datasheets, crystal manufacturers indicate the exact CL required to make the crystal oscillate at the nominal frequency.Table1 gives an example of equivalent crystal circuit component values to have a nominal frequency of 8 MHz. Using equations (2), (3) and (4) we can determine Fs, Fa and Fp of this crystal: and .If the load capacitance CL at the crystal electrodes is equal to 10 pF, the crystal will oscillate at the following frequency: .To have an oscillation frequency of exactly 8 MHz, CL should be equal to 4.02 pF.Table 1.Example of equivalent circuit parametersEquivalent componentValueRm 8 Lm14.7 mHCm0.027 pFC05.57 pFFaFs1CmC0 -- -+=FpFs1Cm 2C 0CL+ ()-- -+ =Fs7988768 Hz=Fa8008102 Hz=Fp7995695 Hz=AN2867Oscillator theory 7/202 Oscillator theoryAn oscillator consists of an amplifier and a feedback network to provide frequency selection. Figure3 shows the block diagram of the basic principle.Figure 3.Oscillator principleWhere: A(f) is the complex transfer function of the amplifier tha January 2009 Rev 11/20AN2867Application noteOscill Application Notes;General Purpose Products;32-Bit Microcontrollers>Stm32 (Arm Cortex M3) - 32-Bit Microcontrollers;32-Bit Microcontrollers>Str9 (Arm) - 32-Bit Microcontrollers;8-Bit Microcontrollers>Stm8 - 8-Bit Microcontrollers;AN2867