Hello:
An 32Khz(20ppm) crystal is used in our project, load capacitance requirement of this crystal is 9pF or 12.5pF.
When the internal capacitor setting is 5.5pF and no external capacitor, I got temperature drift is +25°C (-0.63sec/day), +60°C (-3.47sec/day), -20°C (-5.43sec/day).
With this internal capacitor I add also an external capacitor like 3pF, then drift become bigger, +25°C (-2.61sec/day), +60°C (-4.38sec/day), -20°C (-6.54sec/day). Is that means only internal 5.5pF(no any external capacitor) is already too heavy for crystal. So the only way is to decrease internal capacitor like 2pF, also add some external capacitor to meet +/-1.7sec/day drift in all temperature range requirement.
Could you provide me some formula to calculate the correct internal/external capacitor setting?
Please give me some response, thanks!
Br. LD
