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Well Heck, I Should Have Known It Wouldn’t Work – RobotDyn SD Card Shield is 3.3V Logic

April 1, 2018

I bought this a while ago and could never get consistent results with it. This morning in the shower i realized why. SD cards run on 3.3V and the shield has a level converter that brings the olduino’s 5V signals down to 3.3V but the card’s output on the olduino’s MISO is a bare 3.3V. I guess this is fine with arduino’s and it might even be ok with the 74HC595 used as the olduino’s input shift register except that there’s a 1K resistor between the MISO input on the headers and the shift register. I’m not sure but the Seeed/Radio Shack SD card adapter that I used before must have a proper bi-directional level shifter(or the olduino i used it with had a 74LS595 chip which would be fine). Anyway, it was an excuse to try out my new logic analyzer – so there’s that.

Somewhat ironically by the way, the reason for the resistor that broke the camel’s back is that i had trouble with SD cards not releasing MISO when they’re deselected. I know now that i have to send a bunch of clocks for it to realize it needs to release it.

Below is the schematic of the data logger shield. I have not tried out the real time clock but i will.
18-03-12 robotdyn sd

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