So I'm looking at using the Atmega4809 as an upgrade to the Atmega328P/Atmega32U4, but I need it to run at 3.3V logic level.
I know I could level shift all the ports and be done with it, but if I was to apply 3.3V to the Arduino Nano Every, would I need to change anything in code or fuse bits to make it run as normal? I know it might have to run at a different clock frequency, would this upset a SPI or UART device?
The atmega4809 should run at around 13.33MHz at 3.3V, but using the build-in oscillator and prescaler limit the selection. The wiring.c file has code to handle 2, 4, 8, 16, and 20 MHz clock frequencies, so you can run at any of those by editing the boards.txt file. See the discussion at Clock speed for Nano Every - Nano Every - Arduino Forum for further information on changing the clock frequency, but to run at 8MHz only requires one change in boards.txt, then the compiler should take care of the rest.
nona4809.build.f_cpu=16000000L
change to:
nona4809.build.f_cpu=8000000L
I've tested the serial interface at 115200 baud at all five frequencies, and that much is working.
If you like to experiment a bit, you can modify the boards.txt file to add a selection for clock frequency in the IDE Tools menu by appending the following to the section for the Nano Every:
menu.clock=Clock
nona4809.menu.clock.2internal=2MHz
nona4809.menu.clock.2internal.build.f_cpu=2000000L
nona4809.menu.clock.2internal.bootloader.OSCCFG=0x01
nona4809.menu.clock.4internal=4MHz
nona4809.menu.clock.4internal.build.f_cpu=4000000L
nona4809.menu.clock.4internal.bootloader.OSCCFG=0x01
nona4809.menu.clock.8internal=8MHz
nona4809.menu.clock.8internal.build.f_cpu=8000000L
nona4809.menu.clock.8internal.bootloader.OSCCFG=0x01
nona4809.menu.clock.16internal=16MHz
nona4809.menu.clock.16internal.build.f_cpu=16000000L
nona4809.menu.clock.16internal.bootloader.OSCCFG=0x01
nona4809.menu.clock.20internal=20MHz
nona4809.menu.clock.20internal.build.f_cpu=20000000L
nona4809.menu.clock.20internal.bootloader.OSCCFG=0x02