Dual SPI SD Cards on SC126 using micro SD card adapter

[Mark Pruden]

I have a Small computer Central SC126, and wanted to use 2 SD cards. 

However a well known issue is that the standard micro SD card adapters don't work on a shared SPI bus. The issue is that the MISO signal is always driven out of the level shifter chip (on the adapter) regardless of CS. Thus the two cards thus compete for MOSI output. See this post:

https://forum.arduino.cc/t/issue-with-some-sd-cards-when-used-with-another-spi-device/1264230/2

I came up with a simpler fix than recommended on the forums, by simpler it means less soldering of very small pitch pins on the level shifter IC.

As per the picture below, there are 2 changes

• Cut one of the MOSI pins off, on the main pin header, this isolates the driver output, so only one board is driving the MOSI output
• Connect the Serial output from the micro SD card socket(s) together. This means the output from both the SD cards can be presented back to the micro. The sd cards themselves play nice since they respect the CS signal.  

Its not entirely perfect, since these 2 cards are now joined. But I have glued some perf board to the back of them, so they are physically joined as one adapter, and no chance of the litter wire breaking off. 

Hope this helps.

[Alan Cox]

The SC126 at power up or if you get a software error will select both cards at once sometimes. If that happens you will sometimes blow one or both up.

I ended up doing something a bit more complex as a result which also gives you more devices so you can get 2 SD adapters and ethernet on it.

https://hackaday.io/project/167445-spi-fix-for-sc126

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[Greg Holdren]

Check out my post on retro-comp:

https://groups.google.com/g/retro-comp/c/fcGZEa91NSc/m/7hjddl-WAwAJ

It describes how to modify the SD card to tri-state the MISO line. It is similar to the method described in the Arduino but is really addressing the issue and makes each card independent and not tied together. A little more involved but cleaner.

Greg

[Mark Pruden]

Thanks for feedback.

Greg - Yes I was aware this was the best solution, however lifting that pin and soldering it didn't look achievable with the tools I had

Alan - Tat looks like a great solution. I would love to see a photo of what it turned out like.

Mark