Hi - I have an ATMEGA168PV-10AUR with an Abracon ABS25-32.768KHZ-6-1-T crystal attached. The crystal runs a bit slow - about 150 seconds/month slow without any capacitors on either line. It initially was about 250 seconds/month with a 10pf cap on each line. I tried swapping crystals and that did not change anything.
Reading the ATMEGA168 datasheet more carefully, I see this section:
"The Low-frequency Crystal Oscillator provides an internal load capacitance of typical 6 pF at each TOSC pin."
So that means it presents a load capacitance of 3pf (6pf || 6pf = 3pf) to the crystal. The crystal I'm using has a CL of 6pf, so it is still missing 3pf (ie 6pf - (6pf||6pf) = 3pf). So I should have used something like 4-5pf for my load capacitors ((5pf + stray) || (5pf + stray) = 3pf, if stray is 1pf). And yet without any load capacitance at all, I'm still slow. And we all know adding more capacitance will just slow it down further.
My traces (6 mil wide) are about 20mm long each. 62mil PCB (2 layers), so they should have less than a pf on each of them. They do spend about 10mm right next to my ground plane (same layer), with 6 mils between them and the ground plane. I do not have a formula for calculating how much capacitance that adds. Can't be much.
My current plan is to switch to an Abracon ABS25-32.768KHZ-1-T, which is a 12.5pf part. That should hopefully account for the extra capacitance that is messing with me. My only concern there is that it has a 50K ESR and the ATMEGA168 datasheet says that for a 12.5pf crystal you need your crystal to have an ESR of 30K or less. Thing is - Digi-Key doesn't stock a single 32K crystal that has an ESR of less than 50K.
Can anybody give me any ideas as to why this doesn't work? I am completely confused. Any feelings as to what bad things will happen due to my crystal's ESR being too high?