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I know that running the 328 with 3.3v at 16MHz is "overclocking" it and slightly out of spec. Because of limited board space i'd like to do this though as all other components are 3.3v. I need the 16MHz.

I'm using a TQFP 328 and am planning on burning the arduino bootloader to it and then working with FTDI. I know that FTDI cables come in 3.3v, which should be ok, but i'm wondering what the impact on burning the bootloader will be. Will the bootloader burn fine at 3.3v even if it's actually too low? Other concerns or suggestions?

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    \$\begingroup\$ You should follow the recommendations in the data sheet for a reliable design. \$\endgroup\$ – Leon Heller Feb 22 '13 at 9:47
  • \$\begingroup\$ You overall approach seems to be flawed. Why do you need full 16MHz? Also, there are other ATMEGA controllers that run @>=16MHz @3.3V. The bootloader probably "will burn fine" but this shouldn't be any measure for if the controller is running fine at 3V3@16Mhz in general. \$\endgroup\$ – Rev1.0 Feb 22 '13 at 9:53
  • \$\begingroup\$ I need the 16MHz for fast sampling on the ADC. If i design everything using 3.3v i guess worst case i can just put in a 8MHz crystal and it should be fine? \$\endgroup\$ – Antti Feb 22 '13 at 9:58
  • \$\begingroup\$ If you're going to run the microcontroller out of spec, nothing else matters. Any further discussion is therefore pointless speculation and a waste of time. This "question" needs to be closed. \$\endgroup\$ – Olin Lathrop Feb 22 '13 at 18:27
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Regarding you comment question "If i design everything using 3.3v i guess worst case i can just put in a 8MHz crystal and it should be fine?".

Look at the speed grades in the datasheet where maximum frequency vs voltage is given.

Speed grades

Since the curve is linear between 2.7V and 4.5V you can calculate the maximum speed @3.3V as follows:
fmax@3.3V = (3.3V / 2.7V) * 10MHz = 12.22MHz (i wasn't thinking)
You have to get the linear equation using the two-point form for example (given the two points form the diagram) which results in:
f = 50/9 * U - 5 = 50/9 * 3.3V - 5 = 13.33MHz

So you should be fine running at about 13MHz@3.3V.

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  • \$\begingroup\$ Great, i'll go with a 12MHz crystal \$\endgroup\$ – Antti Feb 23 '13 at 18:02

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