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AN2867
Application note
Oscillator design guide for STM8S, STM8A
and STM32 microcontrollers
Introduction
Most designers are familiar with oscillators (Pierce-Gate topology), but few really
understand how they operate, let alone how to properly design an oscillator. In practice,
most designers do not even really pay attention to the oscillator design until they realize the
oscillator does not operate properly (usually when it is already being produced). This should
not happen. Many systems or projects are delayed in their deployment because of a crystal
not working as intended. The oscillator should receive its proper amount of attention during
the design phase, well before the manufacturing phase. The designer would then avoid the
nightmare scenario of products being returned.
This application note introduces the Pierce oscillator basics and provides some guidelines
for a good oscillator design. It also shows how to determine the different external
components and provides guidelines for a good PCB for the oscillator.
This document finally contains an easy guideline to select suitable crystals and external
components, and it lists some recommended crystals (HSE and LSE) for STM32 and
STM8A/S microcontrollers in order to quick start development. Refer to Table 1 for the list of
applicable products.
Table 1. Applicable products
Type
Product categories
STM8S Series
Microcontrollers
STM8AF Series, STM8AL Series
STM32 32-bit ARM Cortex MCUs.
August 2015
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1
List of tables
AN2867
List of tables
1
Quartz crystal properties and model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2
Oscillator theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3
4
5
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2.1
Negative resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2
Transconductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3
Negative-resistance oscillator principles . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Pierce oscillator design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1
Introduction to pierce oscillators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
3.2
RF feedback resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3
CL load capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.4
Oscillator transconductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.5
Drive level (DL) and external resistor (RExt) calculation . . . . . . . . . . . . . 15
3.5.1
Calculating drive level (DL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.5.2
Another drive level measurement method . . . . . . . . . . . . . . . . . . . . . . . 16
3.5.3
Calculating external resistor (RExt) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.6
Startup time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.7
Crystal pullability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.8
Safety factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.8.1
Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.8.2
Measurement methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.8.3
Safety factor for STM32 and STM8 oscillators . . . . . . . . . . . . . . . . . . . 19
Guidelines for selecting suitable crystal
and external components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.1
Low-speed oscillators embedded into STM32 microcontrollers . . . . . . . . 20
4.2
Detailed steps to select an STM32-compatible crystal . . . . . . . . . . . . . . . 23
Some recommended resonators for
STM32 microcontrollers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.1
STM32-compatible high-speed resonators . . . . . . . . . . . . . . . . . . . . . . . 26
5.2
STM32-compatible low-speed resonators . . . . . . . . . . . . . . . . . . . . . . . . 27
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7
List of tables
Some recommended crystals for STM8A/S microcontrollers . . . . . . . 29
6.1
Part numbers of recommended crystal oscillators . . . . . . . . . . . . . . . . . . 29
6.2
Part numbers of recommended ceramic resonators . . . . . . . . . . . . . . . . 29
Tips for improving oscillator stability . . . . . . . . . . . . . . . . . . . . . . . . . . 30
7.1
PCB design guidelines
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
7.2
PCB design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
7.3
Soldering guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
8
Reference documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
9
FAQs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
10
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
11
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
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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.
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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 microcontrollers . . . . . . . . . . . . . . . . . . . . . . . . . . 22
HSE oscillators embedded in STM32 microcontrollers. . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Recommended crystal resonators for LSE oscillator
embedded in STM32 microcontrollers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
KYOCERA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Recommendable conditions (for consumer) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Recommendable conditions (for CAN-BUS). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Frequently asked questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
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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.
Quartz crystal model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Impedance representation in the frequency domain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
I-V curve of a dipole showing a negative transresistance area (in purple) . . . . . . . . . . . . . . 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
Classification of low-speed crystal resonators available on the market . . . . . . . . . . . . . . . 20
Recommended layout for an oscillator circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
PCB with separated GND plane and guard ring around the oscillator . . . . . . . . . . . . . . . . 32
GND plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Signals around the oscillator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Preliminary design (PCB design guidelines not respected) . . . . . . . . . . . . . . . . . . . . . . . . 33
Final design (all design guidelines have been respected) . . . . . . . . . . . . . . . . . . . . . . . . . 34
GND plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Top layer view. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
PCB guidelines not respected . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
PCB guidelines respected . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
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Quartz crystal properties and model
1
AN2867
Quartz crystal properties and model
A quartz crystal is a piezoelectric device transforming electric energy to mechanical energy
and vice versa. The transformation occurs at the resonant frequency. The quartz crystal can
be modeled as follows:
Figure 1. Quartz crystal model
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C0: represents the shunt capacitance resulting from the capacitor formed by the electrodes
Lm: (motional inductance) represents the vibrating mass of the crystal
Cm: (motional capacitance) represents the elasticity of the crystal
Rm: (motional resistance) represents the circuit losses
The impedance of the crystal is given by the following equation (assuming that Rm is
negligible):
(1)
2
w × Lm × Cm – 1
j
Z = ---- × --------------------------------------------------------------------------------w ( C + C ) – w2 × L × C × C
0
m
m
m
0
Figure 2 represents the impedance in the frequency domain.
Figure 2. Impedance representation in the frequency domain
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Quartz crystal properties and model
Fs is the series resonant frequency when the impedance Z = 0. Its expression can be
deduced from equation (1) as follows:
(2)
1
F s = -----------------------------2π L C
m m
Fa is the anti-resonant frequency when impedance Z tends to infinity. Using equation (1), it
is expressed as follows:
(3)
F
a
= F
Cm
1 + --------s
C0
The region delimited by Fs and Fa is usually called the area of parallel resonance (shaded
area in Figure 2). In this region, the crystal operates in parallel resonance and behaves as
an inductance that adds an additional phase equal to 180 ° in the loop. Its frequency Fp (or
FL: load frequency) has the following expression:
(4)
Cm


F p = F s  1 + ------------------------------
2 ( C 0 + C L )

From equation (4), it appears that the oscillation frequency of the crystal can be tuned by
varying CL load capacitance. This is why in their datasheets, crystal manufacturers indicate
the exact CL required to make the crystal oscillate at the nominal frequency.
Table 2 gives an example of equivalent crystal circuit component values to have a nominal
frequency of 8 MHz.
Table 2. Example of equivalent circuit parameters
Equivalent component
Value
Rm
8Ω
Lm
14.7 mH
Cm
0.027 pF
C0
5.57 pF
Using equations (2), (3) and (4) we can determine Fs, Fa and Fp of this crystal:
F s = 7988768 Hz and F a = 8008102 Hz .
If the load capacitance CL at the crystal electrodes is equal to 10 pF, the crystal will oscillate
at the following frequency: F p = 7995695 Hz .
To have an oscillation frequency of exactly 8 MHz, CL should be equal to 4.02 pF.
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Oscillator theory
2
AN2867
Oscillator theory
Oscillators are one of the backbone components of modern digital ICs. They can be
classified into different sub-families depending on their topology and operating principles. To
each oscillator sub-family corresponds a more suitable 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 not within the
scope of this application note) with a particular focus on Pierce-oscillator topology (see
Section 3: Pierce oscillator design for details). This restricted scope is due to the fact that all
the oscillators embedded in STM32 microcontrollers covered by this document that require
external passive components (external resonator, load capacitors, etc.) are of the previously
mentioned type and topology.
The harmonic oscillator family can be divided into two main sub-families:
•
Negative-resistance oscillators
•
Positive-feedback oscillators.
These two sub-families of oscillators are similar for what regards the output waveform. They
deliver an oscillating waveform at the desired frequency. This waveform is typically
composed of a fundamental sine-waveform at the desired frequency plus a sum of overtone
harmonics (at frequencies multiple of the fundamental one) due to the nonlinearity of some
components of the oscillation loop.
These two sub-families differ in their operating principles. This difference also implies a
different mathematical model to describe and analyze each sub-family.
Positive-feedback oscillators are generally modeled using the famous Barkhausen model
where an oscillator should fulfill the Barkhausen criterion to be able to maintain a stable
oscillation at the desired frequency.
Negative-resistance oscillators could be described by the Barkhausen model. However this
approach is not adequate. The most suitable approach to analyze a negative-resistance
oscillator is by using the negative-resistance model as described in E. Vittoz’s paper ([1]).
Since STM32 low-speed external (LSE) oscillator and high-speed external (HSE) oscillators
were both designed following the negative-resistance principle, this section focuses on the
presentation of the negative-resistance model.
2.1
Negative resistance
Theoretically speaking, a negative resistance would be a dipole that absorbs heat and
converts the energy into an electrical current proportional to the applied voltage but flowing
in the opposite direction (exactly the opposite mechanism of an electrical resistance). In the
real word such a dipole does not exist.
In fact the term “negative resistance” is a misnomer of the “negative transresistance” which
is defined by the ratio of a given voltage variation (∆V) divided by the induced current
variation (∆I). Unlike the resistance which is always positive, the transresistance (also
known as differential resistance) can be either positive or negative. Figure 3 gives the
current-voltage curve for a dipole that shows a negative transresistance region. It is obvious
that the V/I ratio is always positive. However this is not the case for the ∆V/∆I ratio.
8/42
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AN2867
Oscillator theory
The part of the I-V curve represented in purple shows a negative transresistance:
V(D) – V(C )
ΔV
-------- = --------------------------------- < 0
ΔI
I( D) – I(C)
while the parts of the curve in blue shows a positive transresistance:
ΔV
V( B) – V(A )
-------- = -------------------------------- > 0
I( B) – I(A )
ΔI
Figure 3. I-V curve of a dipole showing a negative transresistance area (in purple)
2.2
Transconductance
Like the conductance which is defined as the inverse of the resistance, the
transconductance is also defined as the inverse of the transresistance. Transconductance
can also be defined as the differential conductance which expressed by the formula:
ΔV
-------ΔI
2.3
Negative-resistance oscillator principles
An oscillation loop is made of two branches (see Figure 4):
•
The active branch of the oscillation loop which is composed of the oscillator itself. This
branch is responsible for providing enough energy at startup to make the oscillation
start and build up until it reaches the stable oscillation phase. When a stable oscillation
is reached, the oscillator branch provides enough energy to compensate for the
oscillation loop passive branch losses.
•
The passive branch is mainly composed of the resonator, the two load capacitors and
all the parasitic capacitances.
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Oscillator theory
AN2867
Figure 4. Block diagram of a typical oscillation loop based on a crystal resonator
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Following the small signals theory and when the active branch (oscillator part) is correctly
biased, the latter should have its transconductance equal to the passive branch
conductance in order to maintain a stable oscillation around the oscillator biasing voltage.
However, at startup, the oscillator transconductance should 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 inherent noise of the oscillation loop. Please note that an excessive
oscillator transconductance compared to the oscillation loop passive branch conductance
may also saturate the oscillation loop and cause a startup failure.
In order to ensure the oscillator ability to startup successfully and maintain stable oscillation,
a ratio between the negative resistance of the oscillation loop and the crystal maximal
equivalent series resistance (ESR) is specified: for STM32 and STM8 microcontrollers, it is
recommended to have a ratio higher than x5 for the HSE oscillators and higher than x3 for
the LSE oscillators.
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3
Pierce oscillator design
Pierce oscillator design
This section describes the different parameters and how to determine their values in order
to be compliant with the Pierce oscillator design.
3.1
Introduction to pierce oscillators
Pierce oscillators are variants of Colpitts oscillators which are widely used in conjunction
with crystal resonators. A Pierce oscillator requires a reduced set of external components
which results in a lower final design cost. In addition, the Pierce oscillator is known for its
stable oscillation frequency when paired with a crystal resonator, in particular a quartzcrystal resonator.
Figure 5. Pierce-oscillator circuitry
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Inv: the internal inverter that works as an amplifier
Q: crystal quartz or a ceramic resonator
RF: internal feedback resistor
RExt: external resistor to limit the inverter output current
CL1 and CL2: are the two external load capacitances
Cs: stray capacitance is the addition of the microcontroller pin capacitance (OSC_IN and
OSC_OUT) and the PCB capacitance: it is a parasitic capacitance.
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Pierce oscillator design
3.2
AN2867
RF feedback resistor
In most STMicroelectronics microcontrollers, RF is embedded in the oscillator circuitry. Its
role is to make the inverter act as an amplifier. The feedback resistor is connected between
Vin and Vout so as to bias the amplifier at Vout = Vin and force it to operate in the linear region
(shaded area in Figure 6). The amplifier amplifies the noise (for example, the thermal noise
of the crystal) within the range of serial to parallel frequency (Fa, Fa). This noise causes the
oscillations to start up. In some cases, if RF is removed after the oscillations have stabilized,
the oscillator continues to operate normally.
Figure 6. Inverter transfer function
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Table 3 provides typical values of RF.
Table 3. Typical feedback resistor values for given frequencies
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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
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3.3
Pierce oscillator design
CL 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 CL1 and CL2 and the stray
capacitance of the printed circuit board and connections (Cs). The CL value is specified by
the crystal manufacturer. Mainly, for the frequency to be accurate, the oscillator circuit has to
show the same load capacitance to the crystal as the one the crystal was adjusted for.
Frequency stability mainly requires that the load capacitance be constant. The external
capacitors CL1 and CL2 are used to tune the desired value of CL to reach the value specified
by the crystal manufacturer.
The following equation gives the expression of CL:
C L1 × C L2
C L = -------------------------- + C s
C L1 + C L2
Example of CL1 and CL2 calculation:
For example if the CL value of the crystal is equal to 15 pF and, assuming that Cs = 5 pF,
then:
C L1 × C L2
C L – C s = -------------------------- = 10 pF . That is: C L1 = C L2 = 20 pF .
C L1 + C L2
3.4
Oscillator transconductance
Theoretically, to make the oscillation start and build up until it reaches a stable oscillation
phase, the oscillator should provide sufficient gain that at the same time compensates for
the oscillation loop losses and provide the energy that makes the oscillation build up. When
the oscillation becomes stable, the equality between the oscillator provided power and the
oscillation loop dissipated power is achieved.
Practically speaking and due to tolerances on passive component values and their
dependency on environmental parameters (e.g. temperature), a ratio of x1 between the
oscillator gain and the oscillation loop critical gain is not recommended. This will induce a
too long oscillator startup time and might 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 in order 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 microcontroller datasheet:
•
•
If the oscillation loop maximal critical gain parameter (gm_crit_max) is specified, it is
important to make sure that the oscillation loop critical gain (gmcrit) is smaller than the
specified parameter.
If the oscillator transconductance parameter (gm) is specified, make sure that the gain
margin ratio (gainmargin) is bigger than x5. Below the calculation formulas for both gmcrit
and gainmargin.
gm
gain m arg in = --------------g mcrit
where:
gm is the oscillator transconductance specified in the microcontroller datasheet. Note
that the HSE oscillator transconductance is in the range of a dozens of mA/V while LSE
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Pierce oscillator design
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oscillator transconductance ranges from a few µA/V to a few dozens of µA/V
depending on the product.
gmcrit is defined as the minimal transconductance of an oscillator required to maintain a
stable oscillation when it is a part of the oscillation loop for which this parameter is
relevant. gmcrit is computed from oscillation-loop passive components parameters.
Assuming that CL1 equals CL2, and that the crystal sees the same CL on its pads as the
value given by the crystal manufacturer, gmcrit is expressed as follows:
2
g mcrit = 4 × ESR × ( 2πF ) × ( C 0 + C L )
2
where
ESR = equivalent series resistance
C0 is the crystal shunt capacitance.
CL is the crystal nominal load capacitance.
F is the crystal nominal oscillation frequency.
For example, to design the oscillation loop for the HSE oscillator embedded into an
STM32F1 microcontroller which a transconductance value (gm) equal to 25 mA/V, we
choose a quartz crystal from Fox, with the following characteristics:
frequency = 8 MHz
C0 = 7 pF
CL = 10 pF
ESR = 80 Ω .
To check if this crystal will oscillate with an STM32F1 microcontroller, let us calculate gmcrit:
6 2
– 12
g mcrit = 4 × 80 × ( 2 × π × 8 ×10 ) × ( 7 ×10
– 12 2
+ 10 ×10
) = 0,23 mA ⁄ V
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 greater than 5”
condition is reached. The oscillator is expected to reach a stable oscillation after a typical
delay specified by the microcontroller datasheet.
If an insufficient gain margin is found (gainmargin < 5), the oscillation might start up under
typical conditions (achieved in laboratory conditions) when designing and testing the final
application. However, this does not guarantee that the oscillation will start up in operating
conditions. As a result, 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 or/and a lower CL.
Whatever the specified parameter, the oscillator transconductance (gm) or the oscillation
loop maximal critical gain (gm_crit_max), the conversion between these two parameters is
possible, if need be. The relation between these two parameters is given by the below
formula:
gm
G m_crit_max = ------5
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3.5
Pierce oscillator design
Drive level (DL) and external resistor (RExt) calculation
The drive level (DL) and external resistor value (RExt) are closely related and will be
addressed in the same section.
3.5.1
Calculating drive level (DL)
The drive level is the power dissipated in the crystal. It has to be limited otherwise the quartz
crystal can fail due to excessive mechanical vibration. The maximum drive level is specified
by the crystal manufacturer, usually in mW. Exceeding this maximum value may lead to the
crystal being damaged or to a shorter device lifetime.
2
The drive level is given by the following formula: DL = ESR × IQ , where:
•
ESR is the equivalent series resistor (specified by the crystal manufacturer):
•
IQ 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 (IPP). When using a current probe (as shown in Figure 7), the voltage scale of an
oscilloscope may be converted into 1mA/1mV.
C0 2
ESR = R m ×  1 + -------
CL
Figure 7. Current drive measurement with a current probe
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DLE
So as described previously, when tuning the current with the potentiometer, the current
through the crystal does not exceed IQmax RMS (assuming that the current through the
crystal is sinusoidal).
Thus IQmax RMS is given by:
I Qmax RMS =
DL max
I Qmax PP
----------------- = -----------------------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 current (IQmaxPP) equal to:
2 × DL max
I Qmax PP = 2 × --------------------------ESR
Hence the need for an external resistor (RExt) (refer to Section 3.5.3) when IQ exceeds
IQmaxPP. The addition of RExt then becomes mandatory and it is added to ESR in the
expression of IQmax.
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Pierce oscillator design
3.5.2
AN2867
Another drive level measurement method
The drive level can be computed as:
DL= I²QRMS × ESR, where IQRMS 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 CL1, so we can assume that the current
through the crystal is equal to the current flowing through CL1. Therefore the RMS voltage at
this point is related to the RMS current by:
I QRMS = 2πF × V RMS × C tot , with:
•
F = crystal frequency
•
V pp
V RMS = ----------- , where: Vpp is the voltage peak-to-peak measured at CL1 level
2 2
•
Ctot = CL1 + (Cs/2) + Cprobe where:
–
CL1 is the external load capacitance at the amplifier input
–
Cs is the stray capacitance
–
Cprobe is the probe capacitance)
2
2
ESR × ( π × F × C tot ) × ( V pp )
Therefore the drive level, DL, is given by: DL = -------------------------------------------------------------------------------.
2
This DL value must not exceed the drive level specified by the crystal manufacturer.
3.5.3
Calculating external resistor (RExt)
The role of this resistor is to limit the drive level of the crystal. With CL2, 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 3, 5, 7 etc. times the fundamental frequency). If the
power dissipated in the crystal is higher than the value specified by the crystal manufacturer,
the external resistor RExt becomes mandatory to avoid overdriving the crystal. If the power
dissipated in the selected quartz is less than the drive level specified by the crystal
manufacturer, the insertion of RExt is not recommended and its value is then 0 Ω.
An initial estimation of RExt is obtained by considering the voltage divider formed by
RExt/CL2. Thus, the value of RExt is equal to the reactance of CL2.
1
2πFC 2
Therefore: RExt = ------------------ .
Let us put:
•
oscillation frequency F = 8 MHz
•
CL2 = 15 pF
Then: R Ext = 1326 Ω
The recommended way of optimizing RExt is to first choose CL1 and CL2 as explained earlier
and to connect a potentiometer in the place of RExt. The potentiometer should be initially set
to be approximately equal to the capacitive reactance of CL2. It should then be adjusted as
required until an acceptable output and crystal drive level are obtained.
Caution:
After calculating RExt it is recommended to recalculate the gain margin (refer to Section 3.4:
Oscillator transconductance) to make sure that the addition of RExt has no effect on the
oscillation condition. That is, the value of RExt has to be added to ESR in the expression of
gmcrit and gm >> gmcrit must also remain true:
gm >> gmcrit = 4 × (ESR + RExt) × (2 × PI × F)² × (C0 + CL)²
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AN2867
Pierce oscillator design
Note:
If RExt is too low, there is no power dissipation in the crystal. If RExt is too high, there is no
oscillation: the oscillation condition is not reached.
3.6
Startup time
The startup time is the time required by the oscillation to start up 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 then the startup time will be higher if a ceramic resonator
is used (ceramic resonators are know for their poor Q-factor compared to quartz-crystal
resonators). The startup time also depends on the external components, CL1 and CL2, and
on the crystal frequency. The higher the crystal nominal frequency, the lower the start up
time. In addition the startup problems are usually due to the fact that the gain margin is not
properly dimensioned (as explained previously). This is caused either by CL1 and CL2 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
would typically start up after a delay of few ms.
The startup time of a 32.768 kHz crystal ranges from 1 to 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 usually has more importance when dealing with low-speed oscillators, since they
are used to clock time-keeping functions (such as real-time clock functions).
When the final application is still in design stage, the influence of this parameter on the lowspeed oscillator accuracy (and consequently on all the time-keeping functions clocked by
this oscillator) is not so obvious. This is due to the fact that 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 will directly induce a shift
of the oscillation frequency. Changes in the capacitive load (CL) seen by the crystal may 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 by taken into account by the designer. The main
contributors to the capacitive load (CL) seen by the oscillator are the following:
•
The capacitance of the load capacitors CL1 and CL2
•
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 will limit the influence of such production
uncertainties on the final oscillation frequency accuracy.
Generally speaking, the higher the load capacitance (CL) 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, CL1 and CL2,
with equal capacitances equal to 7 pF. C0G ceramic capacitors have a tolerance value of
DocID15287 Rev 10
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41
Pierce oscillator design
AN2867
± 5% of their nominal value. From crystal point of view, the two load capacitors are mounted
in series which means that their contribution to the CL is (CL1 = CL2)/2. As CL1 equals CL2,
the tolerance on their contribution to CL remains the same and is equal to ± 5%. Now if we
consider that all the remaining contributors to the CL 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 (CL) will either decrease by
0.175 pF or increase by the same value. This will induce 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 the lower the pullability, the lower the impact of small load
capacitance deviation on the frequency shifting. Crystal pullability is an important factor
when defining the final application PPM budget.
6
Pullability ( PPM ⁄ pF )
C m ×10
= -------------------------------------22 × ( C0 + CL )
Where
Cm is the crystal motional capacitance
C0 is the crystal shunt capacitance
CL is the crystal nominal load capacitance
Next 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) are well known to undergo aging effects. They
manifest themselves over time in a deviation of resonator parameters from their initial
values defined by the resonator specifications. Among the affected parameters there is the
resonator ESR which value depends on the surrounding environmental conditions such as
moisture and temperature.
The oscillator transconductance also depends on the microcontroller power supply voltage
and on the temperature.
The safety factor parameter allows to qualify 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 the
resonator ESR. It is given by the below formula:
R ADD + Crystal ESR
Oscillator negative resistance
S f = ------------------------------------------------------------------------------ = --------------------------------------------------------Crystal ESR
Crystal ESR
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AN2867
3.8.2
Pierce oscillator design
Measurement methodology
To measure the oscillator negative resistance, a resistance is added in series with the
resonator as described in Figure 8.
The oscillator negative resistance is the value of smallest series resistance that will prevent
the oscillator from starting up successfully.
In practice, this is achieved by conducting several experiments in which the value of the
series resistance is slightly increased compared to the previous experiment. This sequence
of experiments should stop when the oscillator is not able to start up correctly. This allows
determining the oscillator negative resistance which is equal to the value of the added series
resistance.
Figure 8. Negative resistance measurement methodology description
670
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3.8.3
Safety factor for STM32 and STM8 oscillators
Table 4 gives a summary of the safety factors for the oscillators embedded in STM32 and
STM8 microcontrollers. It should be noted that for the LSE oscillator, the oscillation is
considered safe for a safety factor higher than or equal to x3, while for the HSE oscillator,
the oscillation is considered safe starting from a safety factor higher than or equal to x5.
Table 4. Safety Factor (Sf) for STM32 and STM8 oscillators(1)
Assurance level
Safety Factor (Sf)
Sf ≥ 5
HSE
LSE
Safe
Very Safe
3 ≤ Sf < 5
Sf < 3
Not Safe
Safe
Not Safe
1. Safe and very safe oscillations are shown in green while not safe oscillation is show in orange.
DocID15287 Rev 10
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41
Guidelines for selecting suitable crystal and external components
4
Guidelines for selecting suitable crystal
and external components
4.1
Low-speed oscillators embedded into STM32
microcontrollers
AN2867
The low-speed resonator market provides a wide range of crystal resonators. Selecting the
most adequate one for a given design depends on many parameters. Below a list of the
most important parameters that must be taken into account (only technical factors are
listed):
•
Crystal size or footprint
•
Crystal load capacitance (CL)
•
Oscillation frequency offset (PPM)
•
Startup time.
A trade-off between the above parameters must be found depending on the key design
criteria. Figure 9 shows that the resonators available on the market can be divided into two
categories depending on the above mentioned factors and trade-offs.
Figure 9. Classification of low-speed crystal resonators available on the market
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A resonator with a relatively big load-capacitance (such as 12.5 pF) will require more power
for the oscillator to drive the oscillation loop at the resonator nominal frequency. Designs
targeting low-power consumption (e.g. RTC application powered by coin-batteries requiring
very long autonomy) are consequently more likely to use resonators with relatively small
load capacitance. On the other side, big load capacitance resonators have a much smaller
pullabilty compared to resonators with small load capacitance. As a result, designs without
severe constraints on power consumption tend to use big load capacitance crystals to
reduce pullability.
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AN2867
Guidelines for selecting suitable crystal and external components
One of the key emerging areas where crystal resonators are massively used is the handheld
wearable appliance consumer market (e.g. smart phones, Bluetooth kits). For this market
segment the crystal size is of critical importance. However it is widely known that smallfootprint crystals come always with high crystal ESR. For this kind of designs, the choice
may be harder if the target design has severe constraints in terms of power consumption
(which almost always happens). In this case, choose a crystal with a load capacitance as
small as possible to optimize power consumption even if this compromises pullabilty. In
addition, crystals with high ESR may have a slightly longer startup time. If there are no
constraints on crystal size, then it is recommended to choose a crystal with an ESR as small
as possible.
In noisy environment (which it is almost always the case for industrial applications), if there
are no constraints on power consumption, it is recommended to choose crystals with high
load capacitance. These crystals will require a high-drive current from the oscillator while
being more robust against noise and external perturbations. Another advantage is that the
design pullability will be minimized.
Depending on which STM32 microcontroller is used, all the resonator families listed below
can be compatible with your design or only some of them. STM32 microcontrollers embed
two types of low-speed oscillator (LSE):
•
Constant-gain low-speed oscillators
This type of LSE oscillators features a constant gain which makes them compatible
only with a few crystal groups mentioned above. For example, LSE oscillators
embedded into STM32F2 and STM32L1 microcontrollers target designs with severe
power consumption constraint. The selected crystal should consequently have a low
load capacitance and a moderate ESR. LSE oscillators embedded into STM32F1
microcontrollers target crystal resonators with moderate ESR and moderate load
capacitance.
•
Configurable-gain low-speed oscillators
LSE oscillators belonging to this family have the main advantage to be compatible with
a large number of crystals. Almost no constraint will be induced by the STM32
microcontroller embedding this kind of LSE oscillator. This large list of compatible
resonator crystals allows to focus only on design constraints (e.g. power consumption,
footprint) when selecting a compatible resonator. These LSE oscillators are divided into
two categories:
–
Dynamically (on-the-fly) modifiable gain LSE oscillators
The gain of this type of LSE oscillators can be changed either before starting the
oscillator or after enabling it.
–
Statically modifiable gain LSE oscillators
The gain can be changed only when the LSE oscillator is turned off. If the
oscillator transconductance has to be increased or decreased, the LSE must be
turned off first.
Table 5 gives the list of low-speed oscillators (LSE) embedded into the STM32
microcontrollers.
Caution:
When the gain is modified either statically or on-the-fly, the calibration of the oscillation
frequency must be re-adjusted to estimate the final accuracy uncertainty (PPM) budget.
Caution:
In STM32F0 and STM32F3 MCUs, High drive mode (gm_crit_max = 5 µA/V) should be used
only with 12.5 pF crystals to avoid saturating the oscillation loop and causing a startup
failure. When used with a low CL crystal (eg CL=6 pF), the oscillation frequency jitter and
duty cycle may be distorted.
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41
•
F4_G1: STM32F4 series with LSE generation 1
This category corresponds to STM32F401/405/ 407/427/429xx MCUs that feature LSE oscillators with non-modifiable
transconductance
•
F4_G2: STM32F4 series with LSE generation 2
This category corresponds to STM32F411/446/469/479xx that feature LSE oscillators with statically-modifiable
transconductance.
Table 5. LSE oscillators embedded into STM32 microcontrollers(1)
STM32F0/F3
Drive-level
Medium
High
High
STM32F1/T
STM32F2
F4_G1
F4_G2
STM32L0/L4
STM32L1
STM32F7
NA
NA
NA
Low
High
Low
Medium
Low
Medium
High
High
NA
Low
Unit
DocID15287 Rev 10
Low
Medium
Low
gm_min
5
8
15
25
5
2.8
2.8
2.8
7.5
2.5
3.75
8.5
13.5
3
2.4
3.75
8.5
13.5
gm_crit_max
1
1.6
3
5
1
0.56
0.56
0.56
1.5
0.5
0.75
1.7
2.7
0.6
0.48
0.75
1.7
2.7
Medium Medium
Low
high
High
µA/V
1. Color code:
Blue: LSE oscillators with transconductance modifiable on the fly (dynamically).
Green: LSE oscillators with non-modifiable transconductance.
Gray: LSE oscillators with statically-modifiable transconductance.
Guidelines for selecting suitable crystal and external components
22/42
For simplification purposes, the following terms will be used in Table 5: LSE oscillators embedded into STM32 microcontrollers:
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4.2
Guidelines for selecting suitable crystal and external components
Detailed steps to select an STM32-compatible crystal
This section describes the procedure recommended to select suitable crystal/external
components. The whole procedure is divided into three main steps:
Step 1: Check the resonator compatibility with the selected STM32
microcontroller
To check the compatibility between the selected crystal and the STM32 microcontroller, first
identify which procedure has to be followed among the two procedures described in
Section 3.4: Oscillator transconductance. The decision should be made based on the
oscillator specification provided in the product datasheet:
•
If the oscillator transconductance parameter is specified, then the first procedure
should be applied. Ensure that the gain margin ratio is higher than five (x5) to make
sure that the crystal is compatible with the selected STM32 microcontroller.
•
If Gm_Crit_max is specified instead, make sure Gm_crit for the oscillation loop is smaller
than the specified Gm_Crit_max value.
Step 2: Determine the capacitance value of the load capacitors CL1 and CL2
To determine the right capacitance values for CL1 and CL2 load capacitors, apply the
formula specified in Section 3.3: CL load capacitance. 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, a series of experimental iterations should be performed until
the right capacitance values are found.
During the experimental phase, use an etalon crystal. An etalon crystal is a characterized
crystal which PPM drift is well known when it is loaded by the crystal nominal load
capacitance (CL). This kind of crystals can be provided by the crystal manufacturer upon
request. After the etalon crystal has been chosen, calculate its oscillation frequency (Fetalon)
when the crystal is loaded by its nominal load capacitance. This frequency is given by the
formula:
6
F etalon = F nominal ×  PPM etalon ⁄ 10 


where:
Fetalon is the etalon crystal oscillation frequency when the crystal is loaded by its
nominal load capacitance.
Fnominal is the oscillation nominal frequency specified in the crystal datasheet.
PPMetalon is the oscillation frequency drift of the etalon crystal as it was characterized
by the crystal manufacturer.
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41
Guidelines for selecting suitable crystal and external components
AN2867
When Fetalon is computed, execute the sequence below:
1.
The first experimental iteration should be made with CL1 and CL2 capacitance values
determined by calculation:
–
2.
–
If the oscillation frequency is slower than Fetalon then go to sub-step 2).
–
Otherwise execute sub-step 3).
For this experimental iteration, decrease CL1 and CL2 capacitance values, measure
again the oscillation frequency and compare it to Fetalon:
–
3.
If the oscillation frequency is equal to Fetalon, then CL1 and CL2 are the correct
capacitances. You can therefore skip sub-steps 2) and 3).
If the oscillation frequency is slower than Fetalon, execute sub-step 2).
–
Otherwise execute sub-step 3).
–
If the oscillation frequency is almost equal to Fetalon then the latter CL1 and CL2
capacitance values should be used.
For this experimental iteration, increase CL1 and CL2 capacitance values, measure
again the oscillation frequency and compare it to Fetalon:
–
If the oscillation frequency is slower than Fetalon then execute sub-step 2).
–
Otherwise execute sub-step 3).
–
If the oscillation frequency is almost equal to Fetalon then the latter CL1 and CL2
capacitance values should be used.
Step 3: Check the Safety Factor of the oscillation loop
The safety factor should be assessed as described in Section 3.8: Safety factor to ensure a
safe oscillation of the oscillator under operating conditions.
Note:
Many crystal manufacturers can check microcontroller/crystal pairing compatibility upon
request. If the pairing is judged valid, they can provide a report including the recommended
CL1 and CL2 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: Drive level (DL) and external resistor (RExt)
calculation) and check if it is greater or lower than DLcrystal:
24/42
•
If DL < DLcrystal, no need for an external resistor. Congratulations you have found a
suitable crystal.
•
If DL > DLcrystal, you should calculate RExt in order to have: DL < DLcrystal. You should
then recalculate the gain margin taking RExt into account.
If you find that gain margin > 5, congratulations, you have found a suitable crystal. If
not, then this crystal will not work and you have to choose another. Return to step 1 to
run the procedure for the new crystal.
DocID15287 Rev 10
AN2867
Guidelines for selecting suitable crystal and external components
Step 5 (optional): Calculate the PPM accuracy budget
Finally, you can use the formula below to estimate the PPM accuracy budget for the whole
application:
PPM Budget = PPM crystal + Deviation ( C L ) × Pullability crystal
where:
PPMBudget is the estimated accuracy for the oscillation frequency.
PPMcrystal is the crystal PPM accuracy specified in the datasheet.
Deviation (CL) is expressed in pF. It measures the deviation of the load capacitance
(CL) due to tolerances on load capacitor values and the variation of the stray
capacitance (CS) due to PCB manufacturing process deviation.
Pullabilty is expressed in PPM/pF (refer to Section 3.7: Crystal pullability).
Note:
The PPM budget calculated above does not take into account the temperature variation
which may make the PPM budget bigger.
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Some recommended resonators for STM32 microcontrollers
AN2867
5
Some recommended resonators for
STM32 microcontrollers
5.1
STM32-compatible high-speed resonators
The high-speed oscillator (HSE) embedded into all STM32 microcontrollers are compatible
with almost all the resonators available on the market. They are provided by a wide range of
resonator manufacturer including:
•
ABRACON
•
EPSON (refer to http://www5.epsondevice.com)
•
KYOCERA
•
MICROCRYSTAL
•
MURATA (refer to the Murata part-number selector tool available at
http://ds.murata.com/)
•
NDK (refer to http://www.ndk.com)
•
RIVER
Compatible resonators have various frequencies and technologies (ceramic resonator and
quartz-crystal resonator are all compatible with the HSE oscillator embedded into STM32
microcontrollers. Table 6 summarizes the frequency ranges supported by the HSE oscillator
embedded into STM32 microcontrollers.
Table 6. HSE oscillators embedded in STM32 microcontrollers
Frequency
STM32F0/
STM32F1/T
F3
STM32F2
F4_G1 and
F4_G2
STM32F7
STM32L0
STM32L1
STM32L4
Unit
4-32MHz
4 - 16MHz
4-25MHz
4-26MHz
4-26MHz
1-25MHz
1-24MHz
4-48MHz
gm_min
10
25
5
5
5
3,5
3,5
7,5
gm_crit_max
2
5
1
1
1
0,7
0,7
1.5
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mA/V
AN2867
5.2
Some recommended resonators for STM32 microcontrollers
STM32-compatible low-speed resonators
Table 7 lists a set of low-speed quartz-crystal resonators that are either compatible with the
whole STM32 microcontroller families or with a subset. It shows the STM32 microcontrollers
compatible with each resonator part-number. A set of STM32-compatible resonators with
different footprints is provided to facilitate crystal selection even if there are geometric
constraints for the final application.
Note:
The list of the STM32-compatible resonators is not exhaustive. Only the compatible
resonator part-numbers checked by STMicroelectronics are listed.
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41
Package
Manufacturer
Quartz Ref/ Part number
ESR Max(Ω)
Frequency (Hz)
C0 (pF)
CL (pF)
gm_crit_max (µA/V)
Compatible STM32 series/lines
1.6x1.0mm
RIVER
TFX04
90000
32768
1
5
0,5494
F0,F1,F2,F3,F4,F7,L0,L4,L1
1.6x1.0mm
EPSON
FC1610AN
90000
32768
1,2
5
0,5866
F0,F1,F3,F4_G2, F7,L0,L4,L1
1.6x1.0mm
KYOCERA
ST1610SB32768C0
90000
32768
1,5
7
1,1026
F0,F3,F4_G2,F7,L0,L4
1.6x1.0mm
RIVER
TFX04
90000
32768
1
9
1,5260
F0,F3,F7,L0,L4
DocID15287 Rev 10
2.0x1.25mm
EPSON
FC-12M
90000
32768
1
5
0,5494
F0,F1,F2,F3,F4,F7,L0,L4,L1
2.0x1.2mm
MicroCrystal
CM8V-T1A
75000
32768
1,1
4
0,3308
F0,F1,F2,F3,F4,F7,L0,L4,L1
2.0x1.2mm
ABRACON
ABS06-107-32.768KHz-T
80000
32768
1,7
4
0,4407
F0,F1,F2,F3,F4,F7,L0,L4,L1
2.0x1.2mm
KYOCERA
ST2012SB32768A0
80000
32768
1,3
5
0,5384
F0,F1,F2,F3,F4,F7,L0,L4,L1
2.0x1.2mm
RIVER
TFX03/TFX03L
90000
32768
1
5
0,5494
F0,F1,F2,F3,F4,F7,L0,L4,L1
2.0x1.2mm
MicroCrystal
CM8V-T1A
75000
32768
1,1
9
1,2972
F0,F3,F4_G2,F7,L0,L4
2.0x1.2mm
KYOCERA
ST3215SB32768E0
80000
32768
1,3
9
1,4391
F0,F3,F4_G2,F7,L0,L4
2.0x1.2mm
NDK
NX2012SA - EXS00A-MU00524
80000
32768
1,3
7
0,9345
F1,F0,F3,F7,L0,L4,F4_G2
2.0x1.2mm
NDK
NX2012SA - EXS00A-MU00528
80000
32768
1,3
12,5
2,5833
F0,F3,F7,L0,L4
3.2x1.5mm
ABRACON
ABS07-120-32.768KHz-T
60000
32768
1,2
6
0,5274
F0,F1,F2,F3,F4,F7,L0,L4,L1
3.2x1.5mm
EPSON
FC135R
50000
32768
1,1
6
0,4274
F0,F1,F2,F3,F4,F7,L0,L4,L1
3.2x1.5mm
EPSON
FC135
70000
32768
1
6
0,5816
F0,F1,F2,F3,F4,F7,L0,L4,L1
3.2x1.5mm
KYOCERA
ST3215SB32768A0
70000
32768
0,9
5
0,4132
F0,F1,F2,F3,F4,F7,L0,L4,L1
3.2x1.5mm
KYOCERA
ST3215SB32768E0
70000
32768
0,9
9
1,1633
F0,F3,F4_G2,F7,L0,L4
3.2x1.5mm
MicroCrystal
CM7V-T1A
50000
32768
1,2
7
0,5701
F0,F1,F2,F3,F4,F7,L0,L4,L1
3.2x1.5mm
NDK
NX3215SA - EXS00A-MU00525
50000
32768
1
6
0,4154
F0,F1,F2,F3,F4,F7,L0,L4,L1
NDK
NX3215SA - EXS00A-MU00523
50000
32768
1
7
0,5426
F0,F1,F2,F3,F4,F7,L0,L4,L1
3.2x1.5mm
NDK
NX3215SA - EXS00A-MU00526
70000
32768
1
12,5
2,1631
F0,F3,F7,L0,L4,L4
3.2x1.5mm
RIVER
TFX02
70000
32768
1
7
0,7596
F0,F1,F3,F4_G2,F7,L0,L4
8.0x3.8mm
EPSON
MC306
50000
32768
0,9
6
0,4036
F0,F1,F2,F3,F4,F7,L0,L4,L1
8.0x3.8mm
EPSON
MC306
50000
32768
0,9
12,5
1,5223
F0,F3,F7,L0,L4
8.0x3.8mm
ABRACON
ABS26
50000
32768
1,35
12,5
1,6263
F0,F3,F7,L0,L4
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3.2x1.5mm
Some recommended resonators for STM32 microcontrollers
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Table 7. Recommended crystal resonators for LSE oscillator
embedded in STM32 microcontrollers
AN2867
Some recommended crystals for STM8A/S microcontrollers
6
Some recommended crystals for STM8A/S
microcontrollers
6.1
Part numbers of recommended crystal oscillators
Table 8. KYOCERA
Part number
6.2
Freq.
ESR
CL
Drive level (DL)
CX5032GA08000H0QSWZZ
8 MHz
300 Ω max
12 pF
500 µW max
CX5032GA16000H0QSWZZ
16 MHz
100 Ω max
12 pF
300 µW max
CX8045GA08000H0QSWZZ
8 MHz
200 Ω max
12 pF
500 µW max
CX8045GA16000H0QSWZZ
16 MHz
50 Ω max
12 pF
300 µW max
Part numbers of recommended ceramic resonators
Table 9 and Table 10 give the references of recommended CERALOCK® ceramic
resonators for the STM8A microcontrollers provided and certified by Murata.
Table 9. Recommendable conditions (for consumer)
Part number
Freq.
CL
CSTCR4M00G55B-R0
4 MHz
CL1 = CL2 = 39 pF
CSTCE8M00G55A-R0
8 MHz
CL1 = CL2 = 33 pF
CSTCE16M0V53-R0
16 MHz
CL1 = CL2 = 15 pF
Table 10. Recommendable conditions (for CAN-BUS)
Part number
Freq.
CL
CSTCR4M00G15C**-R0
4 MHz
CL1 = CL2 = 39 pF
CSTCR8M00G15C**-R0
8 MHz
CL1 = CL2 = 33 pF
CSTCE16M0V13C**-R0
16 MHz
CL1 = CL2 = 15 pF
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41
Tips for improving oscillator stability
AN2867
7
Tips for improving oscillator stability
7.1
PCB design guidelines
The 32 kHz crystal oscillator is an ultra-low-power oscillator (transconductance of a few
μA/V). The low oscillator transconductance affects the output dynamics since smaller
transconductance values generates a smaller oscillating current. This results in a lower
peak-to-peak voltage on the oscillator outputs (from a few dozen to a few hundred mV).
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 in order 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 that should be taken when designing
the oscillator PCB is provided below:
•
High values of stray capacitance and inductances should be avoided as they might
lead to uncontrollable oscillation (e.g. the oscillator might resonate at overtones or
harmonics frequencies). Reducing the stray capacitance also decreases startup time
and improves oscillation frequency stability.
•
To reduce high frequency noise propagation across the board, the microcontroller
should have a stable power supply source to ensure noiseless crystal oscillations. This
means that well-sized decoupling capacitor should be used for powering the
microcontroller.
•
The crystal should be mounted as close as possible to the microcontroller to keep short
tracks 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 might affect oscillation stability.
Long tracks/paths might behave as antennas for a given frequency spectrum thus
generating oscillation issues when passing EMI certification tests. Refer to Figure 11:
PCB with separated GND plane and guard ring around the oscillator and Figure 13:
Signals around the oscillator.
30/42
•
Any path conveying high-frequency signals should be routed away from the oscillator
paths and components. Refer to Figure 11: PCB with separated GND plane and guard
ring around the oscillator.
•
The oscillator PCB should 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 microcontroller ground. It prevents interferences between the
oscillator components and other application components (e.g. crosstalk between
paths). Note that if a crystal in a metallic package is used, it should not been connected
to the oscillator ground. Refer to Figure 10: Recommended layout for an oscillator
circuit, Figure 11: PCB with separated GND plane and guard ring around the oscillator
and Figure 12: GND plane.
•
Leakage current might increase startup time and even prevent the oscillator startup. If
the microcontroller is intended to operate in a severe environment (high
moisture/humidity ratio) an external coating is recommended.
DocID15287 Rev 10
AN2867
Tips for improving oscillator stability
Figure 10. Recommended layout for66
an oscillator circuit
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Warning:
It is highly recommended to apply conformal coatings to the
PCB area shown in Figure 10, 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.
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41
Tips for improving oscillator stability
7.2
AN2867
PCB design examples
Example 1
Figure 11. PCB with separated GND plane and guard ring around the oscillator
Figure 12. GND plane
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Figure 13. Signals around the oscillator
DocID15287 Rev 10
AN2867
Tips for improving oscillator stability
Example 2
Figure 14 gives an example of PCB that does not respect the guidelines provided in
Section 7.1:
•
No ground plans around the oscillator component
•
Too long paths
•
No symmetry between oscillator capacitances
•
High crosstalk/coupling between paths
•
Too many test points.
Figure 14. Preliminary design (PCB design guidelines not respected)
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41
Tips for improving oscillator stability
AN2867
The PCB design has been improved to respect the guidelines (see Figure 15):
•
Guard ring connected to the GND plane around the oscillator
•
Symmetry between oscillator capacitances
•
Less test points
•
No coupling between paths.
Figure 15. Final design (all design guidelines have been respected)
Figure 16. GND plane
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DocID15287 Rev 10
Figure 17. Top layer view
AN2867
Tips for improving oscillator stability
Example 3
Figure 18 gives another example of PCB that does not respect the guidelines provided in
Section 7.1:
•
No guard ring around oscillator components
•
Long paths
•
EMC tests failed.
Figure 18. PCB guidelines not respected
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41
Tips for improving oscillator stability
AN2867
The PCB design has been improved to respect the guidelines (see Figure 19):
•
Ground planes around the oscillator component
•
Short paths that link the STM32 to the oscillator
•
Symmetry between oscillator capacitances
•
EMC tests passed.
Figure 19. PCB guidelines respected
7.3
Soldering guidelines
In general, soldering is a very sensitive process for low-frequency crystals more than it is for
high-frequency ones. Hints to reduce the impact of such process on the crystal parameters
are provided below:
36/42
•
Expose crystals to temperatures above their maximum ratings can damage the crystal
and affect the ESR value. Refer to the crystal datasheet for the right reflow temperature
curve. If it is 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 ultralow-power applications).
DocID15287 Rev 10
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.
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41
FAQs
9
AN2867
FAQs
Table 11. Frequently asked questions
Questions
Answers
How can I know if my crystal is compatible with a Refer to Section 4: Guidelines for selecting
given STM32 MCU?
suitable crystal and external components.
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Can I use a 32.768 kHz crystal that is compatible
with STM32 MCU but is not mentioned in
Table 7: Recommended crystal resonators for
LSE oscillator embedded in STM32
microcontrollers?
Yes, you can. Table 7: Recommended crystal
resonators for LSE oscillator embedded in
STM32 microcontrollers is not exhaustive. It is
given as a reference for some selected crystal
manufacturers, footprint size and crystal load
capacitance.
In my application, 32.768 kHz frequency verylow drift and high accuracy are mandatory to
obtain an accurate clock without calibration.
Which crystal load capacitance (CL) can I
choose?
First, you should make 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 CL ≥ 6 pF:
CL= 7 pF is a good compromise between low
drift and moderate power consumption.
9 and 12.5 pF can be used in a noisy
environment but will impact the power
consumption.
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10
Conclusion
Conclusion
The most important parameter is the gain margin of the oscillator, which determines if the
oscillator will start up or not. This parameter has to be calculated at the beginning of the
design phase to choose the suitable crystal for the application. The second parameter is the
value of the external load capacitors that have to be selected in accordance with the CL
specification of the crystal (provided by the crystal manufacturer). This determines the
frequency accuracy of the crystal. The third parameter is the value of the external resistor
that is 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 you should use
components that have exactly the same properties as those that will be used in production.
Likewise, you should work with the same oscillator layout and in the same environment to
avoid unexpected behavior and therefore save time.
Recently MEMS oscillators have emerged on the market with a significant market share.
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 their competitive cost. This kind of oscillators are compatible
with the whole STM32 microcontrollers except for STM32F1 and STM32L1. When a MEMS
oscillator is paired with an STM32 embedded oscillator, this latter should be configured in
bypass mode.
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41
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: STM32compatible high-speed resonators and Section 5.2: STM32compatible low-speed resonators.
Section 5.2: STM32-compatible low-speed resonators added.
Section 5.2: STM32-compatible low-speed resonators added.
27-Apr-2010
3
Added Section 7: Some recommended crystals for STM8A/S
microcontrollers.
4
Updated Section 5.2: STM32-compatible low-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 5.2: STM32-compatible low-speed resonators:
removed Table 13: EPSON TOYOCOM, Table 14: JFVNY®, and
Table 15: KDS; Added Table 6: Recommendable crystals NEW
LANDSCAPE TABLE.
Added Warning: after Figure 10.
30-Mar-2011
5
Section 5.2: STM32-compatible low-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 5.2: STM32-compatible lowspeed resonators and updated hyperlink.
Added Section 7: Tips for improving oscillator stability.
Remove section Some PCB hints.
25-Nov-2010
19-Sep-2014
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Changes
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AN2867
Revision history
Table 12. Document revision history (continued)
Date
19-Dec-2014
19-Feb-2015
17-Aug-2015
Revision
Changes
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 for selecting suitable crystal and
external components.
Updated Section 5: Some recommended resonators for STM32
microcontrollers.
Added Section 8: Reference documents.
Updated Section 10: Conclusion.
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: Detailed steps to select an STM32-compatible crystal.
Note moved from step 2 to 3 and updated.
Renamed Table 7.
10
Updated Figure 9: Classification of low-speed crystal resonators
available on the market.
Added caution notes in Section 4.1: Low-speed oscillators
embedded into STM32 microcontrollers.
Added STM32F7, STM32F446xx, STM32F469/479xx and STM32L4
microcontrollers in Table 5: LSE oscillators embedded into STM32
microcontrollers.
Added STM32F411xx, STM32F446xx, STM32F469/479xx and
STM32L4xx microcontrollers in Table 6: HSE oscillators embedded
in STM32 microcontrollers.
Updated Table 7: Recommended crystal resonators for LSE
oscillator embedded in STM32 microcontrollers.
Added Section 9: FAQs.
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41
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