It's already been stated that you do not need a crystal to use the USART or USB peripherals.
http://lmgtfy.com/?q=xmega+usb+no+crystal first link - section 3.2.2 On Thu, Mar 14, 2013 at 11:04 AM, Rick Mann <[email protected]> wrote: > 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 <[email protected]> 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. > > > > > > _______________________________________________ > > AVR-chat mailing list > > [email protected] > > https://lists.nongnu.org/mailman/listinfo/avr-chat > > _______________________________________________ > AVR-chat mailing list > [email protected] > https://lists.nongnu.org/mailman/listinfo/avr-chat >
_______________________________________________ AVR-chat mailing list [email protected] https://lists.nongnu.org/mailman/listinfo/avr-chat
