Re: [avr-chat] XMEGA clocking questions

[David Kelly]

On Mar 14, 2013, at 12:04 PM, Rick Mann <address@hidden> wrote:

> 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. 

Will it kill you to have to make room for the crystal? You don't have to load 
it if you don't need it. Plus I find Murphy's Law has a heavy hand in such 
things, you can make space for a crystal which you will not need, or don't and 
it will almost be certain the onboard RC oscillator is not accurate enough.

I brought my first Xmega up on an XPlain board. Wanted a 2.500 MHz SPI clock so 
I programmed the oscillator chains for 30.0 MHz as 15 times 2.000 MHz 
supposedly corrected using 32 kHz crystal reference. The result was closer to 
31 MHz but close enough to 30 that other things were more important to make 
work before doting i's and crossing t's. Since, Atmel has updated their 
libraries. I need to drill through the changes to see if the clock error was 
theirs or mine.

Before my PCB is finished I may simply drop a manufactured oscillator on the 
board. Brute force. Hit it over the head with a sledgehammer. Its not worth the 
extra NRE.

--
David Kelly N4HHE, address@hidden
============================================================
Whom computers would destroy, they must first drive mad.