Re: [avr-chat] XMEGA clocking questions

[Graham Davies]

Rick Mann wrote


> It seems that it's really not necessary to include a crystal in 
> XMEGA-based designs, even if using the USART or USB, is that correct? The 
> XPLAINED boards only have a 32kHz crystal, I assume for good real 
> timekeeping.

That is correct, although if you have a 32.768kHz crystal, you might as well 
use it.

> Or do they rely on that and the PLL to generate accurate peripheral and 
> USB clock frequencies?

It's not the PLL, its the DFLLs.  They can synchronize the 2MHz  and 32MHz 
internal oscillators to some other more accurate source.  Then you generate 
the peripheral and system clocks from these oscillators.  For USARTs, you 
can use the internal 32.768kHz clock as the reference.  It is factory 
calibrated to +/- 0.5%, drifts over temperature from 0 to 85 degrees C by 
about another 0.5% and over voltage from 1.8 to 3.3 V by perhaps 0.2%.  So 
in many practical cases this is accurate enough for a UART, though, again, 
if you have a 32.768kHz crystal, there is no reason not to use this as the 
frequency source for the DFLL.

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.)

Graham Davies
ECROS Technology.
AVR Consultant #13660.