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AN2867 Application note

Guidelines for oscillator design on STM8AF/AL/S and STM32 MCUs/MPUs Introduction

Many designers know oscillators based on Pierce-Gate topology (Pierce oscillators), but not all of them really understand how they operate, and only a few master their design. In practice, limited attention is paid to the oscillator design, until it is found that it does not operate properly (usually when the final product is already in production). A crystal not working as intended results in project delays, if not overall failure. The oscillator must get the proper amount of attention during the design phase, well before moving to manufacturing, to avoid the nightmare scenario of products failing in application. This document introduces the Pierce oscillator basics, and provides guidelines for its design. It also shows how to determine the external components, and provides guidelines for correct PCB design and for selecting crystals and external components. To speed up the application development, the recommended crystals (HSE and LSE) for the products listed in Table 1 are detailed in Section 5: Recommended resonators for STM32 MCUs/MPUs and Section 6: Recommended crystals for STM8AF/AL/S MCUs.

Table 1. Applicable products Type Product categories

STM32 Ultra Low Power MCUs STM32 High Performance MCUs Microprocessors February 2026 STM32 Arm ® Cortex ® MPUs AN2867 Rev 24 1/60

www.st.com 1 Contents AN2867

Contents

1 Quartz crystal properties and model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2 Oscillator theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3 2.1 Negative resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2 Transconductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.3 Negative-resistance oscillator principles . . . . . . . . . . . . . . . . . . . . . . . . . 10 Pierce oscillator design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.1 Introduction to Pierce oscillators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 3.2 Feedback resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 3.3 Load capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.4 Oscillator transconductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.5 Drive level and external resistor calculation . . . . . . . . . . . . . . . . . . . . . . . 14 5 2/60 Calculating the drive level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.5.2 Another drive level measurement method . . . . . . . . . . . . . . . . . . . . . . . 16 3.5.3 Calculating the external resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.6 Startup time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.7 Crystal pullability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.8 Safety factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.9 4 3.5.1 3.8.1 Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.8.2 Measurement methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 3.8.3 Safety factor for STM32 and STM8 oscillators . . . . . . . . . . . . . . . . . . . 19 Oscillation modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 3.9.1 What are fundamental and overtone modes? . . . . . . . . . . . . . . . . . . . . 20 3.9.2 Third overtone mode: pros and cons . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 3.9.3 Considerations for crystals interfaced with STM32 products . . . . . . . . . 22 Guidelines to select a suitable crystal and external components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 4.1 Low-speed oscillators embedded in STM32 MCUs/MPUs . . . . . . . . . . . . 23 4.2 How to select an STM32-compatible crystal . . . . . . . . . . . . . . . . . . . . . . 26 Recommended resonators for STM32 MCUs/MPUs . . . . . . . . . . . . . . 29 AN2867 Rev 24 AN2867 6 7 Contents 5.1 STM32-compatible high-speed resonators . . . . . . . . . . . . . . . . . . . . . . . 29 5.2 STM32-compatible low-speed resonators . . . . . . . . . . . . . . . . . . . . . . . . 29 Recommended crystals for STM8AF/AL/S MCUs . . . . . . . . . . . . . . . . 42 6.1 Part numbers of recommended crystal oscillators . . . . . . . . . . . . . . . . . . 42 6.2 Recommended ceramic resonators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Tips for improving oscillator stability . . . . . . . . . . . . . . . . . . . . . . . . . . 45 7.1 PCB design guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 7.2 PCB design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 7.3 Soldering guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 7.4 LSE sensitivity to PC13 activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 8 Reference documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 9 FAQs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 10 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 11 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 AN2867 Rev 24 3/60

3 List of tables AN2867

List of tables

Table 1. Table 2. Table 3. Table 4. Table 5. Table 6. Table 7. Table 8. Table 9. Table 10. Table 11. Table 12.

4/60 Applicable products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Example of equivalent circuit parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Typical feedback resistor values for given frequencies . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Safety factor (Sf) for STM32 and STM8 oscillators. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 LSE oscillators embedded into STM32 MCUs/MPUs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 HSE oscillators embedded in STM32 MCUs/MPUs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products . . . . 31 KYOCERA compatible crystals (not exhaustive list). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 NDK compatible crystals (not exhaustive list). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Recommended conditions (for consumer) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Recommended conditions (for CAN-BUS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 AN2867 Rev 24 AN2867 List of figures

List of figures

Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12. Figure 13. Figure 14. Figure 15. Figure 16. Figure 17. Figure 18. Figure 19. Figure 20. Figure 21. Figure 22.

Quartz crystal model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Impedance in the frequency domain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 I-V curve of a dipole showing a negative trans-resistance area . . . . . . . . . . . . . . . . . . . . . . 9 Block diagram of a typical oscillation loop based on a crystal resonator . . . . . . . . . . . . . . 10 Pierce oscillator circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Inverter transfer function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Current drive measurement with a current probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Negative resistance measurement methodology description . . . . . . . . . . . . . . . . . . . . . . . 19 Fundamental and overtone frequencies of an AT-cut quartz crystal . . . . . . . . . . . . . . . . . 20 Quartz crystal theoretical model with third overtone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Oscillator implementation for third overtone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Classification of low-speed crystal resonators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Recommended layout for an oscillator circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 PCB with separated GND plane and guard ring around the oscillator . . . . . . . . . . . . . . . . 47 GND plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Signals around the oscillator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Preliminary design (PCB design guidelines not respected) . . . . . . . . . . . . . . . . . . . . . . . . 48 Final design (following guidelines) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 GND plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Top layer view. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 PCB guidelines not respected . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 PCB guidelines respected . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 AN2867 Rev 24 5/60

5

Quartz crystal properties and model

1 AN2867

Quartz crystal properties and model

A quartz crystal is a piezoelectric device converting electric energy into mechanical energy, and vice versa. The transformation occurs at the resonant frequency. The quartz crystal can be modeled as shown in Figure 1.

Figure 1. Quartz crystal model C0 Q

Lm Rm Cm MS36117V1 • C 0 represents the shunt capacitance resulting from the capacitor formed by the

electrodes • L m (motional inductance) represents the vibrating mass of the crystal • C m (motional capacitance) represents the elasticity of the crystal • R m (motional resistance) represents the circuit losses The impedance of the crystal (assuming that R m is negligible) is

(1)

Figure 2 shows the impedance in the frequency domain.

Figure 2. Impedance in the frequency domain

Impedance

Area of parallel resonance: Fp Inductive behavior: the quartz oscillates Capacitive behavior:

no oscillation Fs Fa Frequency Phase (deg)

+90 Frequency –90

ai15834c 6/60 AN2867 Rev 24 AN2867 Quartz crystal properties and model F s is the series resonant frequency when Z = 0. Its expression can be deduced from equation (1) as follows:

F a is the antiresonant frequency when Z tends to infinity. Using equation (1), it is expressed as follows:

(3)

The region delimited by F s and F a (shaded area in Figure 2) is the area of parallel resonance. In this region, the crystal operates in parallel resonance and behaves as an inductance that adds an additional 180° phase to the loop. Its frequency F p (or F L : load frequency) has the following expression:

(4)

According to this equation, the oscillation frequency of the crystal can be tuned by varying the load capacitance C L . This is why, in their datasheets, crystal manufacturers indicate the exact C L required to make the crystal oscillate at the nominal frequency. Table 2 gives an example of equivalent crystal circuit component values for an 8 MHz nominal frequency.

Table 2. Example of equivalent circuit parameters Equivalent component Value

R m 8 Ω L m 14.7 mH C m 0.027 pF C 0 5.57 pF Using equations (2), (3), and (4), it is possible to calculate F s , F a , and F p of this crystal:

If the load capacitance C L is equal to 10 pF, the crystal oscillates at F p = 7995695 Hz. To have an oscillation frequency of exactly 8 MHz, C L must be 4.02 pF.

AN2867 Rev 24 7/60

59 Oscillator theory

2 AN2867

Oscillator theory

Oscillators are among the backbone components of modern digital ICs. They can be classified into different subfamilies, depending upon their topology and operating principles. For each subfamily there is a mathematical model that can be used to study the oscillator behavior, and theoretically determine its performance. This section deals only with harmonic oscillators (relaxation oscillators are out of the scope of this document), with particular focus (see Section 3) on Pierce oscillators. This is because all the oscillators requiring external passive components (resonator, load capacitors, etc.) covered by this document are of the previously mentioned type and topology. The harmonic oscillator family can be divided into two main subfamilies: • negative-resistance oscillators • positive-feedback oscillators. These two subfamilies of oscillators are similar for what concerns the output waveform. They deliver an oscillating waveform at the desired frequency. This waveform is typically composed of a fundamental sine wave of the desired frequency, plus a sum of overtone harmonics (at frequencies multiple of the fundamental one), due to the nonlinearity of components of the oscillation loop. The two subfamilies differ in their operating principles. A specific mathematical model describes and analyzes each of them. Positive-feedback oscillators are usually modeled using the Barkhausen model, where an oscillator must fulfill the Barkhausen criterion to maintain a stable oscillation at the desired frequency. The Barkhausen model is not fully adequate to describe negative-resistance oscillators: the most suitable approach is to use the negative-resistance model described in [1]. STM32 microcontrollers and microprocessors (based on Arm ®(a) cores) feature low-speed external (LSE) and high-speed external (HSE) oscillators designed according to the negative-resistance principle, hence this section focuses on the presentation of this model.

2.1 Negative resistance

Theoretically speaking, a negative resistance is a dipole that absorbs heat and converting it in electrical current, proportional to the applied voltage, but flowing in the opposite direction (the opposite mechanism of an electrical resistance). In the real world, such a dipole does not exist.

a. Arm and Cortex are registered trademarks of Arm Limited (or its subsidiaries or affiliates) in the US and/or

elsewhere. The Arm word and logo are trademarks of Arm Limited (or its subsidiaries) in the US and/or elsewhere. All rights reserved..

8/60 AN2867 Rev 24 AN2867 Oscillator theory The term negative resistance is a misnomer of negative transresistance, defined as the ratio between a given voltage variation (∆V) and the induced current variation (∆I). Unlike the resistance, always positive, the transresistance (also known as differential resistance) can be either positive or negative. Figure 3 shows the current-voltage curve for a dipole with a negative transresistance region. It is obvious that the V/I ratio is always positive, this is not the case for the ∆V/∆I ratio . The portion of the I-V curve in purple shows a negative transresistance:

The portions in blue feature a positive transresistance:

AN2867 Rev 24 9/60

59 Oscillator theory

2.3

AN2867

Negative-resistance oscillator principles

An oscillation loop is made of two branches (see Figure 4): • The active branch, composed by the oscillator itself, provides the energy to make the

oscillation start and build up until it reaches a stable phase. When a stable oscillation is reached, this branch provides the energy to compensate for the losses of the passive branch. • The passive branch is mainly composed by the resonator, the two load capacitors and

all the parasitic capacitances.

Figure 4. Block diagram of a typical oscillation loop based on a crystal resonator

Passive branch Active branch Xtal STM32 C MSv36188V1

According to the small signals theory, when the active branch (oscillator part) is correctly biased, to maintain a stable oscillation around the oscillator biasing voltage the latter must have its transconductance equal to the passive branch conductance. However, at startup, the oscillator transconductance must be higher than (multiple of) the conductance of the passive part of the oscillation loop, to maximize the possibility to build up the oscillation from the inherent noise of the loop. An excessive oscillator transconductance compared to the oscillation loop passive branch conductance can saturate the oscillation loop, and cause a startup failure. To ensure the successful oscillator start, and to maintain stable oscillation, the ratio between the negative resistance of the loop and the crystal maximal equivalent series resistance (ESR) is specified for STM32 and STM8 products. It is recommended to have a ratio higher than 5 for the HSE oscillators, and higher than 3 for the LSE oscillators.

10/60 AN2867 Rev 24

AN2867

3 Pierce oscillator design

Pierce oscillator design

This section describes the different parameters, and how to determine their values to be compliant with the Pierce oscillator design.

3.1 Introduction to Pierce oscillators

Pierce oscillators are variants of Colpitts oscillators, widely used with crystal resonators. A Pierce oscillator (see Figure 5) requires a reduced set of external components, this 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 Microcontroller

RF In v OSC_IN OSC_OUT R Ext Q C L1 Cs C L2 ai15836b

3.2

• Inv: the internal inverter that works as an amplifier • Q: crystal quartz or a ceramic resonator • R F : internal feedback resistor • R Ext : external resistor to limit the inverter output current • C L1 and C L2 : are the two external load capacitances • C s : stray capacitance, sum of the device pin (OSC_IN and OSC_OUT) and the PCB (a

parasitic) capacitances.

Feedback resistor

In most MCUs/MPUs manufactured by ST, R F is embedded in the oscillator circuitry. Its role is to make the inverter act as an amplifier. The feedback resistor is connected between V in and V out to bias the amplifier at V out = V in , and force it to operate in the linear region (shaded AN2867 Rev 24 11/60

59 Pierce oscillator design AN2867 area in Figure 6). The noise (for example, the thermal noise of the crystal) is amplified within the range of serial to parallel frequency (F a , F p ), thus starting the oscillation.

Figure 6. Inverter transfer function Linear area: the inverter acts as an amplifier Vout VDD Saturation region Saturation region ~VDD/2 V DD Vin ai15837b

Table 3 provides typical values of R F .

Table 3. Typical feedback resistor values for given frequencies

3.3

Frequency Feedback resistor range

32.768 kHz 10 to 25 MΩ

1 MHz 5 to 10 MΩ

10 MHz 1 to 5 MΩ

20 MHz 470 kΩ to 5 MΩ

Load capacitance

The load capacitance is the terminal capacitance of the circuit connected to the crystal oscillator. This value is determined by the external capacitors C L1 and C L2 , and the stray capacitance of the printed circuit board and connections (C s ). The C L value is specified by the crystal manufacturer. For the frequency to be accurate, the oscillator circuit must show the same load capacitance to the crystal as the one the crystal was adjusted for. Frequency stability requires that the load capacitance be constant. The external capacitors C L1 and C L2 are used to tune the desired value of C L, to reach the value specified by the crystal manufacturer. The following equation gives the expression of C L :

12/60 AN2867 Rev 24 AN2867 Pierce oscillator design For example, with C L = 15 pF and C s = 5 pF ,

3.4 Oscillator transconductance

Theoretically, to make the oscillation start and reach a stable phase, the oscillator must provide sufficient gain to compensate for the loop losses and to provide the energy for the oscillation buildup. When the oscillation becomes stable, the power provided to the oscillator and the one it dissipates in the loop are equal. Practically, because of tolerances on passive component values and their dependency upon environmental parameters (such as temperature), the ratio between oscillator gain and oscillation loop critical gain cannot just exceed 1. This would induce a too long oscillator startup time, and even prevent the oscillator from starting up. This section describes the two approaches that can be used to check if an STM32 oscillator can be paired with a given resonator, to ensure that the oscillation is started and maintained under the specified conditions for both resonator and oscillator. The approach depends on how the oscillator parameters are specified in the device datasheet: • • If the oscillation loop maximum critical transconductance parameter (G m_crit_max ) is specified, ensure that it is greater than the oscillation loop critical crystal gain (g mcrit , see the formula below). Note that the maximum critical crystal transconductance can be named either G m_crit_max or G m , depending on the STM32 product documentation. If the oscillator transconductance parameter (g m ) is specified, make sure that the gain margin ratio (gain margin ) is bigger than 5. The gain margin ratio is determined by the formula gain margin = g m / g mcrit , where • g m is the oscillator transconductance specified in the STM32 datasheet. The HSE

oscillator transconductance is in the range of a dozen of mA/V, while the LSE oscillator transconductance ranges from a few to a few dozens of µA/V, depending upon the product. • g mcrit is defined as the minimal transconductance required to maintain a stable

oscillation when it is a part of the oscillation loop for which this parameter is relevant. g mcrit is computed from oscillation loop passive components parameters. Assuming C L1 = C L2 , and that the crystal sees the same C L on its pads as the value given by the crystal manufacturer, g mcrit is expressed as follows:

2 2 g mcrit = 4 × ESR × ( 2πF ) × ( C 0 + C L )

where: • ESR is the equivalent series resistance • C 0 is the crystal shunt capacitance • C L is the crystal nominal load capacitance. • F is the crystal nominal oscillation frequency AN2867 Rev 24 13/60

59 Pierce oscillator design AN2867 For example, to design the oscillation loop for the HSE oscillator embedded in an STM32F1 microcontroller with a transconductance value (g m ) of 25 mA/V, we choose a quartz crystal from Fox, with the following characteristics: • Frequency = 8 MHz

To check if this crystal oscillates, let us calculate g mcrit :

Calculating the gain margin gives:

gm 25 gain

m arg in = ----------------- = ----------- = 107 0.23 g mcrit

The gain margin is sufficient to start the oscillation and the gain margin > 5 condition is met. The oscillator is expected to reach stable oscillation after the typical delay specified in the datasheet. If an insufficient gain margin is found (gain margin < 5), the oscillation can start when designing and testing the final application, but this does not guarantee that the oscillation starts in operating conditions. It is highly recommended that the selected crystal has a gain margin higher than or equal to 5 (try to select a crystal with a lower ESR and/or a lower C L ). In a second example of the case where the maximal critical crystal transconductance is given, the HSE oscillator embedded in STM32G0 microcontrollers has G m = 1.5 mA/V. g mcrit for the implemented oscillator must stay below this value. The Fox quartz crystal described above respects this condition. The conversion between the oscillator transconductance (g m ) and the oscillation loop maximal critical transconductance (G m_crit_max ) is given by G m_crit_max = g m / 5. Note: Before any verification, the crystal chosen must vibrate at a frequency that respects the

oscillator frequency range given in the STM32 datasheet.

3.5 Drive level and external resistor calculation

The drive level (DL) and external resistor value (R Ext ) are closely related, and are addressed in the same section.

3.5.1 Calculating the drive level

The drive level is the power dissipated in the crystal. It must be limited, otherwise the quartz crystal can fail because of excessive mechanical vibrations. The maximum drive level is specified by the crystal manufacturer, usually in mW. Exceeding this value can lead to crystal damage, or to a shorter device lifetime.

14/60 AN2867 Rev 24 AN2867 Pierce oscillator design 2 The drive level is given by the formula: DL = ESR × I

Q , where: • ESR is the equivalent series resistor (specified by the crystal manufacturer):

• I Q is the current flowing through the crystal in RMS. This current can be displayed on

an oscilloscope as a sine wave. The current value can be read as the peak-to-peak value (I PP ). When using a current probe (as shown in Figure 7), the voltage scale of an oscilloscope may be converted into 1 mA / 1 mV.

ai15838b

So, as described previously, when tuning the current with the potentiometer, the current through the crystal (assuming it is sinusoidal) does not exceed I Qmax RMS, given by:

I Qmax RMS = I Qmax PP DL max ----------------- = ----------------------- ESR

2 2

Therefore, the current through the crystal (peak-to-peak value read on the oscilloscope) should not exceed a maximum peak-to-peak (I Qmax PP) equal to:

I Qmax PP = 2 × 2 × DL max --------------------------- ESR Hence, the need for an external resistor R Ext (refer to Section 3.5.3) when I Q exceeds I Qmax PP. The addition of R Ext becomes mandatory, and is added to ESR in the expression of I Qmax .

AN2867 Rev 24 15/60

59

Pierce oscillator design

3.5.2

AN2867

Another drive level measurement method

The drive level can be computed as DL= I² QRMS × ESR (I QRMS is the RMS AC current). This current can be calculated by measuring the voltage swing at the amplifier input with a low-capacitance oscilloscope probe (no more than 1 pF). The amplifier input current is negligible with respect to the current through C L1 , so we can assume that the current through the crystal is equal to the current flowing through C L1 . Therefore, the RMS voltage at this point is related to the RMS current by I QRMS = 2 π F x V RMS x C tot , with: • F = crystal frequency • V pp V RMS = ---------- , where V pp is the peak-to-peak voltage measured at C L1 level

2 2 • C tot = C L1 + (C s / 2) + C probe where:

– C L1 is the external load capacitance at the amplifier input – C s is the stray capacitance – C probe is the probe capacitance

This value must not exceed the drive level specified by the crystal manufacturer. Note: Use special care when measuring voltage swing at LSE input, as it is very sensitive to load

capacitance. It is recommended to use a 0.1 pF input capacitance probe.

3.5.3 Calculating the external resistor

The role of this resistor is to limit the drive level of the crystal. With C L2 , it forms a low-pass filter that forces the oscillator to start at the fundamental frequency and not at overtones (prevents the oscillator from vibrating at the odd harmonics of the fundamental frequency). If the power dissipated in the crystal is higher than the value specified by the crystal manufacturer, the external resistor R Ext becomes mandatory to avoid overdriving the crystal. If the power dissipated in the selected quartz is lower than the drive level specified by the crystal manufacturer, the insertion of R Ext is not recommended, and its value is then 0 Ω. An initial estimation of R Ext is obtained by considering the voltage divider formed by R Ext and C L2 . Thus, the value of R Ext is equal to the reactance of C L2 . Therefore, R Ext = 1 / (2 π F C L2 ), and so, with an oscillation frequency of 8 MHz and C L2 = 15 pF, we have R Ext = 1326 Ω. The recommended way for optimizing R Ext is to first choose C L1 and C L2 as explained before, and to connect a potentiometer in the place of R Ext . The potentiometer should be initially set to be approximately equal to the capacitive reactance of C L2 . It should then be adjusted as required, until an acceptable output and crystal drive level are obtained. Caution: After calculating R Ext , it is recommended to recalculate the gain margin (refer to Section 3.4)

to make sure that the addition of R Ext has no effect on the oscillation condition. The value of R Ext must be added to ESR in the expression of g mcrit , and g m » g mcrit must also remain true:

g m » g mcrit = 4 × (ESR + R Ext ) × (2 π F)² × (C 0 + C L )² Note:

16/60 If R Ext is too low, there is a considerable increase of the power dissipation by the crystal. If, on the other hand, R Ext is too high, there is no oscillation.

AN2867 Rev 24 AN2867

3.6

Pierce oscillator design

Startup time

This is the time required by the oscillation to start and then build up, until it reaches a stable oscillation phase. The startup time depends, among other factors, on the Q-factor of the resonator used. If the oscillator is paired with a quartz-crystal resonator characterized by its high Q-factor, the startup time is higher when ceramic resonators are used (these are known for their poor Q-factor, compared to quartz-crystal resonators). The startup time also depends upon the external components, C L1 and C L2 , and on the crystal frequency. The higher the crystal nominal frequency, the lower the startup time. In addition, startup problems usually arise because the gain margin is not properly dimensioned (as explained previously). This is caused either by C L1 and C L2 being too small or too large, or by the ESR being too high. As an example, an oscillator paired with a few MHz nominal frequency crystal resonator typically starts up after a delay of a few ms. The startup time of a 32.768 kHz crystal ranges between 1 and 5 s.

3.7 Crystal pullability

Crystal pullabilty, also known as crystal sensitivity, measures the impact of small variations of the load capacitance seen by the crystal on the oscillation frequency shifting. This parameter has more importance when dealing with low-speed oscillators, as they are used to clock time-keeping functions (such as real-time clock). When the final application is still in the design stage, the influence of this parameter on the low-speed oscillator accuracy (and consequently on all the time-keeping functions clocked by this oscillator) is not obvious. This is because the designer fine tunes the load capacitors until the desired oscillation frequency is obtained. When the design reaches production stage it is frozen, and all the passive components including the load capacitors have their values well defined. Any change of the load capacitance induces a shift of the oscillation frequency. Changes in the capacitive load (C L ) seen by the crystal can be thought of as due to inadequate operation environment, and only happening when the final design is not properly operated. In practice, this is not true since changes of the load capacitance are rather frequent and must be taken into account by the designer. The main contributors to the capacitive load (C L ) seen by the oscillator are • the capacitance of the load capacitors C L1 and C L2 • the stray capacitance of the PCB paths • the parasitic capacitance of the oscillator pins. Any change on the capacitances listed above directly shifts the oscillation frequency. When the design is in production stage, many of these capacitance values cannot be accurately controlled. Selecting a crystal with low pullability limits the influence of such production uncertainties on the final oscillation frequency accuracy. Generally speaking, the higher the load capacitance of a crystal, the lower its pullability. As an example, let us consider a crystal with a pullability of 45 PPM/pF. To fine-tune the oscillation frequency, this crystal is loaded by two C0G ceramic capacitors (with a ± 5% tolerance of their nominal value), C L1 and C L2 , with the same 7 pF capacitance. From the crystal point of view, the two load capacitors are mounted in series, which means that their contribution to C L is (C L1 = C L2 ) / 2. The tolerance on their contribution to C L AN2867 Rev 24 17/60

59 Pierce oscillator design AN2867 remains the same, and is equal to ± 5%. If we consider that all the remaining contributors to the C L are maintained to their nominal values at design stage (to assess the frequency shift magnitude induced only by load capacitor tolerances), then the load capacitance seen by the crystal (C L ) either decreases by 0.175 pF, or increases by the same value. This induces an oscillation shift of:

0.175 pF × 45 PPM / pF = ~7.8 PPM (~0.7 s/day for a time-keeping function such as RTC)

The above example shows that lower pullability results in lower impact of small load capacitance deviation on the frequency shifting. Crystal pullability is an important factor when defining the final application PPM budget.

where • C m is the crystal motional capacitance (in pF) • C 0 is the crystal shunt capacitance (in pF) • C L is the crystal nominal load capacitance (in pF) The following sections give a more detailed description on how to calibrate the oscillation frequency, and how to estimate the final accuracy uncertainty (PPM) budget.

3.8 Safety factor

3.8.1 Definition

Resonators (such as crystal resonators) undergo aging effects that manifest themselves over time in deviations of resonator parameters from the values defined by the specifications. Among the impacted parameters there is the resonator ESR, whose value depends upon the environment conditions, such as moisture and temperature. The oscillator transconductance depends upon the power supply voltage and upon the temperature. The safety factor parameter enables to determine the oscillator safe operation under the operating conditions and during the application life. It measures the ability of the oscillator not to fail under operating conditions. The safety factor is defined as the ratio between the oscillator negative resistance and its ESR:

R ADD + Crystal ESR Oscillator negative resistance S f = ----------------------------------------------------------------------------- = -------------------------------------------------------- Crystal ESR Crystal ESR 18/60 AN2867 Rev 24 AN2867

3.8.2

Pierce oscillator design

Measurement methodology

To measure the oscillator negative resistance, a resistance is added in series to the resonator, as indicated in Figure 8.

Figure 8. Negative resistance measurement methodology description STM32 Q R ADD C L1 C L2 MSv37268V1

Assurance level Safety factor (S f )

S f < 3 HSE LSE

Safe Very safe Not safe Safe

Not safe

1. Safe and very safe oscillations are shown in green, unsafe oscillation in yellow.

AN2867 Rev 24 19/60

59 Pierce oscillator design AN2867

3.9 Oscillation modes

3.9.1 What are fundamental and overtone modes?

Equation (4) gives the oscillation frequency F p of a crystal, which depends on the series resonant frequency F s for which the crystal impedance is null. The oscillator is said to operate in fundamental mode when vibrating around F p . Fs (and hence Fp) depend upon the parameters of the crystal theoretical model illustrated in Figure 1. These parameters, given by the crystal manufacturer, define the frequency for which the crystal is designed to oscillate around the fundamental frequency. In real life, an AT-cut quartz crystal impedance reaches a zero value for several frequencies, which correspond to the odd multiples of its fundamental vibration frequency. A crystal can also vibrate around one of those odd multiples, these are the overtone oscillation modes. Figure 9 represents the cancellation of an AT-cut crystal impedance for the fundamental frequency, its following odd multiples (third and fifth overtones are represented), as well as some spurious frequencies.

20/60 AN2867 Rev 24

AN2867 Pierce oscillator design Figure 10. Quartz crystal theoretical model with third overtone C 0 = L m Q R m C m

Fundamental mode Third overtone mode

L m3OT R m3OT C m3OT

For example, it is possible to use the third overtone mode by implementing the oscillator as shown in Figure 11, to suppress the fundamental frequency (theoretically each overtone mode can be selected by suppressing the previous ones).

Figure 11. Oscillator implementation for third overtone Microcontroller R

FInv OSC_IN OSC_OUT R Ext Q C L1 L L1 C S C L2 MS55484V1

3.9.2 Third overtone mode: pros and cons

Because of the thickness, the crystals designed for a high frequency fundamental mode are very expensive, requiring high-end cutting technologies and a lot of caution for implementation. Practically, it becomes impossible to operate in fundamental mode for a frequency above 50 MHz. This is why most of the high frequency crystals are designed to work in the third overtone mode (it is possible to cut the crystal for a frequency three times lower than the one it oscillates at).

AN2867 Rev 24 21/60

59 Pierce oscillator design AN2867 The model for a crystal operating in third overtone mode (Figure 11) shows a resistance R m approximately three times higher and a capacitance C m nine times lower than those associated with the fundamental mode. For the third overtone mode, these differences mean a higher Q-factor since the quality factor for an RC series circuit is 1 / ωRC (less energy loss, more stable performances, better jitter, and lower pullability, see Section 3.7). A lower pullability means a lower frequency shift when the application is deployed in the field, at the expense of lower tunability of the oscillation frequency.

3.9.3 Considerations for crystals interfaced with STM32 products

The oscillators integrated in the STM32 products have been validated for use in the fundamental mode, respecting the implementation of Figure 5. If a third overtone crystal is used with this implementation, the theory indicates that it does not start vibrating at the third harmonic frequency, but at the fundamental one. Note:

22/60 The startup mode of an oscillator can even involuntarily balance between the two modes if its external components have not been chosen according to what indicated in this document.

AN2867 Rev 24 AN2867 Guidelines to select a suitable crystal and external components

4 Guidelines to select a suitable crystal and external components

4.1 Low-speed oscillators embedded in STM32 MCUs/MPUs

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. The most important parameters to be taken into account (only technical factors are listed) are: • Crystal size or footprint • Crystal load capacitance (C L ) • Oscillation frequency offset (PPM) • Startup time A trade-off between the above parameters must be found, depending on the key design criteria. Figure 12 shows that the resonators available on the market can be divided into two categories, depending upon the above-mentioned factors and trade-offs.

Crystal load capacitance Figure 12. Classification of low-speed crystal resonators High (12.5 pF)

Low drift Incresed robustness against noisy environments Medium (7 pF)

Low power Low (”S)

Low Medium High

LSE drive level / G m_crit_max MSv36189V3

A resonator with a relatively high load-capacitance (such as 12.5 pF) requires more power for the oscillator to drive the oscillation loop at the resonator nominal frequency. Designs targeting low power consumption (for example, 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. One of the key areas where crystal resonators are massively used is the hand-held and wearable appliance consumer market (such as smartphones, Bluetooth ® kits). For this AN2867 Rev 24 23/60

59 Guidelines to select a suitable crystal and external components AN2867 market segment, the crystal size is of critical importance. However, it is widely known that small-footprint crystals come with high crystal ESR. The choice may be harder if the target design has severe constraints in terms of power consumption (the usual scenario). 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 the smallest possible ESR. In noisy environments (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 require a high-drive current from the oscillator, but are more robust against noise and external perturbations. Another advantage is that the design pullability is minimized. Depending on the device used, all the resonator families listed below can be compatible with your design, or only some of them. STM32 devices embed two types of low-speed oscillator (LSE): • Constant gain

This type of LSE oscillator features a constant gain, which makes them compatible only with a few crystal groups mentioned above. For example, LSE oscillators embedded in STM32F2 and STM32L1 MCUs target designs with severe power consumption constraints. The selected crystal should consequently have a low load capacitance and a moderate ESR. LSE oscillators embedded in STM32F1 MCUs target crystal resonators with moderate ESR and moderate load capacitance. • Configurable gain

The main advantage of LSE oscillators belonging to this family is the compatibility with a large number of crystals. Almost no constraint comes by the device embedding this kind of oscillator. The large list of compatible resonator crystals allows the designer to focus on design constraints (such as power consumption, footprint) when selecting a compatible resonator. These 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 in STM32 devices. Caution: When the gain is modified statically or on-the-fly, the calibration of the oscillation frequency

must be readjusted to estimate the final accuracy uncertainty (PPM) budget. Caution: In STM32F0 and STM32F3 MCUs, High drive mode (g m = 25 µA/V) must be used only with

12.5 pF crystals, to avoid saturating the oscillation loop and causing a startup failure. When used with a low C L crystal (for example 6 pF), the oscillation frequency jitters and duty cycle can be distorted.

24/60 AN2867 Rev 24 AN2867 Guidelines to select a suitable crystal and external components Table 5. LSE oscillators embedded into STM32 MCUs/MPUs (1) Series C0 F0, F3 F1, T F2, F4_g1 (2)

F4_g2 (3)

F7 L1

G0, G4 H7 L0, L4, L4+, L5 MP1, MP2 N6 U0, U3, U5 (4) WB, WB0, WBA (5) , WL C5, H5 Drive level g m (min) G m_crit_max Medium high 8.5 1.7

High 13.5 2.7 Low 5 1.0 Medium low 8 1.6 Medium high 15 3

High 25 5

5 1 2.8 0.56 Low 2.8 0.56 High 7.5 1.5 Low 2.4 0.48 Medium low 3.75 0.75 Medium high 8.5 1.7

High 13.5 2.7 Not available 3 0.6

Low 2.5 0.5

Medium low 3.75 0.75

Medium high 8.5 1.7

High 13.5 2.7

Medium low 3.75 0.75 Medium high 8.5 1.7

High 13.5 2.7 Not available Unit µA/V µA/V µA/V µA/V µA/V µA/V

1. Color code:

• Light blue: LSE oscillators with transconductance modifiable on the fly (dynamically) - Yellow: LSE oscillators with non-modifiable transconductance - Gray: LSE oscillators with statically-modifiable transconductance

2. STM32F4 series with LSE generation 1 (STM32F401/405/407/415/417/427/429/437/439xx MCUs

featuring LSE oscillators with non modifiable transconductance). 3. STM32F4 series with LSE generation 2 (STM32F410/411/412/413/423/446/469/479xx MCUs featuring

LSE oscillators with statically modifiable transconductance).

4. STM32U575/585 rev. X devices do not support low drive mode. 5. STM32WBA devices do not support low drive mode.

AN2867 Rev 24 25/60

59

Guidelines to select a suitable crystal and external components

4.2

AN2867

How to select an STM32-compatible crystal

This section describes the procedure recommended to select suitable crystal/external components. The procedure is based on the following steps:

Step 0: Choose a fundamental mode designed resonator

Choose a fundamental mode designed resonator (as explained in Section 3.9, the STM32 oscillators are validated to work with resonator vibrating in fundamental mode and while using the Pierce oscillator circuitry presented in Figure 5), and make sure that the resonator chosen is designed to work in fundamental mode.

Step 1: Check the resonator compatibility with the selected STM32

To check the compatibility between the selected crystal and the STM32 MCU/MPU, first identify the procedure to follow among the two described in Section 3.4. The decision must be made based on the oscillator specification provided in the datasheet. • If the oscillator transconductance parameter is specified, then apply the second

procedure. Ensure that the gain margin ratio is higher than five (x5) to make sure that the crystal is compatible with the selected STM32 part. • If G m_crit_max is specified instead, make sure that gmcrit for the oscillation loop is

smaller than the specified G m_crit_max value.

Step 2: Determine the capacitance value of the load capacitors C L1 and C L2

To determine the correct capacitance values for C L1 and C L2 load capacitors, apply the formula specified in Section 3.3. The values obtained are approximations of the exact capacitances to be used. In a second phase, to fine-tune the values of the load capacitors, go through a series of experimental iterations, until the right capacitance values are found. During the experimental phase, use a standard crystal, one whose PPM drift is well known when it is loaded by the crystal nominal load capacitance (C L ). This kind of crystal can be provided by the manufacturer upon request. After this crystal has been chosen, calculate its oscillation frequency (F standard ) when the crystal is loaded by its nominal load capacitance. This frequency is given by the formula:

6 F s tan dard = F nominal × ⎛ ⎝ PPM s tan dard ⁄ 10 ⎞ ⎠

where:

26/60 • F standard is the standard crystal oscillation frequency when it is loaded by its nominal

load capacitance • F nominal is the oscillation nominal frequency specified in the crystal datasheet • PPM standard is the oscillation frequency drift of the standard crystal, as characterized by

the crystal manufacturer AN2867 Rev 24 AN2867 Guidelines to select a suitable crystal and external components When F standard is computed, go through the following sequence: 1.

Make the first experimental iteration with C L1 and C L2 capacitance values determined by calculation: – If the oscillation frequency is equal to F standard , C L1 and C L2 are the correct

capacitances. The user can therefore skip substeps 2 and 3. – If the oscillation frequency is slower than F standard go to substep 2, otherwise

execute substep 3. For this experimental iteration, decrease C L1 and C L2 capacitance values, measure again the oscillation frequency, and compare it to F standard : – If the oscillation frequency is slower than F standard , execute substep 2, otherwise

execute substep 3. – If the oscillation frequency is almost equal to F standard , use the latter C L1 and C L2

capacitance values. For this experimental iteration, increase C L1 and C L2 capacitance values, measure again the oscillation frequency, and compare it to F standard : – If the oscillation frequency is slower than F standard , execute substep 2, otherwise

execute substep 3. – If the oscillation frequency is almost equal to F standard , use the latter C L1 and C L2

capacitance values.

Step 3: Check the safety factor of the oscillation loop

The safety factor must be assessed as described in Section 3.8 to ensure a safe oscillation of the oscillator under operating conditions. Note: Many crystal manufacturers can check device/crystal pairing compatibility upon request. If

the pairing is judged valid, they can provide a report including the recommended C L1 and C L2 values, as well as the oscillator negative resistance measurement. In this case steps 2 and 3 can be skipped.

Step 4: Calculate the drive level and external resistor

Compute the drive level (DL) (see Section 3.5) and check if it is greater or lower than DL crystal : • If DL < DL crystal , no need for an external resistor (a suitable crystal has been found). • If DL > DL crystal , the user should calculate R Ext to have: DL < DL crystal . The user should

then recalculate the gain margin taking R Ext into account. If gain margin > 5, a suitable crystal has been found. If not, then this crystal does not work, another one must be chosen. Return to Step 1: Check the resonator compatibility with the selected STM32 to run the procedure for the new crystal.

Step 5 (optional): Calculate the PPM accuracy budget

Use the following formula to estimate the PPM accuracy budget for the application:

PPM Budget = PPM crystal + Deviation ( C L ) × Pullability crystal

where: • PPM Budget is the estimated accuracy for the oscillation frequency • PPM crystal is the crystal PPM accuracy specified in the datasheet AN2867 Rev 24 27/60

59 Guidelines to select a suitable crystal and external components AN2867 Deviation (C L ) is expressed in pF. It measures the deviation of the load capacitance (C L ) due to tolerances on load capacitor values and the variation of the stray capacitance (C S ) due to PCB manufacturing process deviation. Pullabilty is expressed in PPM / pF (refer to Section 3.7). Note:

28/60 The PPM budget calculated above does not take into account the temperature variation, which can make the PPM budget bigger.

AN2867 Rev 24 AN2867 Recommended resonators for STM32 MCUs/MPUs

5 Recommended resonators for STM32 MCUs/MPUs

5.1 STM32-compatible high-speed resonators

The high-speed oscillator (HSE) embedded into STM32 products (a) is compatible with almost all the resonators available on the market. The recommended resonators are provided by a wide range of manufacturers, including: • ABRACON • ECS (www.ecsxtal.com) • EPSON (http://www5.epsondevice.com) • KYOCERA • Micro Crystal • muRata (www.murata.com) • NDK (http://www.ndk.com) • RIVER (http://www.river-ele.co.jp) Several tools are available to select the more recent and high-demand crystals, among them the STM32 Crystal Selection Tool from ECS, and the IC Matching Information provided by NDK. Compatible resonators come with various frequencies and technologies (ceramic resonators and quartz-crystal resonators working in fundamental mode are all compatible with the HSE oscillator embedded in STM32 MCUs/MPUs). Table 6 summarizes the supported frequency ranges.

Table 6. HSE oscillators embedded in STM32 MCUs/MPUs Series F0 F3 F1 T F2 F4 F7 L0 L1 C0 L4, L4+

L5 H7 G0, G4 MP1 U0

C5 H5 U3 U5 MP2 N6 Unit MHz mA/V

STM32-compatible low-speed resonators

Table 7 contains a not exhaustive list (only the compatible resonator part numbers checked by ST are included) of low-speed quartz-crystal 32.768 kHz resonators that are either compatible with the whole STM32 portfolio, or with a subset.

a. This is not applicable for STM32WB, STM32WB0, STM32WBA and STM32WL series due to the RF

constraints. For information about HSE oscillator compatibility with these devices, refer to Precise HSE frequency and startup time tuning for STM32 wireless MCUs (AN5042), available on www.st.com.

AN2867 Rev 24 29/60

59 Recommended resonators for STM32 MCUs/MPUs AN2867 Different footprints are provided to facilitate crystal selection, even if there are geometric constraints for the final application.

30/60 AN2867 Rev 24

31/60

Recommended resonators for STM32 MCUs/MPUs

AN2867 Rev

24 ECS 90 C 0 (pF)

6 ECS-.327-9-1210-TR 1.2 x 1.0

mm 2 ESR max (kΩ)

AN2867 Table 7. Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products

Package

size Manufacturer Micro Crystal Quartz reference

Part number ESR max (kΩ)

CM9V-T1A / CM9V-

T1A 0.3 C 0 (pF)

1.4 ECS-.327-6-16-TR3 ECS ECS-.327-7-16-C-TR 90

1.3 ECS-.327-9-16-TR

ECS-.327-12.5-16-TR

AN2867 Rev 24

NX1610SA 32.768 kHz

EXS00A-MU00658 NDK NX1610SE 32.768 kHz

EXS00A-MU01501

NX1610SE 32.768 kHz

EXS00A-MU01500

NX1610SE 32.768 kHz

EXS00A-MU01499 60 (4)

EPSON SC-16S FC1610AN 32.768000

kHz 9

FC1610AN 32.768000

kHz 12.5 4 0.4449 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

6 0.8356 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

7 1.0768 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

9 1.6506 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 2.9484 F0, F3

6 0.8132 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

7 1.0513 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

9 1.6190 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 2.9062 F0, F3 6 0.8132 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 12.5 2.9062 F0, F3 6 0.5799 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+, L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 7 0.7437 C0, F0, F1, F3, F4_g2, L0, L4, L4+, L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 9 1.1323 C0, F0, F3, F4_g2, L0, L4, L4+, L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 12.5 2.0083 C0, F0, F3, L0, L4, L4+, L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 6 0.7910 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 2.8642 F0, F3

9 1.5877 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 90 1.2 12.5 2.8642 F0, F3 6 0.7910 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

7 1.0261 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

9 1.5877 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 2.8642 F0, F3 CM1610H AN2867 CITIZEN STM32 series compatibility (2)(3)

1.55 NX1610SE 32.768 kHz

EXS00A-MU01498 SII g mcrit(1) (µA/V)

1.3 NX1610SA 32.768 kHz

EXS00A-MU01367 1.6 x 1.0

mm 2 C L

(pF) Recommended resonators for STM32 MCUs/MPUs 32/60 Table 7. Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products (continued)

Package

size Manufacturer Quartz reference

Part number ABS05-32.768 kHz 9

pF ABRACON C 0 (pF)

90 1.3

ABS05-32.768 kHz

ABS05W-32.768 kHz-D 1.6 x 1.0

mm 2 ESR max (kΩ)

TFX-04 70 90 2 1.3 RIVER TFX-04C 60 1.5 g mcrit(1) (µA/V)

9 1.6190 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 12.5 2.9062 F0, F3

4 0.4272 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

5 0.6056 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

7 1.0513 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

9 1.6190 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 2.9062 F0, F3

5 0.4298 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

7 0.7350 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

9 1.1216 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 1.9940 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 STM32 series compatibility (2)(3)

33/60

Recommended resonators for STM32 MCUs/MPUs

AN2867 Rev 24

C L

(pF) AN2867 Table 7. Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products (continued)

Package

size Manufacturer g mcrit(1) (µA/V)

ABS06-127-32.768kHz 12.5 2.3844 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

ABS06-32.768kHz-4P 4 0.2441 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

ABS06-32.768kHz-6 ABRACON ESR max (kΩ)

C L (pF)

Quartz reference

Part number C 0 (pF)

6 0.5493 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

7 0.7477 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

ABS06-32.768kHz-9 9 1.2361 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

ABS06-32.768kHz 12.5 2.3844 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 4 0.4103 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 6 0.8312 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

7 1.0513 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

9 1.6190 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 2.9062 F0, F3

4 0.4103 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

6 0.7372 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

7 0.8983 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 ABS06-32.768kHz-7 ABS06-107- 32.768kHz-T 90 80 - 1.5 ECS-.327-6-12-TR

ECS-.327-7-12-TR

AN2867 Rev 24

2.0 x 1.2

mm 2 ECS-.327-9-12-TR 90 1.3 80 1.5 ECS-.327-12.5-12-TR

ECS-.327-CDX-1082 ECS STM32 series compatibility (2)(3)

ECS-.327-6-12RR-TR

ECS-.327-7-12RR-TR

1.7 ECS-.327-9-12RR-TR

ECS-.327-12.5-12R-TR 70

ECS-.327-6-12R-TR

ECS-.327-9-12R-TR

ECS-.327-12.5-12R-TR 1.3 9 1.3589 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 2.3933 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

6 0.4517 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

9 0.8994 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 1.6145 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

Recommended resonators for STM32 MCUs/MPUs 34/60 Table 7. Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products (continued) AN2867

Recommended resonators for STM32 MCUs/MPUs

AN2867 Rev 24

2.0 x 1.2

mm 2 CC8V-T1A ESR max

(kΩ) AN2867 Table 7. Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products (continued)

Package

size Manufacturer RIVER Quartz reference

Part number TFX-03C ESR max (kΩ)

60 C 0 (pF)

1.8 5 0.4704 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

7 0.7878 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

1.1866 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

2.0804 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 6 0.7228 C0, F0, F1, F3, F4_g2, L0, L4, L4+, L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 7 0.9344 C0, F0, F1, F3, F4_g2, L0, L4, L4+, L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 NX2012SA 32.768 kHz

EXS00A-MU00528 12.5 2.5833 C0, F0, F3, L0, L4, L4+, L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

NX2012SE 32.768 kHz

EXS00A-MU01260 6 0.5026 C0, F0, F1, F3, F4_g2, L0, L4, L4+, L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 7 0.6416 C0, F0, F1, F3, F4_g2, L0, L4, L4+, L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 9 0.9706 C0, F0, F1, F3, F4_g2, L0, L4, L4+, L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 12.5 1.7095 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 6 0.6231 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

7 0.8138 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

9 1.2717 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 2.3177 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

6 0.6325 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 2.2604 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

6 0.6325 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

7 0.8176 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

9 1.2592 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 2.2604 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

AN2867 Rev 24

NX2012SE 32.768 kHz

EXS00A-MU01259

NX2012SE 32.768 kHz

EXS00A-MU01611 80 50 (4)

1.3 1.7 NX2012SE 32.768 kHz

EXS00A-MU01612 SC-20T 75 1

SII SC-20S CITIZEN STM32 series compatibility (2)(3)

9 NX2012SA 32.768 kHz

EXS00A-MU00524 2.0 x 1.2

mm 2 g mcrit(1) (µA/V)

12.5

NX2012SA 32.768 kHz

EXS00A-MU00527 NDK C L (pF)

CM2012H 70 70 1.3 1.3

Recommended resonators for STM32 MCUs/MPUs 36/60 Table 7. Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products (continued) AN2867

Package

size Manufacturer ESR max (kΩ)

C L (pF)

g mcrit(1) (µA/V)

7 0.9751 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 9 1.6472 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 ABS07L-32.768kHz 12.5 2.6208 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 ABS07-32.768kHz 6pF 6 0.5815 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 ABS07-32.768kHz 7pF 7 0.7596 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 9 1.1869 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 ABS07 32.768kHz 12.5 2.1631 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 ABS07-166- 32.768kHz-T 7 0.7596 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 Quartz reference

Part number C 0 (pF)

ABS07L-32.768kHz

7pF ABS07L-32.768kHz

9pF ABRACON 70 1.4 1.1 ABS07-120- 32.768kHz-T 60 1.2 6 0.5273 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 ABS07W-32.768kHz-D 55 1.15 3 0.1606 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 ABS07-LR-32.768 kHz-

6 50 1.3 6 0.4517 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 6 0.5733 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

7 0.7501 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

9 1.1751 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 2.1471 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

4 0.2678 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

6 0.5407 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

8 0.9087 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

9 1.1283 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 2.0838 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 CM315D CITIZEN 0,95 70 CM315E 0,75

37/60

Recommended resonators for STM32 MCUs/MPUs

AN2867 Rev 24

3.2 x 1.5

mm 2 ABS07-32.768kHz 9pF 80 STM32 series compatibility (2)(3) AN2867

Table 7. Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products (continued)

Package

size Manufacturer CITIZEN Quartz reference

Part number CM315DL CC7V-T1A CM7V-T1A (low profile)

AN2867 Rev 24

Micro Crystal CM7V-T1A 0.3 ESR max (kΩ)

50 70 60 C 0 (pF)

1,3 1,2 1,4 3.2 x 1.5

mm 2 CM7V-T1A 50 1,3 FC-135 32.768000 kHz

7

FC-135 32.768000 kHz

9

EPSON FC-135 32.768000 kHz 12.5

FC-13A 32.768000 kHz

9

FC-13A 32.768000 kHz

12.5 1.0 70 C L (pF)

g mcrit(1) (µA/V) STM32 series compatibility

Recommended resonators for STM32 MCUs/MPUs 38/60 Table 7. Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products (continued)

0.9

AN2867

Package

size Manufacturer ESR max (kΩ)

g mcrit(1) (µA/V) STM32 series compatibility

(2)(3)

7 0.5562 C0, F0, F1, F3, F4_g2, L0, L1, L4, L4+, L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 9 0.8648 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 12.5 1.5681 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 6 0,5815 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 2.1631 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

6 0,4154 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 1.5451 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

6 0.5983 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

7 0.7787 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

9 1.2108 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 12.5 2.1953 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 6 0.5407 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 12.5 2.0838 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 ECS-.327-6-34S-TR 6 0.5899 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

ECS-.327-12.5-34S-

TR 12.5 2.1792 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 2.1792 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

ECS-.327-7-34B-TR 7 0.7691 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

ECS-.327-9-34B-TR 9 1.1988 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 2.1792 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

6 0.6511 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 C 0 (pF)

FC-135R 32.768000

kHz 7

EPSON FC-135R 32.768000 kHz 9 50 1.1

FC-135R 32.768000

kHz 12.5 SC-32S 70

SII 1.0

SC-32P 50 ECS-.327-6-34QS-TR

AN2867 Rev 24

ECS-.327-7-34QS-TR 1,1 ECS-.327-9-34QS-TR 3.2 x 1.5

mm 2 ECS-.327-12.5-34QS-

TR

ECS-.327-6-34G-TR

ECS-.327-12.5-34G-

TR ECS 0,75

70 ECX-.327-CDX-1293 1,05 ECS-.327-12.5-34B-TR

ECS-.327-CDX-1128 60 2

39/60

Recommended resonators for STM32 MCUs/MPUs

C L (pF)

Quartz reference Part number AN2867

Table 7. Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products (continued)

Package

size Manufacturer ECS Quartz reference

Part number ESR max (kΩ)

C 0 (pF)

ECS-.327-6-34R-TR

ECS-.327-7-34R-TR

ECS-.327-9-34R-TR 50 0.5840 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 6 0.3614 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

9 0.7195 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

ECS-.327-12.5-34RR-

TR 12.5 1.2916 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

NX3215SA 32.768 kHz

EXS00A-MU00525 6 0.5815 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+, L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 7 0.7596 C0, F0, F1, F3, F4_g2, L0, L4, L4+, L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 12.5 2.1631 C0, F0, F3, L0, L4, L4+, L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 6 0.3323 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+, L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 12.5 1.2361 C0, F0, F3, F4_g2, L0, L4, L4+, L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 6 0.6152 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

7 0.7980 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 1,3 40 70 1.0

AN2867 Rev 24

NX3215SA 32.768 kHz

EXS00A-MU00526 40 CC5V-T1A ECS-.327-6-49-TR 1.3 1.2 70 ECS-.327-7-49-TR

1.1 ECS-.327-9-49-TR

ECS-.327-12.5-49-TR ABRACON 7 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 NX3215SE 32.768 KHz

EXS00A-MU00989 6.9 x 1.4

mm 2 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 NX3215SE 32.768 KHz

EXS00A-MU00990 ECS 0.4517 1.6145 NX3215SA 32.768 kHz

EXS00A-MU00523 4.1 x 1.5

mm 2 6 0.8994 ECS-.327-9-34RR-TR Micro Crystal STM32 series compatibility (2)(3)

9 ECS-.327-6-34RR-TR NDK g mcrit(1) (µA/V)

12.5 ECS-.327-12.5-34R-TR 3.2 x 1.5

mm 2 C L (pF)

ABS13 -32.768kHz-

7pF - 9 1.2349 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 2.2277 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

6 0.5983 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

7 0.7787 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

9 1.2108 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 2.1953 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 7 0.5400 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 1.7221 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 AN2867 ABS13 -32.768kHz 65

Recommended resonators for STM32 MCUs/MPUs 40/60 Table 7. Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products (continued)

Package

size Manufacturer ESR max (kΩ)

g mcrit(1) (µA/V)

7 0.6878 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 1.9790 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

3.7 0.2332 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

4.4 0.3095 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

6 0.5247 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

MC-164 32.768000kHz

7 7 0.6705 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

MC-164 32.768000kHz

9 9 1.0585 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 12.5 1.9496 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 6 0.5096 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

7 0.6705 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

ECS-.327-9-38-TR 9 1.0585 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

ECS-.327-12.5-38-TR 12.5 1.9496 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

6 0.5096 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

7 0.6705 C0, F0, F1, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5 C 0 (pF)

SSP-T7-F 0.9 SII

SSP-T7-FL EPSON 7.0 x 1.5

mm 2 MC-164 32.768000kHz

12.5 65

AN2867 Rev 24

ECS-.327-6-38-TR ECS CITIZEN ECS 8.0 x 3.8

mm 2

ABRACON ECS-.327-7-38-TR 0.8 CM130 ECS-.327-6-17X-TR

ECS-.327-12.5-17X-TR

ABS25-32.768kHz-6-T 50 ABS25-32.768kHz-T 1.35 STM32 series compatibility (2)(3)

9 1.0585 C0, F0, F3, F4_g2, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 1.9496 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

6 0.4580 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 1.6263 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

6 0.4580 C0, F0, F1, F2, F3, F4, L0, L1, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

12.5 1.6263 C0, F0, F3, L0, L4, L4+,L5, G0, G4, F7, C5, H5, H7, N6, WB, WB0, WBA, WL, MP1, MP2, U0, U3, U5

1. As defined in Section 3.4: Oscillator transconductance.

2. F4_g1: STM32F4 series with LSE generation 1, corresponds to STM32F401/405/ 407/415/417/427/429/437/439xx MCUs featuring LSE oscillators with non-modifiable transconductance. 3. F4_g2: STM32F4 series with LSE generation 2, corresponds to STM32F410/411/412/413/423/446/469/479xx MCUs, featuring LSE oscillators with statically-modifiable transconductance.

4. Low_ESR: -40 to +85 °C.

41/60 Note: STM32U575/585 rev. X devices do not support crystal/MEMS resonators with C L ≤ 6 pF. Recommended resonators for STM32 MCUs/MPUs

C L (pF)

Quartz reference Part number AN2867

Table 7. Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products (continued) Recommended crystals for STM8AF/AL/S MCUs AN2867

6 Recommended crystals for STM8AF/AL/S MCUs

6.1 Part numbers of recommended crystal oscillators

Table 8. KYOCERA compatible crystals (not exhaustive list) Part number CX3225GA8000D0PTVCC CX3225SA8000D0PTVCC 42/60 ESR max 8 MHz 500 Ω CX2016SA16000D0GSSCC 200 Ω CX2016GR16000D0GTVCC 300 Ω CX3225CA16000D0PSVCC CX3225GA16000D0PTVCC Note: Frequency 16 MHz C L Drive level (DL) 200 µW max 8 pF 300 µW max 100 Ω 200 µW max CX3225SB16000D0GSSCC 80 Ω 100 µW max CX3225SA16000D0PTVCC 60 Ω 200 µW max Contact provider regarding recommended crystals for new designs. AN2867 Rev 24 AN2867 Recommended crystals for STM8AF/AL/S MCUs Table 9. NDK compatible crystals (not exhaustive list) Part number Frequency ESR max NX3225GD 8MHz EXS00A-CG04874 8 MHz 500 Ω NX3225GA 16MHz EXS00A-CG04815 NX2016SA 16MHz EXS00A-CS07826 NX3225GA 24MHz EXS00A-CG04818 NX2016SA 24MHz EXS00A-CS10820 NX3225GA 25MHz EXS00A-CG06750 NX2016SA 25MHz EXS00A-CS11321 80 Ω C L Drive level (DL) 8 pF 16 MHz 200 Ω 50 Ω 8 pF 200 µW max 24 MHz 80 Ω 6 pF 50 Ω 8 pF 80 Ω 6 pF 80 Ω 6 pF 25 MHz NX2016SA32MHz EXS00A-CS10925 32 MHz NX1612SA 40MHz EXS00A-CS13836 40 MHz NX1612SA 48MHz EXS00A-CS13734 48 MHz 6 pF 80 Ω AN2867 Rev 24 100 µW max 7 pF 43/60

59

Recommended crystals for STM8AF/AL/S MCUs

6.2

AN2867

Recommended ceramic resonators

Table 10 and Table 11 give the references of recommended CERALOCK ® ceramic resonators for the STM8A microcontrollers provided and certified by muRata. Table 10. Recommended conditions (for consumer) Part number Frequency C L

Table 11. Recommended conditions (for CAN-BUS) Part number 44/60 Frequency C L

AN2867 Rev 24 AN2867 Tips for improving oscillator stability

7 Tips for improving oscillator stability

7.1 PCB design guidelines

Keeping the signal-to-noise ratio (SNR) below acceptable limits for a perfect operation of the oscillator means more severe constraints on the oscillator PCB design to reduce its sensitivity to noise. Therefore, great care must be taken when designing the PCB to reduce as much as possible the SNR. A non exhaustive list of precautions to take when designing the oscillator PCB is provided below: • Avoid high values of stray capacitance and inductances, as they can lead to uncontrollable oscillation (the oscillator can resonate at overtones or harmonic frequencies). Reducing the stray capacitance also decreases startup time and improves oscillation frequency stability. • To reduce high frequency noise propagation across the board, the MCU/MPU must have a stable power supply source, to ensure noiseless crystal oscillations. This means that well-sized decoupling capacitor must be used for powering the device. • Mount the crystal as close as possible to the MCU/MPU, to keep tracks short, and to reduce inductive and capacitive effects. A guard ring around these connections, connected to the ground, is essential to avoid capturing unwanted noise, which can affect oscillation stability. Long tracks/paths behave as antennas for a given frequency spectrum, generating oscillation issues when passing EMI certification tests. Refer to Figure 14 and Figure 16. • Any path conveying high-frequency signals must be routed away from the oscillator paths and components. Refer to Figure 14. • The oscillator PCB must be underlined with a dedicated underneath ground plane, distinct from the application PCB ground plane. The oscillator ground plane should be connected to the nearest MCU/MPU ground. It prevents interferences between the oscillator components and other application components (for example, crosstalk between paths). If a crystal in a metallic package is used, do not connect it to the oscillator ground. Refer to Figure 13, Figure 14, and Figure 15. • Leakage current can increase startup time and even prevent the oscillator start. If the device operates in a severe environment (high moisture/humidity ratio), an external coating is recommended. AN2867 Rev 24 45/60

59

Tips for improving oscillator stability AN2867 Figure 13. Recommended layout for an oscillator circuit VSS paths C L1

Ground shield Microcontroller OSC_IN

Quartz

OSC_OUT R Ex t(1)

C L2

Local ground plane (other layer)

ai15839b Warning:

46/60 It is highly recommended to apply conformal coatings to the PCB area shown in Figure 13, especially for the LSE quartz, CL1, CL2, and paths to the OSC_IN and OSC_OUT pads as a protection against moisture, dust, humidity, and temperature extremes that may lead to startup problems.

AN2867 Rev 24 AN2867

7.2

Tips for improving oscillator stability

PCB design examples

Example 1

Figure 14. PCB with separated GND plane and guard ring around the oscillator

ee

eFigure 15. GND plane Figure 16. Signals around the oscillator

ee AN2867 Rev 24 47/60

59 Tips for improving oscillator stability AN2867

Example 2

Figure 17 is an example of a PCB that does not respect the guidelines provided in Section 7.1, for the following reasons: • no ground planes around the oscillator component • too long paths • no symmetry between oscillator capacitances • high crosstalk / coupling between paths • too many test points.

Figure 17. Preliminary design (PCB design guidelines not respected)

48/60 AN2867 Rev 24 AN2867 Tips for improving oscillator stability The PCB design has been improved according to the guidelines (see Figure 18): • guard ring connected to the GND plane around the oscillator • symmetry between oscillator capacitances • less test points • no coupling between paths.

Figure 18. Final design (following guidelines)

eFigure 19. GND plane

AN2867 Rev 24 Figure 20. Top layer view 49/60

59 Tips for improving oscillator stability AN2867

Example 3

Figure 21 is another example of PCB that does not respect the guidelines provided in Section 7.1 (EMC tests likely to fail): • no guard ring around oscillator components • long paths.

Figure 21. PCB guidelines not respected

e50/60 AN2867 Rev 24

AN2867 Tips for improving oscillator stability The layout has been improved to respect the guidelines (see Figure 22), EMC tests are likely to be passed: • ground planes around the oscillator component • short paths that link the STM32 to the oscillator • symmetry between oscillator capacitances.

Figure 22. PCB guidelines respected

ee

7.3 Soldering guidelines

In general, soldering is a sensitive process, especially for low-frequency crystals. To reduce the impact of such process on the crystal parameters user should consider that

7.4

• Exposing crystals to temperatures above their maximum ratings can damage the

crystal and affect their ESR value. Refer to the crystal datasheet for the right reflow temperature curve (if not provided, ask the manufacturer). • PCB cleaning is recommended to obtain the maximum performance by removing flux

residuals from the board after assembly (even when using “no-clean” products in ultra-low power applications).

LSE sensitivity to PC13 activity

The OSC32_IN is sensitive to PC 13 activity, When PC13 is active (toggling) the LSE clock may shift, depending upon the LSE drive configuration. For example, using PC_13 as RTC_OUT calibration for 512 Hz or 1 Hz can disturb the LSE oscillation loop, hence shift the calibration frequency.

AN2867 Rev 24 51/60

59 Tips for improving oscillator stability AN2867 On products having the possibility to remap RTC_OUT on another pin (for example PB2), it is recommended to use this remapping to avoid LSE disturbances.

52/60 AN2867 Rev 24 AN2867

8 Reference documents

Reference documents

[1] E. Vittoz High-Performance Crystal Oscillator Circuits: Theory and Application

IEEE Journal of solid State Circuits, Vol 23, No 3, June 1988 pp 774 - 783.

AN2867 Rev 24 53/60

59 FAQs

9 AN2867

FAQs

Question

How can I know if my crystal is compatible with a given STM32 part?

Answer

Refer to Section 4: Guidelines to select a suitable crystal and external components.

Question

Can I use a 32.768 kHz crystal compatible with STM32 parts but not mentioned in Table 7?

Answer

Yes, you can. Table 7 is not exhaustive, it is given as a reference for some selected crystal manufacturers, footprint size, and crystal load capacitance.

Question

In my application, 32.768 kHz frequency very-low drift and high accuracy are mandatory to obtain an accurate clock without calibration. Which crystal load capacitance (C L ) should I choose?

Answer

First, you must be sure that your crystal is compatible with the selected STM32 LSE. Then, it is highly recommended to use a crystal with low pullability, that is with C L ≥ 6 pF:

54/60 • 7 pF is a good compromise between low drift and moderate power consumption • 9 and 12.5 pF can be used in noisy environments, but impact the power consumption.

AN2867 Rev 24 AN2867

10

Conclusion

Conclusion

The most important parameter is the gain margin of the oscillator, which determines if the oscillator starts up or not. This parameter must be calculated at the beginning of the design phase, to choose a crystal suitable for the application. The second parameter is the value of the external load capacitors, to select in accordance with the C L specification of the crystal (provided by the manufacturer). This determines the frequency accuracy. The third parameter is the value of the external resistor used to limit the drive level. In the 32 kHz oscillator part, however, it is not recommended to use an external resistor. Because of the number of variables involved, in the experimentation phase it is recommended to select components that have exactly the same properties as those that will be used in production, and operate with the same oscillator layout and in the same environment, to avoid unexpected behavior. Recently, MEMS oscillators have emerged on the market. They are a good alternative to resonators-based oscillators, thanks to their reduced power consumption, small size (they do not require additional passive components such as external load capacitors), and cost. This kind of oscillator is compatible with all STM32 MCUs/MPUs, except for the STM32F1 and STM32L1 series. When a MEMS oscillator is paired with an STM32 embedded oscillator, configure the latter in bypass mode.

AN2867 Rev 24 55/60

59

Revision history

11

AN2867

Revision history

Table 12. Document revision history

Date Revision

20-Jan-2009 1 Initial release.

10-Nov-2009 2 DL formula corrected in Section 3.5.2: Another drive level measurement method. Package column added to all tables in Section 6: Some recommended crystals for STM32 microcontrollers. Recommended part numbers updated in Section 5.1: STM32-compatible high-speed resonators. 27-Apr-2010 3 Added Section 7: Some recommended crystals for STM8A/S microcontrollers.

4 Updated Section 5.1: STM32-compatible high-speed resonators: removed Table 7: Recommendable condition (for consumer) and Table 8: Recommendable condition (for CAN bus); added Table 8: Recommendable conditions (for consumer); updated Murata resonator link. Updated Section 5.1: STM32-compatible high-speed resonators: removed Table 13: EPSON TOYOCOM, Table 14: JFVNY ® , and Table 15: KDS; Added Table 6: Recommendable crystals. Added Warning: after Figure 13.

30-Mar-2011 5 Section 5.1: STM32-compatible high-speed resonators: updated “STM32” with “STM8”. Table 16: Recommendable conditions (for consumer): replaced ceramic resonator part number “CSTSE16M0G55A-R0” by “CSTCE16M0V53-R0”. 17-Jul-2012 6 Whole document restricted to STM32 devices.

7 Changed STM32F1 into STM32 throughout the document. Added STM8AL series in Table 1: Applicable products. Replace STM8 by STM32 in Section : and updated hyperlink. Added Section 7: Tips for improving oscillator stability. Remove section Some PCB hints.

8 Updated Section 2: Oscillator theory. Updated Section 3: Pierce oscillator design. Renamed section “Gain margin of the oscillator” into Section 3.4: Oscillator transconductance and content updated. Updated Section 3.6: Startup time. Updated Section 3.7: Crystal pullability. Updated Section 4: Guidelines to select a suitable crystal and external components. Updated Section 5: Recommended resonators for STM32 MCUs/MPUs. Added Section 8: Reference documents. Updated Section 10: Conclusion.

25-Nov-2010 19-Sep-2014 19-Dec-2014 56/60 Changes AN2867 Rev 24

AN2867 Revision history Table 12. Document revision history (continued)

Date 19-Feb-2015 17-Aug-2015 31-May-2017 21-Jan-2020 Revision Changes 9 Updated Section 2.3: Negative-resistance oscillator principles to specify the ratio between negative resistance and crystal ESR for STM8 and STM32 microcontrollers. Added Section 3.8: Safety factor. Added Check the Safety Factor of the oscillation loop step in Section 4.2: How to select an STM32-compatible crystal. Note moved from step 2 to 3 and updated. Renamed Table 7: Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products.

10 Updated Figure 12: Classification of low-speed crystal resonators. Added caution notes in Section 4.1: Low-speed oscillators embedded in STM32 MCUs/MPUs. Added STM32F7, STM32F446xx, STM32F469/479xx and STM32L4 microcontrollers in Table 5: LSE oscillators embedded into STM32 MCUs/MPUs. Added STM32F411xx, STM32F446xx, STM32F469/479xx and STM32L4xx microcontrollers in Table 6: HSE oscillators embedded in STM32 MCUs/MPUs. Updated Table 7: Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products. Added Section 9: FAQs.

11 Updated document title, Introduction, Section 9: FAQs, Section 10: Conclusion, title of Section 6: Recommended crystals for STM8AF/AL/S MCUs, and revision of text across the whole document. Updated Table 5: LSE oscillators embedded into STM32 MCUs/MPUs, Table 6: HSE oscillators embedded in STM32 MCUs/MPUs and Table 7: Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products. Updated caption of Table 8: KYOCERA compatible crystals (not exhaustive list), and added Table 9: NDK compatible crystals (not exhaustive list). Updated Figure 12: Classification of low-speed crystal resonators.

12 Updated document title, Section 2: Oscillator theory, Section 5.1: STM32-compatible high-speed resonators and Section 5.2: STM32- compatible low-speed resonators. Updated Table 1: Applicable products, Table 5: LSE oscillators embedded into STM32 MCUs/MPUs, Table 6: HSE oscillators embedded in STM32 MCUs/MPUs, Table 7: Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products, Table 8: KYOCERA compatible crystals (not exhaustive list), Table 9: NDK compatible crystals (not exhaustive list), Table 10: Recommended conditions (for consumer) and Table 11: Recommended conditions (for CAN-BUS). Minor text edits across the whole document.

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Revision history AN2867 Table 12. Document revision history (continued)

AN2867 Rev 24

AN2867 Revision history Table 12. Document revision history (continued)

Date Revision Changes 20 Updated document title. Updated Table 5: LSE oscillators embedded into STM32 MCUs/MPUs, Table 6: HSE oscillators embedded in STM32 MCUs/MPUs, and Table 7: Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products. Updated footnote a. Updated Section 7.1: PCB design guidelines. Minor text edits across the whole document.

21 Updated Section 5.1: STM32-compatible high-speed resonators and footnote a. Updated Table 5: LSE oscillators embedded into STM32 MCUs/MPUs and Table 7: Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products. Updated Figure 10: Quartz crystal theoretical model with third overtone. Minor text edits across the whole document.

22 Updated Section 3.5.3: Calculating the external resistor. Updated Table 5: LSE oscillators embedded into STM32 MCUs/MPUs, Table 6: HSE oscillators embedded in STM32 MCUs/MPUs, and Table 7: Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products.

28-Jan-2025 23 Introduced STM32U3 series, hence updated Table 5: LSE oscillators embedded into STM32 MCUs/MPUs, Table 6: HSE oscillators embedded in STM32 MCUs/MPUs, and Table 7: Recommended crystal / MEMS resonators for the LSE oscillator in STM32 products. 23-Feb-2026 24 Introduced STM32C5 series.

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AN2867

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