AN2867 datasheet(21/41 Pages) STMICROELECTRONICS

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AN2867

Guidelines for selecting suitable crystal and external components

One of the key emerging areas where crystal resonators are massively used is the handheld

wearable appliance consumer market (e.g. smart phones, Bluetooth kits). For this market

segment the crystal size is of critical importance. However it is widely known that small-

footprint crystals come always with high crystal ESR. For this kind of designs, the choice

may be harder if the target design has severe constraints in terms of power consumption

(which almost always happens). In this case, choose a crystal with a load capacitance as

small as possible to optimize power consumption even if this compromises pullabilty.

In addition, crystals with high ESR may have a slightly longer startup time. If there are no

constraints on crystal size, then it is recommended to choose a crystal with an ESR as small

as possible.

In noisy environment (which it is almost always the case for industrial applications), if there

are no constraints on power consumption, it is recommended to choose crystals with high

load capacitance. These crystals will require a high-drive current from the oscillator while

being more robust against noise and external perturbations. Another advantage is that the

design pullability will be minimized.

Depending on which STM32 microcontroller is used, all the resonator families listed below

can be compatible with your design or only some of them. STM32 microcontrollers embed

two types of low-speed oscillator (LSE):

•

Constant-gain low-speed oscillators

This type of LSE oscillators features a constant gain which makes them compatible

only with a few crystal groups mentioned above. For example, LSE oscillators

embedded into STM32F2 and STM32L1 microcontrollers target designs with severe

power consumption constraint. The selected crystal should consequently have a low

load capacitance and a moderate ESR. LSE oscillators embedded into STM32F1

microcontrollers target crystal resonators with moderate ESR and moderate load

capacitance.

•

Configurable-gain low-speed oscillators

LSE oscillators belonging to this family have the main advantage to be compatible with

a large number of crystals. Almost no constraint will be induced by the STM32

microcontroller embedding this kind of LSE oscillator. This large list of compatible

resonator crystals allows to focus only on design constraints (e.g. power consumption,

footprint) when selecting a compatible resonator. These LSE oscillators are divided into

two categories:

–

Dynamically (on-the-fly) modifiable gain LSE oscillators

The gain of this type of LSE oscillators can be changed either before starting the

oscillator or after enabling it.

–

Statically modifiable gain LSE oscillators

The gain can be changed only when the LSE oscillator is turned off. If the

oscillator transconductance has to be increased or decreased, the LSE must be

turned off first.

Table 5 gives the list of low-speed oscillators (LSE) embedded into the STM32

microcontrollers.

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