Class 4 sd card

Hi Guys and the rest,

we have got a lot of system deployed now running the EMX module without any real problems. But the urgent issue that we are facing now is that it doesn’t seem to be able to read class 4 sd cards. We had some problems with the sd cards we always use. So we started looking for the differences (because we just always order the same) and the only difference is that the old ones are class 2 and the new ones are class 4. We took a different brand (class 2) and they also run perfect. But the class 4 from that different brand also doesn’t work.

Is this a know problem?



This is not a “class” problem. Some cards need more capacitance on SD power pin. Adding a 22uF capacitor on SD power pin fixes the problem usually. You can also connect the SD to 3.3V directly instead of SD power pin on EMX.

Hi Gus,

but we tested 4 different brands now, From all of them the class2 cards work and the class4 cards don’t. Isn’t that a bit to regulair to say that some cards need the capacitor? I will try it though ofcourse! Or is there something with the newer class 4 and 6 cards that makes them draw more power?



As the class defines the minimum speed of the card in MB/s, it would be possible that you need more power if you want to read/write faster. Connecting SD power directly to 3v3 as suggested by Gus should sort this out. If that works, I would prefer to use a FET that is controlled by the CPU SD power output.

Hi Wouter,

yes i thought so also after reading some extra documentation on I woul prefer something with a fet also, but it means a re-design of the , already very compact, board design… But we will see, i will try the suggestion with the capacitor or the direct 3.3v first to see if it solves the problems.

Controlling power on SD is a “nice” feature but not required. Most systems just leave the SD powered constantly.

I’ve been struggling with similar SD card issues for the last couple of days, until I found this post. Additional capacitors solved the problem immediately.

Well until recently I thought different cards needed different voltages. I would have tied it to 3V3 in my design to if I knew this earlier :slight_smile:

Hi all,

so we hung the scope on the power supply pin and you are correct Gus. With a class4 card the ripple is bigger and the voltage drops down to 2.88v

So thank you ver much!!!



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