We just discovered that our external RTC, Maxim DS1374C, is not to be trusted in low temperatures. The internal clock divider of the RTC units fails for some units when temperatures falls below 0 C, which is verified by their support. Crazy, but a fact that we have to handle for our sensor boards that we manufacture. They say that it's probably a manufacturing error in their component.
So, we need to move away from using the external RTC to time our events. Specifically we need an accurate wake up interrupt every 24 hours as well as an interrupt to handle sampling of our sensors at 512 Hz. We have ween using the external RTC for both during the last few years, but now have to find a new solution.
A nice alternative seems to be the internal RTC of the Xmega (we're using the Xmega 256a3). If we could use that we could fully eliminate the need for the external RTC. However, from what I understand it needs a 32 kHz crystal to clock it. Right now we only have a 4 Mhz crystal connected to the XTAL pins. Isn't there some way we could use that to clock the internal RTC? We use the PLL to clock the processor at 32 Mhz from the 4 Mhz clock today.