Re: [avr-chat] XMEGA clocking questions

[Erik Walthinsen]

On 03/14/2013 05:38 AM, Graham Davies wrote:
> For USB, you re-purpose the 32MHz internal oscillator and use its DFLL
> to increase its frequency to 40MHz and synchronize it to the
> Start-of-Frame packet on the USB bus.  This cannot be used as the CPU
> clock, so you use (for example) the 2MHz oscillator as the basis for
> that and the PLL can then increase the frequency to any multiple,
> including 32MHz.  (Note that Atmel training sometimes gives a different
> story for USB clocking, however that is not correct; I am 100% certain
> that what I just wrote is the real situation.)

In theory you can use the 48MHz clock divided by 2 to drive the CPU at 
24Mhz, but there's no way to get 32Mhz out of the USB SOF+DFLL ;-(

Also, the 2Mhz DFLL can *only* be driven from internal or external 
32.768KHz, which means you can't use the SOF to calibrate the CPU clock 
as driven by the 2MHz * PLL.