how to select capacitance for crystal

hi in this diagram u may have seen that 27pf is used for 10MHz crystal

how is that possible?

how capacitance is decided for crystal ???

this Capacitance is used for supplying charge for crystal ..am i right???
if it not tell me the exact reason for providing those capacitance


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is any formula involved for calculating Capacitance for crystal??

I think the crystal was designed to use that capacitance. There's no equation. Crystals of the same frequency might need different load capacitances. It's just in the datasheet of the crystal.

It's generally in the range of 20-30pf changing the capacitance with a small trimmer can tune the frequency juuust slightly if need be. I'm not sure how the capacitance is calculated for a crystal but whomever you buy your crystals from should specify the cap value with the crystal like Dk says.