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| From: | Graham Davies |
| Subject: | Re: [avr-chat] Urgent clock-related questions |
| Date: | Mon, 08 Apr 2013 08:40:23 -0400 |
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.
Graham Davies. ECROS Technology.
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