Arduino Pro Mini (5v) ext. crystal running at high freq?

I have a Arduino Pro Mini (5v) board that seems to behaving "interesting". First off, if I run with internal RC oscillator @ 8MHz, F_CPU=8MHz then it seems to be OK. I have an LED flashing and serial output @ 9600. However, when I switch to ext. oscillator (8MHz) it looks like the oscillator is running at 16MHz. The LED flashes twice as fast and the serial port is outputting at twice the baud rate.

These are the settings I have:

For Internal RC 8MHz operation:

Fuses: E:FD, H:DA, L:E2
baud = 9600
UBRRn = 103
U2Xn = 1

For external oscillator 8MHz operation:

Fuses: E:FD, H:DA, L:FF
baud = 9600
UBRRn = 103
U2Xn = 1

If I open the serial device @ 9600 baud I get garbage but with 19200 it looks fine.

So looks like the oscillator is running at 16MHz?

Note, the board came with a program that was blinking the LED and the fuses were by default set to E:FD, H:DA, L:FF
I also believe I had it working fine for a bit with this configuration....

I don't have a LA right now so I cannot check the actual frequency of the crystal.


Can you post a link to the Pro Mini you bought? Typically, 5V ones do indeed run at 16MHz.

I use both 3 and 5volt prominis and the 3 runs at 8 and the 5 runs at 16 MHz
The IDE uses these settings and will need the serial baud rate setting half or double if the wrong board voltage is selected. Very useful to have serial print to see that that the correct board has actually been selected.

There seems to be an influx of incorrect pro minis. Marked 5V but with 8MHz fitted, marked 3.3V but with 16MHz fitted.

Can you decypher the crystal / ceramic resonator on the board?

If those are official Sparkfun ones, you can ask them. If they are clones, that happens when you buy cheap stuff :wink:

Have you selected the correct board type and voltage in the IDE?
I have not had any problem with both types of promini as long as the board selected and the chip/voltage is selected correctly in the IDE. If there are dubious clones about I have not had one - yet.

Thanks for all the responses. Really appreciate the community.

So this is a cheap Chinese clone :slight_smile: [1]. It does say 16MHz/5v so I assume it is running at 16MHz then. I just now realize that I misread the fuse configuration. I was looking at [2] and it shows no "16MHz ext. oscillator" fuse configuration, or so I though. I didn't notice that the actual configuration said "Ext. Crystal Osc,; Frequency 8- MHz...". I didn't notice the "-" after 8 so I thought it was configured as 8MHz (and not 8MHz or higher). It makes sense now.

What is the "full swing" oscillator fuse configuration used for?

There aren't any useful (as far as I can tell) markings on the crystal. It says either "A.G." or "A.Q" or something like that.

I am using Makefiles and not the IDE btw.

Again, thanks for the help. "Mystery" solved.


Thanks for the update, it's good to know you got it sorted.