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SDcard 3 wire interface?

Discussion in 'Electronic Design' started by Jan Panteltje, May 24, 2007.

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  1. I have the software, but in this existing system can only come up
    with 2 outputs and 1 programmable input/output (3.3V logic).

    Is it OK to tie DO and DI of the SD card together via a resistor?

    As far as I can see data output appears after the neg edge of the clock,
    and input data is sampled at pos edge of the clock,
    so I could switch direction on my input line?


    So you get this:

    SYSTEM CARD

    out1 ------------------------------------ Chip select
    out2 ------------------------------------ Clock
    -------------------- DI
    |
    programmabe IO ------------- R ---------- DO


    Sequence:

    CS low
    IO pin to input
    clock high // sample card DO
    read DO
    IO pin to output
    data to output
    clock low // sample data on DI to card


    Only drawback I can see is that data on DI will change when clock is high,
    but according to the specs I found this is OK after enough delay.

    Anybody done this?
     
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