Are they always the same or not? It looks that they are. But I don't see this clearly spelled in the (ATmega4809) datasheet. Interestingly, one of the older ATtiny417/814/816/817 datasheet says "On this device, CLK_CPU and CLK_PER share the prescaler setting, and hence, always have the same frequency". But for some reason they removed that note from newer datasheets.
They seem to be always the same. The only difference is that CLK_CPU is disabled in idle mode, while CLK_PER is not.
Same problem in ATtiny 1604. Despite what the documentation says, the frequency of these two signals CLK_CPU & CLK_PER is the same and depends on the Main Clock Prescaler settings. Because of this, a problem arises, either the program is executed at a clock frequency of 20/16 MHz, but the USART does not work, or the USART works, but the program is executed at a clock frequency of 10 MHz or lower. By the way, there is not a word in the documentation that USART does not work if CLK_PER > 10 MHz. I wonder if Microchip is aware of these issues?
awneil 
There was a thread about this recently: https://www.avrfreaks.net/forum/meaning-clkmain
Specifically: https://www.avrfreaks.net/commen...
I wonder if Microchip is aware of these issues?
Please raise a support ticket - then they will be aware!
By the way, there is not a word in the documentation that USART does not work if CLK_PER > 10 MHz.
What? I'm pretty sure I've used the USART at 20MHz, not exactly on the ATtiny 1604 but on other chips of the AVRxt (xtiny) series.
