[Bug 1001764] Enhancement of MMC/SD over SPI driver

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http://bugs.ecos.sourceware.org/show_bug.cgi?id=1001764

--- Comment #22 from Mike Jones <mjones@linear.com> ---
I have had more bench time on the MMC, so I have more information about its
operation under heavy use.

First, my setup is atypical. I have a cable on J5 of the Kinetis tower board
that connects to another board 30cm away. This board has its own power supply
off line. Therefore, because there is a ground connection in J5, there is a
ground loop. Furthermore, I am using the I2C lines in the J5 connector.

I have seen four problems:

1) Can't mount Samsung microSD
2) Can't mount any microSD
3) IO failure after three hours of intense operation (logging MB to card)
4) Can't get DMA to work reliably

I have some new information. I was debugging the TWR-DACADC-LTC and noticed
problem 2. When I removed the ground clips from my scope, problem 2 went away.
This leads me to the conclusion that problems 2 and 3 are related to grounding
problems. I can't prove grounding is causing 3 until I have a single board
version of my solution.

Problems 1 and 4 still seem to be problems that only affect me. I work around
them by not using DMA and not using the Samsung card. I am certain that problem
1 is not related to grounding. I have tested for that. I am reasonable certain
problem 4 is not related to grounding, I am pretty sure I have seen the problem
with my cable removed.

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