Arduino Pro Mini 5v/16MHz working at 8MHz

Hi all.

I have an issue with my Pro Mini (supposedly) 16MHz.

I select ATmega328P (5V, 16MHz) and upload the sketch below

void setup(){

void loop(){

I see gibberish unless I set the baud rate to 57600...

But, if I change the board to ATmega328P (3.3V, 8MHz) and upload the same sketch, I can see "test" coming from the serial port at 115200

The board is powered with 5V. The chip label reads MEGA328P U-PH

Has anyone experienced this before?


Double check that your board is the 3V 8 Mhz Pro Mini and be completely sure that it's selected in the IDE boards menu. I have had this exact same issue before and the wrong IDE choice is what causes the baud mismatch.

Moderators: I previously posted a reply to this issue on a very old post before seeing this one. My bad, sorry. I'm getting the hang of this but as a slow learner I suppose. Sorry.

Why do you think this is a 16mhz unit rather than the 8mhz unit?

As per the Tech Specs:

Clock Speed = 8 MHz (3.3V versions) or 16 MHz (5V versions)

But at the same time:

Board Power Supply = 3.35 -12 V (3.3V model) or 5 - 12 V (5V model)

So maybe I'm powering a 3.3V version with 5V.... ???

Double check that your board is the 3V 8 Mhz Pro Mini

How can I do this? At this point, I'm thinking it is 3.3V, but not completely sure. Thx

You probably got a 3.3v@8MHz version instead of 5v@16MHz. Is it an official board or a cheap clone? The people selling cheap clones definitely get them mixed up sometimes - since the cheap clone manufacturers never mark the boards as 3.3v or 5v (I often see boxes on the back on the silkscreen, meant to be marked after manufacture, but I have never seen them filled in! And I have bought, uhm, a lot of different pro mini and nano clones)

So, before I suggest something more, please clarify what board you have. That is, where did you buy it - specifically. If you can attach a photo of the top and bottom of the board, that may help. Otherwise, see if there is a check-mark or other marking on the bottom that indicates the voltage.

If this info doesn't give a solution, there are ways to measure some things to determine the model but for now, keep it simple.

Regarding your facts about the wide supply voltage range, this is because whether you have the 3V or the 5V model, RAW supply voltage (sometimes marked BAT) can be anywhere in the wider range and the board has a voltage regulator that precisely outputs either 3.3v or 5v depending on which model you have. As long as we're to this detail, you can determine which board you have by simply connecting a power supply of at least 6 volts to the RAW or BAT terminal and then measure the voltage OUTPUT of the Vcc pin of the Pro Mini. The voltage reading will match the board that you have, i.e., 3V/8MHZ or 5V/16MHz.

Hope this helps.

Good luck,

You probably got a 3.3v@8MHz version instead of 5v@16MHz. Is it an official board or a cheap clone?

Yeap, it's a clone

Based on your comments and this video Arduino Pro Mini Identification - 5V, 3.3V, 8MHz or 16MHz - YouTube

I found out it is an 8MHz @ 3.3V:

3.3V regulator

8.0MHz crystal (?)

Thanks for your help guys!