Re: [avr-chat] Urgent clock-related questions

[Graham Davies]

Rick Mann wrote:

> I only need [the system clocks] to be "accurate enough" to do USB and 
> serial reliably .... So I'm putting in a MHz crystal and it should be 
> fine.

Yes, this will be fine.  The following is for information only and I'm not 
suggesting you deviate from your plan, but you can meet your goals with no 
crystal at all.  Synchronize the 32 MHz oscillator to the USB start-of-frame 
and run it at 48 MHz to drive the USB sub-system.  Then, synchronize the 2 
MHz oscillator to the 32.768 kHz internal oscillator and use the PLL to 
multiply by 16 to get a 32 MHz CPU and peripheral clock.  This will be 
accurate enough for UARTs because the 32.768 kHz internal oscillator is 
factory calibrated and is fairly stable with temperature and voltage. 
Again, this is just so that you know that this is possible (I have done 
exactly this) but I'm not suggesting you do it.  Your plan with the external 
crystal is fine, probably better.

Graham.