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I am thinking about getting both the Pro Mini 3.3V and the 5V boards.

Example (but I may end up going generic, not sure yet) 3.3v: https://www.sparkfun.com/products/11114 5v: https://www.sparkfun.com/products/11113

Any way, I understand I can supply up to 12V of unregulated power through the RAW pins on both boards. But I need a very energy efficient set up since I'll be using batteries and I did some research that the voltage regulator wastes some power. Therefore, I'd like to send power direct to the VCC pin. What is the min/max voltage I can send through VCC for these two models? Is it exactly 3.3 and 5 respectively, no tolerance or margin? or is there some?

thanks!

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    \$\begingroup\$ Have you looked at the datasheet? \$\endgroup\$ – Matt Young Mar 29 '14 at 4:57
  • \$\begingroup\$ It's right on the first page. Both important parts. \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 29 '14 at 5:06
  • \$\begingroup\$ Hi yes I did, (dlnmh9ip6v2uc.cloudfront.net/datasheets/Dev/Arduino/Boards/…) but they say the max voltage is 16V but I understand that is only to the RAW pin and not VCC. VCC, i have seen on some forums that you should give properly regulated voltage to VCC - so I am curious what the tolerance is for this pin for the 3.3v and 5v versions. thanks! \$\endgroup\$ – Chris Drumgoole Mar 29 '14 at 7:54
  • \$\begingroup\$ That is the schematic for the arduino PCB, not the datasheet for the ATmega328P. That's not a datasheet, and not the relevant document anyways. \$\endgroup\$ – Connor Wolf Mar 29 '14 at 9:01
  • \$\begingroup\$ @ChrisDrumgoole Answer below is correct on the microcontroller voltage. Not sure about that particular regulator (the MIC5205) but personally I'd remove it, some regulators can draw excessive current if a voltage is on the output without an input. \$\endgroup\$ – PeterJ Mar 29 '14 at 10:11
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Page 313 in the ATMEGA328P datasheet: Vcc 1.8V to 5.5V. Absolute maximum 6V, but then a lot of parameters are not specified.

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  • \$\begingroup\$ thank you very much for pointing me in the right direction! \$\endgroup\$ – Chris Drumgoole Mar 29 '14 at 11:31
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In addition to the VCC ratings of 1.8V to 5.5V, max 6V as mentioned by Dejvid_no1, there is another issue with voltage levels on VCC.

As your raw, unregulated battery voltage is being used as the system VCC reference, this means that any logic signals on the inputs and outputs will be affected along with it. If at full battery your voltage is 4.2V for example, off a single cell Li-Po battery, you will not be able to receive logic "HIGH" from 1.8V devices like GPS transmitters etc. However as the battery drops in voltage, the comparison level for "HIGH" and "LOW" changes and will eventually pick up the signals. This happened to me in a low power battery into VCC rail application!

The other issue is clock speed. The ATMEGA328 datasheet specifies minimum VCC voltage levels for certain clock speeds, to guarantee correct operation. If you want < 10MHz, or want to use the internal 1Mhz oscillator, then you can give it whatever you want in that allowable VCC range - but if you want to use 10+Mhz and external crystals etc, you must supply it with 4.5V+. Please double check the datasheet for the exact voltage requirements and clock speed - there is a table/chart with the speed and voltage listings.

Obviously you'd need the full 5V for the maximum rated clock speed of 20Mhz but your application being low power battery operated hopefully does not need such high clock speeds.

edit: Also, the regulator on those boards are linear regulators, which literally burn the difference in voltage as heat. I strongly suggest you avoid putting anything more than 12V on any "arduino" product, as they all use linear regulators - dropping 7V and using any reasonable power will blow up the regulator pretty fast.

If you ever get into your own PCB making, for example your own custom pro-mini style board, I suggest you look at the Texas Instruments "Simple Switcher" and similar lines of DC-DC Switchmode power converters which are very low part count, and easy to use. They are 85-90% efficient in most cases, rather than 35-40% like the linear ones with inputs at ~12V. DC-DC converters often allow quite high inputs, like 36-40V, down to your 5V or 3.3V output, so they are pretty amazing once you learn how to use them.

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