AN2867 datasheet(11/41 Pages) STMICROELECTRONICS

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AN2867

Pierce oscillator design

This section describes the different parameters and how to determine their values in order

to be compliant with the Pierce oscillator design.

3.1

Introduction to pierce oscillators

Pierce oscillators are variants of Colpitts oscillators which are widely used in conjunction

with crystal resonators. A Pierce oscillator requires a reduced set of external components

which results in a lower final design cost. In addition, the Pierce oscillator is known for its

stable oscillation frequency when paired with a crystal resonator, in particular a quartz-

crystal resonator.

Figure 5. Pierce-oscillator circuitry

Inv: the internal inverter that works as an amplifier

Q: crystal quartz or a ceramic resonator

RF: internal feedback resistor

RExt: external resistor to limit the inverter output current

CL1 and CL2: are the two external load capacitances

Cs: stray capacitance is the addition of the microcontroller pin capacitance (OSC_IN and

OSC_OUT) and the PCB capacitance: it is a parasitic capacitance.

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