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SPI buffer

Kevin Ginunas
Expert 5835 points
Other Parts Discussed in Thread: SN65LVDT14

Hi Team,

Can any of our logic portfolio work as a buffer for off-board SPI sensor running at 10MHz over 3ft cable?

-Kevin 

  • +1
    Guru 317000 points

    You can use pretty much any logic buffer, but 90 cm is long enough that you will get transmission line effects and need proper termination.

    The cable is likely to be sensitive to cross-coupling and especially environmental noise, so I'd estimate that you cannot use single-ended logic but must use differential signals, e.g., LVDS. There are integrated transceivers for SPI (SN65LVDT14/41), but you could also use smaller transmitters/receivers.

  • 0 in reply to Clemens Ladisch
    TI__Expert 5835 points

    Thanks Clemens, if customer also wants to run PDM audio interface (clock/data) over 3ft cable at 10MHz, do you have a similar LVDS buffer that can support PDM audio interface over LVDS? I would suspect using logic buffer single ended with PDM interface would have the same risks as SPI that you mention above. Even if the LVDS buffer isn't branded as PDM compatible, I would think 2 ch LVDS buffer could work here? 

  • 0 in reply to Kevin Ginunas
    Guru 317000 points

    What is the total number of signals, in each direction? You can combine LVDS drivers/reveivers/transceivers as desired.

  • +1 in reply to Clemens Ladisch
    TI__Guru 74282 points

    Clemens,

    Thanks for the help as always!

    Kevin,

    The Interface Transceiver portfolio only has RS-485 transceivers that can work at 10MHz. Similarly to what Clemens has suggested with LVDS transceivers, this would extend the signal using a differential scheme for each single-ended signal. The RS-485 transceivers have higher output to account for noisier environments, but with the high margin of data rate, and a quieter environment, and LVDS transceiver will be a better choice.

    Regards,

    Eric Hackett

  • +1 in reply to Kevin Ginunas
    TI__Mastermind 22910 points

    Hello Kevin,

    For SPI it's not recommended to have that long of a transmission line. As Clemens mentioned, over 90 cm will result in transmission line effects. To increase transmission length and avoid signal integrity issues, differential signaling like LVDS can be used. We have an App Note that talks about this topic; Transmitting SPI Over LVDS Interface.

    I'm not too familiar with PDM audio interface but it seems like a typical logic signal. It that case LVDS would still work here.

    Regards,

    Josh

  • 0 in reply to Clemens Ladisch
    TI__Expert 5835 points

    Clemens, Joshua - we looked at SN65LVDT14/41 but due to cable limitations they need to run 3-wire SPI. What other SPI buffers can we suggest that meet below requirements?

    "I need to run 3-wire SPI (+3.3V/GND, CS, SCK, SDI/SDO for the 4 twisted pairs).  This means the data line (SDI/SDO) needs to be bi-directional, which it doesn’t look like SN65LVDT14/41 will support unless there’s a valid way to hook the MISO/MOSI driver pair back-to-back.  I don’t see that referenced anywhere in the app notes.  Is there a configuration to use SN65LVDT14/41 in a 3-wire SPI configuration?  Or is there a better suited part? "

    Thanks,

    -Kevin

  • 0 in reply to Kevin Ginunas
    Guru 317000 points

    Is there a direction control signal available on both sides? If not, then bidirectional differential transmission or even buffering is not possible.

    You could run the SDI/SDO signals separately as single-ended signals and hope for the best.