STM32F105xx, STM32F107xx
Electrical characteristics
(1) (2)
Table 22. HSE 3-25 MHz oscillator characteristics
Symbol Parameter Conditions
fOSC_IN Oscillator frequency
Min
Typ
Max Unit
3
25
MHz
RF
Feedback resistor
200
30
kΩ
Recommended load capacitance
versus equivalent serial
C
RS = 30 Ω
pF
resistance of the crystal (RS)(3)
VDD = 3.3 V, VIN = VSS
with 30 pF load
i2
HSE driving current
1
mA
gm
Oscillator transconductance
Startup time
Startup
25
mA/V
ms
(4)
tSU(HSE
VDD is stabilized
2
1. Resonator characteristics given by the crystal/ceramic resonator manufacturer.
2. Based on characterization, not tested in production.
3. The relatively low value of the RF resistor offers a good protection against issues resulting from use in a
humid environment, due to the induced leakage and the bias condition change. However, it is
recommended to take this point into account if the MCU is used in tough humidity conditions.
4. tSU(HSE) is the startup time measured from the moment it is enabled (by software) to a stabilized 8 MHz
oscillation is reached. This value is measured for a standard crystal resonator and it can vary significantly
with the crystal manufacturer
For C and C , it is recommended to use high-quality external ceramic capacitors in the
L1
L2
5 pF to 25 pF range (typ.), designed for high-frequency applications, and selected to match
the requirements of the crystal or resonator (see Figure 16). C and C are usually the
L1
L2
same size. The crystal manufacturer typically specifies a load capacitance which is the
series combination of C and C . PCB and MCU pin capacitance must be included (10 pF
L1
L2
can be used as a rough estimate of the combined pin and board capacitance) when sizing
and C . Refer to the application note AN2867 “Oscillator design guide for ST
C
L1
L2
microcontrollers” available from the ST website www.st.com.
Figure 16. Typical application with an 8 MHz crystal
Resonator with
integrated capacitors
C
L1
f
OSC_IN
HSE
Bias
controlled
gain
8 MHz
resonator
R
F
STM32F10xxx
OSC_OUT
(1)
R
EXT
C
L2
ai14128b
1. REXT value depends on the crystal characteristics.
Low-speed external clock generated from a crystal/ceramic resonator
The low-speed external (LSE) clock can be supplied with a 32.768 kHz crystal/ceramic
resonator oscillator. All the information given in this paragraph are based on characterization
results obtained with typical external components specified in Table 23. In the application,
the resonator and the load capacitors have to be placed as close as possible to the oscillator
pins in order to minimize output distortion and startup stabilization time. Refer to the crystal
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