Power arduino UNO with 3.3v to 4.0 v

Hello, can i power arduino uno with 3.3 v or 4.0 volts without reduce the MHz. On the board the crystal are 16 MHz. Can arduino run stable with this supply and this frequency ?

Figure 29-1 in the datasheet of the 328P.

For 3.3V, no.
For 4V, yes.

Thanks, i will try.

That said, people seem to report that the 328p works at 16mhz 3.3v, or at least most units do at room temperature.

However, it is outside spec and may not work reliably or over full temperature range, or with all individual parts.

Note that you would need to put that voltage on the 5v pin, and if the board was plugged into USB, 5v would be applied to that pin, which could damage the power supply and/or board.

I'd use a pro mini and power via vcc pin on 6-pin ftdi header, thus keeping you from plugging it into a potentially 5v serial adapter at the same time as the lower voltage supply. They even make 3.3v pro mini with 8mhz crystal installed...