I'm using Attiny45s with Abracon ABS10-32.768KHZ-1-T crystals. It seems like some of them - and I've only just come to this conclusion, so I can't quantify it yet - run way, way fast. Like, ~200 ppm fast.
Now, according to the datasheet, there isn't really any trick to hooking these crystals up. The crystal is physically located right next to the chip, the traces are really short, and there's nothing on the other side of the board.
These crystals supposedly have a tolerance of 10 ppm. Is there something about them that disqualifies them for this application?
For unrelated reasons, I am considering the Citizen CM315D32768EZFT as a potential replacement. Is there anything I've missed about either or both of these that suggest one is going to be more accurate than the other?
Oh, the fuse settings: -U lfuse:w:0xe6:m -U hfuse:w:0xd7:m -U efuse:w:0xff:m