AN2867 datasheet(20/41 Pages) STMICROELECTRONICS

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Guidelines for selecting suitable crystal and external components

AN2867

20/41

DocID15287 Rev 9

Guidelines for selecting suitable crystal

and external components

4.1

Low-speed oscillators embedded into STM32

microcontrollers

The low-speed resonator market provides a wide range of crystal resonators. Selecting the

most adequate one for a given design depends on many parameters. Below a list of the

most important parameters that must be taken into account (only technical factors are

listed):

•

Crystal size or footprint

•

Crystal load capacitance (CL)

•

Oscillation frequency offset (PPM)

•

Startup time.

A trade-off between the above parameters must be found depending on the key design

criteria. Figure 9 shows that the resonators available on the market can be divided into two

categories depending on the above mentioned factors and trade-offs.

Figure 9. Classification of low-speed crystal resonators available on the market

A resonator with a relatively big load-capacitance (such as 12.5 pF) will require more power

for the oscillator to drive the oscillation loop at the resonator nominal frequency. Designs

targeting low-power consumption (e.g. RTC application powered by coin-batteries requiring

very long autonomy) are consequently more likely to use resonators with relatively small

load capacitance. On the other side, big load capacitance resonators have a much smaller

pullabilty compared to resonators with small load capacitance. As a result, designs without

severe constraints on power consumption tend to use big load capacitance crystals to

reduce pullability.

AN2867 Datasheet(PDF) 20 Page - STMicroelectronics