I have been searching around this forum, other forums, farnell, digikey, etc without results! So, fellow Avrfreaks, I ask you:
What is the optimal SMD type 32.768kHz crystal for Atmega164/324/644p for RTC purposes? I see that the Atmel datasheet recommends Max ESR of 65kOhms for 9pF CL and 30kOhms for 12.5pF CL. I have found many 12.5pF SMD crystals, but all of them have ESR of 50kOhms. I also found a lot of 12.5pF trough-hole cylinder types that had only 35kOhms, but I'd rather not face the assembly effort and space requirements for them. What crystals are tested to work well?
Or would it be a better idea to use 6pF CL crystals? Does lower CL value mean weaker signal for the oscillator? The system around the crystal will be quite noisy no matter how well I try to protect it...