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Convert Arduino Pro Mini 5v to use 3.3v

Id like to make my 5v pro mini use 3.3 v instead of 5 because i want to use it in an existing piece of electronics that uses 3v throughout.

Looking at the specs for the pro mini it says the 3.3v version can accept higher voltages.

So is there a regulator i can bypass/bridge that will convert my 5v version to a 3.3v version? or am i wasting my time?

Answer*

I have good news, and I have bad news.

The good news is that only **two changes** to the board are required to change it from a 5V board to a 3.3V (or any voltage within spec) board.

The bad news is that there's *no way in hell* you'll be able to do them.

So, we work around them and get the board up and running.

The first is obvious: a 3.3V board should be running off of 3.3V. Since we can't replace the SOT23-5 regulator on the board (the little 5-pin device towards the end) we need a voltage regulator to supply this, directly to the VCC pin. Find yourself a linear or switching regulator that provides this voltage.

The second is that the microcontroller, an Atmel ATmega168/P, is not specced to run at 16MHz when the supply voltage is only 3.3V; the maximum speed is 12MHz. Since we can't replace the crystal on the board (the little silver package next to the MCU itself) we need to perform some trickery to tell the MCU divide the frequency down to something way below safe, and then boost it up after. Use a programmer (see some of my other answers for more info about this) to set the CKDIV8 fuse and then use the routines in <a href="http://www.nongnu.org/avr-libc/user-manual/group__avr__power.html">`avr/power.h`</a> in your code to "reduce" the divisor to 2, bringing the clock speed back up to 8MHz. While you're at it, check the BOD fuses to make sure they're not set to something useless like 0b100.

As always, see the datasheet.

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wow great. thanks. i may attempt this, but it sounds like it would be easier for me to just get another (3.3v) pro board. theyre cheap and i can use the 5v version some other time.

S Rosam
– S Rosam

2014年12月10日 08:59:45 +00:00
Commented Dec 10, 2014 at 8:59
I'm curious: Why is there no way in hell the OP will be able to do this? It sounds like the 3V3 supply won't be an issue ("because i want to use it in an existing piece of electronics that uses 3v throughout"). Is setting the fuses properly much more difficult than setting up an ArduinoISP and telling your Arduino IDE to write a bootloader for that configuration?

Christopher Creutzig
– Christopher Creutzig

2014年12月14日 15:24:36 +00:00
Commented Dec 14, 2014 at 15:24
@ChristopherCreutzig: The changes described can be performed by replacing two components on the board. However, even if one had the equipment and skill to replace them it would be a difficult task due to the density of the board. Replacement by someone that doesn't know what's required in the first place? Not happening.

Ignacio Vazquez-Abrams
– Ignacio Vazquez-Abrams

2014年12月14日 16:57:38 +00:00
Commented Dec 14, 2014 at 16:57

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