I've picked up some m328p based development boards that have an external crystal (or resonator) fitted and a 3.3V regulator fitted. The regulator supplies the m328p, an RFM69 radio transceiver and an optional flash chip. I used Nick Gammons code (reply #14) to determine a rough approximation of the external crystal/resonator frequency. All indications are that it's around 16MHz, which is outside the manufacturers spec for 3.3V operation.
So, can I bring it in spec as it were, by setting the system clock prescaler to "divide by 2", giving an 8MHz clock to the rest of the chip? Obviously a very small portion of the chip will have to handle the 16MHz clock to divide it by 2.
The datasheet (Microchip 2018 - DS40002061A) hints that this approach might be ok. Section 9.6 - Calibrated Internal RC Oscillator - table 9-11 note 2 says "If 8MHz frequency exceeds the specification of the device (depends on VCC), the CKDIV8 Fuse can be programmed in order to divide the internal frequency by 8."
Ok, that scenario is talking about the 8MHz internal RC oscillator being too fast for the chosen operating voltage.
But, can I infer that dividing a 16MHz external clock/resonator by 2 is ok for 3.3V?