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Topic: Difference between SD Card and Micro SD Card? (Read 925 times) previous topic - next topic

vwlowen

Sep 11, 2014, 03:11 pm Last Edit: Sep 11, 2014, 03:14 pm by vwlowen Reason: 1
Hi,

I'm trying to write a simple data logger using an LC Studio SD Card breakout board. ( http://tinyurl.com/pjas5wj ).

I'm using the standard Arduino SD Card library to write a text file to the card and read it back again.

What I'm finding is that microSD cards in an adapter work every time but SD cards fail every time.   The cards are identical in size - 4 GB, Class 4.

Looking at the LC Studio card schematic  ( http://tinyurl.com/kg8mem4 ), it's only using resistors rather than having a proper level shifter on the data lines but I cannot understand why SD cards should fail consistently but micro SD cards work every time.

Edit:  I should add that, although the SD cards fail in the LC Studio board, they still work perfectly is a standard PC card reader so they're not being damaged.

To get the project under way, I can just use a microSD card but I'm curious what the difference may be. Any ideas, please?

Thanks,

John.

SurferTim

That SD card does not use any type of level shifter. Those are pullup resistors, not voltage dividers. That means 5 volts is being applied to the 3.3 volt inputs of the SD card.

fat16lib

As SuferTim posted while I wrote this:

The resistors on the LC Studio card are not level shifters, just pull to 3.3V.

The full 5V Arduino signals are applied to MOSI, SCK, and SDCS.  This works on some cards because the input protection on the card causes a lot of current to be drawn and the signals never reach 5V.  You may be exceeding the max pin current of 40 ma for arduino pins.

You may damage cards and often the cards will be hot since you are stressing them beyond spec.

So the difference is a random draw of microSD cards that will tolerate the poor design of the LC Studio card and standard cards that fail with the LC Studio card.

vwlowen

Thanks for your replies.  You've more or less confirmed what I thought although it's confusing why they bother fitting the 5 - 3.3 v regulator at all.

I've got an Adafruit microSD card breakout board which does use proper level shifters so I may use that instead. The LC Studio board would have been better as it's the only breakout board I can find which has the socket sensibly protruding from the edge of the board.  Adafruit's is flush and Sparkfun's is even worse. It makes it impossible to mount sensibly in an enclosure and still be able to remove the card.

I may have a play using the LC Studio board with an external 4050 or something.

Thanks,

John.



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