AVR Freaks

Basically the "32MHz RC oscillator" can actually be tuned to run up to 48MHz. It's more like a "high frequency RC oscillator", the name is misleading. The ASF code loads a value into the calibration register that produces 48MHz.

Atmel state that the maximum CPU frequency for XMEGA parts is 32MHz. So when using the "RC32M" oscillator as the CPU clock and USB clock you need to set the divider to 2. That then gives you 48MHz for USB and 24MHz for the CPU and other peripherals. The limit is given in the datasheet for the part you have, e.g. in the E5 datasheet it is table 36-2.

Of course, if you actually try to run the XMEGA CPU at 48MHz it will probably work. I've tested one up to 64MHz, and found it to work up to about 60MHz without any noticeable issues. I didn't do a thorough test though, and that's the problem. Atmel don't guarantee anything above 32MHz, so for example you might find that the ADC gives bad readings or TWI doesn't work. Best to stick to 32MHz maximum.

Very simple. Just output system clock on pin PC7 and measure it with scope.

PORTC.DIRSET = 0x80;
PORTCFG.CLKEVOUT = 0x01;

You can also in theory output clock X2 and X4, but for my XMEGA128A1U X2 clock is npt outputted on pin, though internally everything works OK.

I wish, I could... Unfortunately you can multiply by any integer number in range 2..31, but you can divide PLL by 2 only. 48 * 4 = 188MHz would do, but PLL is 128MHz only :(

Hmm. As I understand you can run CPU at 32MHz and have onboard USB enabled, but you must use detailed above clocking (XTAL -> PLL -> CPU, RC32M -> RC48M -> USB). You can not use single clock source and have USB and max CPU speed, 24MHz only.

PLL should have had "DIVIDE BY 3" option :(

I don't argue about calculation and your intension. I simply do not know how to divide PLL output by 3.

Have a look at PLLCTRL register:

7:6 PLLSRC (XTAL/RC)

   5 PLLDIV (factor of 2 only)

4:0 PLLFAC (multiplication only)

You can divide this clock further by 2 or 4, but not by 3.