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Display with SD card - pullup resistor issue? — Parallax Forums

Display with SD card - pullup resistor issue?

wmosscropwmosscrop Posts: 409
in General Discussion
I have one of those ebay color LCD displays that also has an SD card slot. The display works fine, the SD card doesn't.

After much hair pulling, it appears that the reason that I can't get the SD card slot to work is that there are 3 10K SMT resistors mounted on the board, each in the CS, MOSI, and CLK circuit paths. MISO is a direct path.

Yes, I said in the paths. These appear to have been meant for pullups, but they are in line with the signals and are not connected in any way to VCC.

I suspect that this additional resistance is blocking or otherwise preventing signals from getting to the card.

Questions:
Has anyone seen this before?
I should be able to unsolder and then jumper these resistors... correct?

Comments

  • VonSzarvasVonSzarvas Posts: 3,450
    edited 2017-12-09 09:07
    wmosscrop wrote: »
    Has anyone seen this before?
    I should be able to unsolder and then jumper these resistors... correct?

    1. No
    2. Yes -although I'd dbl-check first that those I/O's are not being shared with the display, or some other circuit that might lead to a dead-short in the right (wrong!) combination! Otherwise, removing the series resistance would seem like a plan. If you can't easily short them, then replacing with a shunt (or less than ~50 ohm) resistor could also work nicely.
  • wmosscropwmosscrop Posts: 409
    Here's a couple of images of the board before I attempt any surgery:

    9078470220cc9a093f45cf2d7541bdd2?AccessKeyId=B7FAF256C0F76A0B59ED&disposition=0&alloworigin=152785c8988f44236ffc72c2bb9e7d086?AccessKeyId=B7FAF256C0F76A0B59ED&disposition=0&alloworigin=1
  • kwinnkwinn Posts: 8,697
    Good idea to very carefully check the other side of the board to make sure the pins of the sd connector and input pins are not connected to anything else. If this is a photo display there probably are more connections.
  • wmosscropwmosscrop Posts: 409
    Thanks to all for the suggestions - especially about checking the other side of the board - but as the display panel covers the other side that wasn't practical. I had to go with checking continuity.

    As it turns out, removing the resistors, jumpering over the pads, and properly connecting the pullup resistors works!

    Note: I do not recommend this procedure to anyone else. I was probably very lucky that I didn't ruin the display or SD circuitry.

  • Cluso99Cluso99 Posts: 18,069
    Can you measure the resistance between the MISO pin and the SD socket? Since it's tracked on the other side, perhaps there is a series resistor there too.
  • wmosscropwmosscrop Posts: 409
    MISO to SD socket is zero ohms. I didn't have to modify the board at all to get this pin working.
  • Peter JakackiPeter Jakacki Posts: 10,193
    Best way to remove smd passives is to simply get a blog of fresh solder on the end of an iron and dap and swipe the part off the board in one quick and easy motion. Alternatively blob some solder on and end and hold the iron on it for a couple of seconds until it comes loose. Normally you have enough surface tension on the solder blob for it to pickup the part.

    Then if you happen to have some low ohm resistors you can just use those to bridge it otherwise I find either stripped wire-wrap wire or even a single strand from some idc or hookup wire.

    Nothing delicate, nothing complicated, and almost no risk to the pcb if handled deftly.
  • JRetSapDoogJRetSapDoog Posts: 954
    Seems odd. I assume that the board seems well-designed overall. So, you think that those SMD resistor pads were intended for low-value (e.g. 200 Ohm) resistors for short protection and the production line just used the wrong components?

    I just opened a thread the other day in the Propeller section asking if such in-line (not pull-up) resistors might be a good idea. Guess the producer of this LCD board thought that they were. Anyway, thanks for sharing, and congrats on getting things working.
  • wmosscropwmosscrop Posts: 409
    JRetSapDoog wrote: »
    Seems odd. I assume that the board seems well-designed overall. So, you think that those SMD resistor pads were intended for low-value (e.g. 200 Ohm) resistors for short protection and the production line just used the wrong components?

    The board is fine other than this issue.

    I have no idea what the intention was, but as there are no obvious traces to provide the signals with 3.3V.

    I'm just wondering how the boards passed QA.

  • Cluso99Cluso99 Posts: 18,069
    Think it may have been a mistake that was not ever verified. Probably meant to be pull-ups.
  • wmosscropwmosscrop Posts: 409
    It appears that a newer version of the display has some changes to the sd card circuitry:

    current ebay item

    Still looks to me like it has the issue.
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