I have the following SD card reader on the left side from Sparkfun. In the tutorial they have it connected directly into an Arduino Mini, which has BLK and GRN pins> I would like to connect it to an Arduino UNO. Where do I connect these pins or can I ignore them? Thanks for your help.

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  • 1
    This must be one of the weirdest connection diagrams ever. If you flipped one of the boards over, none of the wires would cross. This is by design, as you are supposed to physically plug one into the other.
    – Nick Gammon
    Oct 21, 2016 at 2:23

2 Answers 2


If you read the page about the OpenLog board you can see (on the schematic link) what the pins mean:

OpenLog pins

I've added the labelling on the board in blue. The GRN pin (DTR in the schematic) is actually an input, which resets the chip:

OpenLog reset


  • GRN should be connected to RESET on your Uno, so that when you reset the Uno, you rest the OpenLog.

  • TXO (transmit out) on the OpenLog would be connected to serial input on the Uno (ie. digital pin D0 - labelled RX).

  • RXI (receive input) on the OpenLog would be connected to serial output on the Uno (ie. digital pin D1 - labelled TX).

  • You can see that GND and BLK are both ground pins, and thus should be connected to ground.

  • VCC (RAW on their schematic) is the voltage input to the 3.3V very-low-dropout voltage regulator MIC5205 on the OpenLog. This will accept input voltages in the range 3.3V to 12V. (The datasheet seems to suggest a maximum of 16V, but it might get rather hot if you did that). Connecting VCC to +5V on the Uno would be satisfactory.


It quite simple and stupid that they named the pins with those name. You simply connect:

BLK > ground


The weird pin abreviations can cause quite a bit of confusion.

Extra links for help.

  • Your link is to a guide for connecting an Arduino and an FTDI, which is not what this is. The correct guide is at learn.sparkfun.com/tutorials/openlog-hookup-guide and while it is unclear if it will matter in practice the official recommendation (ie, from the people who designed the openlog) is different from yours. Oct 20, 2016 at 20:22
  • Well, Its written at the bottom of my link what BLK and GRN means. Well I think the issue is resolved so yeah. You are welcome to write your own anwser.
    – Dat Ha
    Oct 20, 2016 at 21:24
  • "Its written at the bottom of my link what BLK and GRN means" again, your link is not about the board in question. If you go by your link, GRN is an output but on the board this question is actually about, GRN is an input. Oct 20, 2016 at 21:33

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