Arduino Pro Mini 3.3V 8MHz?

Development Tools IDE 1.x
1

Using a Arduino Pro Mini 3.3V 328P the board selector definitions are here:

## Arduino Pro or Pro Mini (3.3V, 8 MHz) w/ ATmega328
## --------------------------------------------------
pro.menu.cpu.8MHzatmega328=ATmega328 (3.3V, 8 MHz)

pro.menu.cpu.8MHzatmega328.upload.maximum_size=30720
pro.menu.cpu.8MHzatmega328.upload.maximum_data_size=2048
pro.menu.cpu.8MHzatmega328.upload.speed=57600

pro.menu.cpu.8MHzatmega328.bootloader.low_fuses=0xFF
pro.menu.cpu.8MHzatmega328.bootloader.high_fuses=0xDA
pro.menu.cpu.8MHzatmega328.bootloader.extended_fuses=0xFD
pro.menu.cpu.8MHzatmega328.bootloader.file=atmega/ATmegaBOOT_168_atmega328_pro_8MHz.hex

pro.menu.cpu.8MHzatmega328.build.mcu=atmega328p
pro.menu.cpu.8MHzatmega328.build.f_cpu=8000000L

so by selecting this board one would expect that the selected frequency is indeed 8MHz.

However, when opening the serial monitor, the only way to get meaningfull output is to select a monitor baud rate that is double the one specified in Serial.begin(1200); being in this case a monitor speed of 2400 baud.

Or if any other serial speed is selected, than the monitor speed has to be double that speed.

What is wrong?

2

It looks like your board has a 16 Mhz crystal. It's always possible that when you ordered a "3.3 V Pro Mini", they send you the "5 V Pro Mini" instead.

3

pert:
It looks like your board has a 16 Mhz crystal. It's always possible that when you ordered a "3.3 V Pro Mini", they send you the "5 V Pro Mini" instead.

The crystal has no frequency marking. Is there a way to find out at what frequency the oscillator is running?
I have an oscilloscope but connecting directly to the oscillator will distort the result.

4

I think you already discovered the way.

5

pert:
I think you already discovered the way.

:grin: So these are not 3.3V MCU's?

6

The MCU is the same ATmega328P on either board. The thing is that the ATmega328P is not rated to run at 16 MHz at 3.3 V. This is why the 3.3 V Pro Mini has an 8 MHz clock source. You can run the ATmega328P at 8 MHz with 5 V supply no problem. However, if you run the ATmega328P at 16 MHz with a 3.3 V supply, it might not work reliably (though people certainly do it).

In the ATmega328P datasheet, there is a chart that shows the maximum rated speed at any given supply voltage.