Equivalent Circuit and Paramenters of Crystal
Many digital circuit it needs external crystal for microprocessor. And usually the datasheet tells you its input and output capacitance. So how you choose a crystal with correct parameters? The following descriptions can give you a formula.
 Equivalent Circuit and Paramenters:
Equivalent Series Resistance ( R1 ):
The resistance is motional resistance ( series ) in the equivalent electric circuit.
Motional inductance ( L1 ):
The inductance is the motional inductance ( series ) in the equivalent electric circuit.
Motional Capacitance ( C1 ):
The capacitance is the motional capacitance ( series ) in the equivalent electric circuit.
Shunt Capacitance ( Co ):
Shunt ( parallel ) capacitance ( Co ) is the capacitance between the crystal terminals. It is defference in package, usually it is less than 7.0pF.
Load Capacitance ( CL ):
The load capacitance is the circuit equivalent capacitance looking the circuit system from the two wire leads of crystal. The operation frequency in the circuit is defined by load capacitance and crystal.
U1 : ASIC
C1C2: capacitor which is connected
CL = [(C1*C2)/(C1+C2)] + Cstray + CU1
Cstray : stray capacitance
CU1: IC’s input/output capacitance
Cstray and CU1 may varies from 2 to 6pF