1

It looks like the 5 volt version used the 16MHz external crystal while the 3.3V one uses the 8MHz crystal. I need to use 3.3v as my operating voltage.

Can I just remove the linear regulator and bring in 3.3V. I still have to select the 5V 16MHz version in the Arduino IDE, right?

https://www.arduino.cc/en/uploads/Main/Arduino-Pro-Mini-schematic.pdf

  • Don't make it work at 5V @16MHz, since it is out of the safe operating area – frarugi87 Mar 10 '16 at 21:31
2

The ATMega328p is not rated to operate at 16 MHz on a 3.3v supply.

In practice it may sometimes work, but cannot be considered reliable.

The data sheet will show the actual clock rate vs. voltage tradeoffs, so there might be an intermediate value that is workable for you.

The Arduino toolchain needs to know the CPU clock frequency for the serial baud rates, timing delay code, and any time-based use of the timer peripherals to come out right.

There is a special case where if the configured clock rate and the actual clock rate differ by a factor of two, you may be able to cover that by selecting a similarly wrong baud rate - however, the baud rate in the bootloader was set in comparison to the expected clock frequency when it was compiled before flashing, and the baud rate expected of the bootloader is set in the boards file, so less readily changed than it is in something like the serial monitor.

  • And if I wanted to use the same arduino board, can I simply replace the crystal with an 8MHz one and bypass the linear regulator with my own 3.3V supply? I know it seems like a lot of work... lol but i have spare parts around. – Samee87 Mar 11 '16 at 1:18
  • Some components on the board may be expecting 5v (LEDs immediately springs to mind); you may also need to change the fuse settings - engbedded.com/fusecalc - in fact, if clock accuracy isn't terribly important, then you can change the fuse to use the internal 8mhz clock, save yourself some soldering. – AMADANON Inc. Mar 11 '16 at 1:59
  • PS - setting fuses is advanced black magic - it's easy to brick your Arduino that way (as I did) if you get it wrong. – AMADANON Inc. Mar 11 '16 at 2:00
  • Changing the fuses isn't necessary when changing the crystal. What you do need to change is the bootloader to the 8MHz version. – Gerben Mar 11 '16 at 17:03
  • If the brown out detector is already either disabled or set at 2.7v or below that may be the case (seems to be in the boards.txt). If it is set at 4.3v there would be a problem. Also if the "3.3v" is really two AA cells that will drain to a lower voltage in operation, the chip might be fine but the 2.7v BOD will shorten the usable life. – Chris Stratton Mar 11 '16 at 17:14

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