[SOLVED] 5V Pro Mini cannot send or receive messages via I2C

I am attempting to send data from an arduino pro mini (ATmega328 5V 16MHz) and an arduino uno
(mini = slave, uno = master)

Unfortunately, the pro minis I have been using for testing can no longer communicate with the uno board, or each other for that matter.

To test communications, I used the code and circuit architecture found here: https://www.arduino.cc/en/Tutorial/MasterReader Wherein the slave board sends a “hello” message to the master, and the master prints this message to the serial output. When I ran the slave code on the mini and the master code on the uno, the master received nothing. I added a few Serial.println(“texthere”); calls to verify that the script was running and to check where it failed, and it revealed that the uno was constantly requesting data from the mini, and the mini was constantly waiting for a request, with neither able to communicate. I tried this same test code and setup with two arduino unos, and they communicated successfully (“hello” was printed to serial output) however anytime I used the arduino pro minis, I2C did not work (I tried two pro minis, and the pro mini as both the master and the slave). This same setup has worked in the past (two months ago), but something must have changed because I can no longer get the code to function.

Things I have tried:

  • new pro mini boards
  • new arduino uno boards
  • new wires
  • different power supplies
  • updating libraries
  • resetting the boards
  • quadruple checked that SDA and SCL were connected correctly

I am at a loss as to what could be going on that would cause these boards to fail to communicate. Is my code wrong? could I be using the wrong type of board? am I plugging things in incorrectly?

If anyone has any clues that could point me in the right direction I would really appreciate it!

This is my first project using Arduino boards, it is for an engineering testing project I am working on, data is collected passively and sent at set intervals to the arduino uno to be logged to an SD card

Attached is an example of how I connected the boards, (sorry for the crap picture, it should show the gist of how I am connecting things) In this example, the mini is being used as the master and the arduino uno is the slave. Normally I have the arduino uno plugged into power and powering the mini, with the uno being the master, however neither configurations are working atm.


arduino example.JPG

Did you notice the smoke?


I wish

The boards were fine and there was definitely no smoke or bad smell

just a fuzzy image I snapped quickly for this forum post

You can use this example as a guide.

Are you using pullup resistors, 4K7, on A4 and A5?

Thanks, I have been using that guide for the code with no luck

I am not using pull up resistors (I was using the wiring diagram shown in the guide and assumed they were unnecessary with a direct connection and no bus), but I will give this a try and get back to you

While connecting the boards today I realized I was using A3 and A2 on the pro mini, and did not realize A4 and A5 were offset and marked underneath the board

Everything works as expected now, I guess this is why I am still in school

Thanks all for the help!