luhan Monat Guest

    SPI devices have a maximum clock rate, but no minumum. If you have
    problems with cabling, try just running it slower.
     
  3. If EMI is a concern, you can always go differential
    over a cable. These could be RS485, LVDS, ECL .
    Of importance is also the intended common mode voltage.

    Rene
     
  4. Mac

    Mac Guest

    I agree. I'm not a real EMI/EMC expert or veteran, but I picked up a
    little from some EMI engineers I used to work with at a big company that
    did its own certifications.

    If possible, I would put a series resistor at the clock driver (there is
    only one clock driver, right?), and a small shunt cap at every clock load
    (if you have control of that). Probably you will be able to just use a
    zero ohm jumper for the resistor and leave the caps unpopulated, but if a
    scan reveals problems, you'll be able to tweak the edge rate of the clock.

    The data lines aren't as critical, but depending on the topology, you
    could put series resistors near the data driver chips, too.
    Actually, you should incorporate EMI from fairly early in the design
    phase. It affects pinouts, and chassis design and all kinds of things.
    It sounds as though you are not using any cable, but just stacked boards
    and board-to-board connectors. That's an assumption built-in to the
    following comments.

    Make sure the connector pinout provides good referencing for the clock
    signal. You may want to put ground pins adjacent to the clock pin to
    prevent cross-talk, and to provide good referencing. In the connector
    pinout, you can use power pins as ground references, too, provided that
    you have good decoupling of power to ground on both boards near the
    connector.

    Are the cards going to be inside of a metal box? A properly designed metal
    enclosure is or can be a big part of the EMC solution. For example, my
    first real design job was a memory mezzanine card which could be
    stacked up to two high for a single-board computer. It had multiple single
    ended 100 MHz clock lines which never shut off. When stacked one-high,
    there was an unterminated 100 MHz clock pin. But the SBC goes inside a
    chassis, and it passed EMI/EMC without any drama.

    I think you will be able to get your board through testing (or at least I
    don't think the SPI will stop you).

    But if you would rather be safe than sorry, or if the signals are going
    over cables, you could always switch to LVDS signalling, and put a
    common-mode choke on each diff-pair right where it passes off-board.

    If you use LVDS, you won't need to be quite as careful about referencing,
    so you can get by with fewer ground and power pins. This will
    partially offset the fact that you need two pins for every signal.

    HTH,
    Mac
     
  5. Dave Boland

    Dave Boland Guest

    Mac,

    Thank you for a wonderful response! I really appreciate the
    insight. To answer your question, this is just a card
    stack, like PC-104, and there are no cables. The card stack
    will go into a metal box or have a metal cover and into a
    rack mountable chassis (Euro card size).

    Dave,
     
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