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Thread: Extern micro SD card on T4

  1. #1
    Junior Member
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    Extern micro SD card on T4

    Hello!
    I'm using an extern micro SD on T4. I've tried to find out if i need to use pull-up/pull-down on the CS,MISO,MOSI and SCLK , currently I'm using pullup ( to 3V3) resistor on MISO only. Should i use any other pull-up or pull-down on the other pins?

    Here is the schematic Click image for larger version. 

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  2. #2
    Junior Member RetroD's Avatar
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    Here's a nice guide from PJRC themselves: https://www.pjrc.com/better-spi-bus-design-in-3-steps/

    Since the T4 digital pins are 3.3V, you don't need a tristate buffer in the MISO, MOSI and SCLK pins, unless you wanted to use it for another device that's 5V.

    I believe your MISO pin should float halfway between 3.3V and GND when not in use, according to the guide.

  3. #3
    Senior Member blackketter's Avatar
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    Any reason to not use the SD pins available on the bottom of the T4? There are a few breakout board designs that ease connection to those pads:

    https://forum.pjrc.com/threads/57122...0-Breakout-Kit
    https://forum.pjrc.com/threads/57672...Breakout-Board
    https://forum.pjrc.com/threads/57913...breakout-board

    I suspect you'll get better performance and not use up any of the other pins on the board.

  4. #4
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    Hi RetroD!
    Thx for your tips. I don't need level shifter and the MISO is floating as you recommend!

  5. #5
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    Hi balckketter!

    The reason is actually the placement of those pins, I'm a new beginner in this field! I thought it would be easier to don't use the SD pins since it i use SPI to communicate with SD.

    Here is the 3D view of the PCB:Click image for larger version. 

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    Click image for larger version. 

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  6. #6
    Senior Member blackketter's Avatar
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    I totally understand, those pins are tricky to access! And your board looks excellent.

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