Comments: Using the Arduino Pro Mini 3.3V

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  • I am using one of these in a project to control one of the RN-52 Bluetooth audio modules (w/ sparkfun breakout). The project has it’s own 3.3v power supply for the Arduino and the RN-52, which leads me to my question:

    Is it safe for me to allow the project power supply 3.3v regulator to provide 3.3v to VCC with a 3.3v SparkFun FTDI Basic Breakout hooked up to the programming header? I noticed the first time I hooked up the FTDI with the power supply off that it fed 3.3v through and lit up the “on” LED on the project power supply.

    I considered placing a diode on the VCC connection that feeds the Arduino when on project power, but at 3.3v I really don’t want the foward voltage drop, and I have worries about overpowering something like a 1N6263 schottky diode.

    • Copied from product page comments:

      M-Short (SparkFun) said: “You should only have one power supply on the VCC/3.3V line at any given time. I would see about cutting the trace on the back of the FTDI. There is a solder jumper that that lets you select 3.3V or 5V. It is basically 3 pads with VCC in the center and 3.3V on one side and 5V on the other. Usually you connect one of the sides to the center. If you just isolate the center though you should end up with no power on VCC. Note: I haven’t checked the schematic to double check this will work so proceed with some caution. If you still have questions feel free to email techsupport@sparkfun.com”

      KevinGreenBMX said: “Thank you for confirming what I suspected! I looked at the schematic for the FTDI, and it looks like clipping that trace would leave the FTDI’s VCCIO pin floating, which I imagine would cause some problems. I think what I will need to do instead is just desolder/clip the power pin of the header (JP1, Pin 3). This will leave the FTDI unable to power other projects, but that is ok. Thanks for your help!

  • I found that trying to make (and keep) a good connection between the ftdi and pro mini with only a little pressure is very difficult. This method lead to sketches not uploading. As soon as I soldered some header pins (pointing up) all of my sketches upload without error. Then you can just pull the programmer off and watch the pro mini go!

  • You do not have to solder the header pins to the board as long as you have contact between the FTDI and Pro Mini’s plated through holes with the header pin. Usually a little pressure against the pins should be sufficient enough to make contact between the FTDI and Pro Min in order to program the microcontroller.


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