Crystal Load Capacitance Calculator

Choose C1/C2 for your crystal load capacitance.

Values

Required load capacitance C_L (pF) Stray capacitance C_stray (pF) Capacitor configuration

Result

C1 = C2 = 18 pF → C_L = 12 pF

Crystal load capacitance

Every crystal specifies a load capacitance C_L (e.g. 12 pF, 20 pF). The two external capacitors plus stray capacitance must present that value to the crystal: C_L = (C1·C2)/(C1+C2) + C_stray. Stray (pads + pin) is typically 2–5 pF.

With equal caps: C1 = C2 = 2·(C_L − C_stray). For C_L = 12 pF, C_stray = 3 pF → 18 pF each. Wrong C_L pulls the frequency off — exactly what your Desk Companion firmware would fight.