I've a project using an XMEGA 64 (or 128) A4U.
It's gonna be subject to large temperature swings, from -40C to maybe +40C. It'll be asleep 99% of its life.
For accurate use of USB/Serial, I need the baud rate to remain pretty consistent over this swing.
To use the DFLL to get the system clock spot-on I've the choice of the 32KHz RC oscillator (RC?? At those extremes? Shudder!) or a 'watch crystal'.
Now, on the board I have a DS3232MZ 'Extremely accurate RTC'. It uses a MEMS, is temperature compensated, with correction in its EEPROM. Couple of minutes a year error, max.
The datasheet says that those inputs are the only choices I have. Strangely, the TOSC1 pin is the same as XTAL1, when configured thus, it can have an external clock shoved up its jacksy. BUT, configured so (EXT-Clock) DFLL isn't on offer. Crazy, but...
Unfortunately, to use an external clock to drive the PLL (as distinct from the DFLL) needs a minimum soource frequency of 0,4MHz (or, if you turn to the very next page in the datasheet, 0,44MHz...). My 'plan-B' until I saw that paragraph.
Question: Has anyone ever 'told' an XMEGA to use the TOSC as a crystal oscillator, but shoved 32,768Hz into TOSC1 instead, and used the DFLL as calibration for 32MHz osc? I'd try, but really don't wanna unsolder a 44-pin chip off the board if it causes the Blue Smoke Fairy to visit..
EDIT: Or am I misunderstanding the A4U manual? The A4U DATASHEET has a diagram (Fig. 7.6) implying I CAN do this, but the MANUAL doesn't give this option...