On Apr 8, 2013, at 05:40 , Graham Davies <[email protected]> wrote:
> Rick Mann wrote: > >> ... I'm not sure I can calibrate the internal oscillators via DFLL using an >> external crystal [on the XTAL pins] ... > > You can't. The only calibration sources for the DFLLs are the 32.768 kHz > internal oscillator, the 32.768 kHz crystal oscillator using the TOSC pins > and, for the 32 MHz DFLL only, the USB start-of-frame signal. > >> There doesn't seem to be a way to drive the RTC from the XTAL pins ... > > No. > >> ... nor can you calibrate the internal 32 kHz clock from an external crystal >> (on XTAL1 & 2). > > Yes you can, but you have to do this with a firmware DFLL using the RC32KCAL > register. > >> So, I guess the best thing is to just stick a 16 MHz crystal on the XTAL >> pins and be done with it? I need USB. If I understand things correctly, I >> can PLL that up to 96 MHz, then divide by 2 to get 48 MHz. > > This is correct. You could use any crystal or resonator that will multiply > up to 48 or 96 MHz and meets your frequency accuracy requirements. However, > the maximum CPU clock frequency will be 24 MHz. You could also generate the > USB clock independently of the CPU clock using the DFLL to synchronize the 32 > MHz clock to the USB start-of-frame signal, which will cause it to run at 48 > MHz. Then you have more options available for the CPU clock. You do not say > how accurate you need the CPU clock to be. Thanks, Graham, it's very helpful to get confirmation. I only need it to be "accurate enough" to do USB and serial reliably. A future project that requires RTC will get a proper crystal for that, but this one doesn't. So I'm putting in a MHz crystal and it should be fine. -- Rick _______________________________________________ AVR-chat mailing list [email protected] https://lists.nongnu.org/mailman/listinfo/avr-chat
