Arduino Pro Mini 3.3V

[BlakeM]

I purchased a couple of cheap 3.3V Pro Mini 328P "CLONE" boards to use in a project, I also purchased some FTDI FT232RL units to program the mini boards with.

I have used the RAW input pin as the FT232RL output is 5v & this RAW input pin uses the on board 3V regulator to supply the 328P.

I have it all working & have experimented a bit & found that if I supply 5V to the 328P I get 5 Volt signals at the output without any issues.

The datasheet says that the 328P will work with up to 5.5V Vcc.

When selecting the Processor in The Arduino IDE it has Atmega328 (5V - 16MHZ) or Atmega328 (3.3V - 8MHZ) to select from.
I have been using the Atmega328 (3.3V - 8MHZ) option.
If I use the Atmega328 (5V - 16MHZ) option the 328P runs very slow, if you try to just blink the on board LED one second on & one second off it takes about 2 seconds on & 2 seconds off--Slow.

My question is (Finally) why doesn't the 328P operate correctly when using the Atmega328 (5V - 16MHZ) option?
The 328P is fine with 5V to Vcc.

Is it due to the 328P is set to 8MHZ clock frequency?

If so, can I change this, I would like to experiment a little with this for something to do but need some advice.

Thanks

 

[BlakeM]

After a bit of reading if I have this correct, I would need to use a 16MHz Crystal Oscillator on pins 7 & 8 of the 328P to make this work, Pin Out Picture attached.
Do I have this correct?

Thanks

 

[DirtyLude]

Yes, you have this correct. The 5v and the 3.3v arduino mini differ in the external crystal used. The 328 can only run at 8mhz at 3.3v reliably. If you put in a 16mhz crystal you wont be able to use it at 3.3v reliably.

 

[BlakeM]

I see, thanks for the confirmation on this, it's basically to try to get a better understanding of things & how they work.
At a dollar each I can learn a lot.

Thank You