Re: [avr-chat] XMEGA clocking questions

[Rick Mann]

Hmm. Okay. I'm still not clear, though, on whether I need a crystal to ensure 
reliable USART and USB operation.

I see that the 32.768 kHz crystal on the dev boards could be used to 
automatically calibrate the 32MHz internal clock, although I'm not sure if 
they're actually doing that. 

I'm laying out a board now, need to know if I should include the crystal or 
not. 

Sent from my iPhone

On Mar 14, 2013, at 9:29, Erik Walthinsen <address@hidden> wrote:

> 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.
> 
> 
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