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AN3320
2.1.1
External source (HSE bypass)
In this mode, an external clock source must be provided. It can have a frequency from 1 to
16 MHz (refer to STM32F20xxx/21xxx datasheets for actual max value).
The external clock signal (square, sine or triangle) with a duty cycle of about 50%, has to
drive the OSC_IN pin while the OSC_OUT pin must be left in the high impedance state (see
Figure 7
2.1.2
External crystal/ceramic resonator (HSE crystal)
The external oscillator frequency ranges from 4 to 26 MHz.
The external oscillator has the advantage of producing a very accurate rate on the main
clock. The associated hardware configuration is shown in
oscillator frequency is a good choice to get accurate Ethernet, USB OTG high-speed
peripheral, and I
The resonator and the load capacitors have to be connected as close as possible to the
oscillator pins in order to minimize output distortion and startup stabilization time. The load
capacitance values must be adjusted according to the selected oscillator.
For C
and C
L1
25 pF range (typ.), designed for high-frequency applications and selected to meet the
requirements of the crystal or resonator. C
crystal manufacturer typically specifies a load capacitance that is the series combination of
C
and C
L1
C
(10 pF can be used as a rough estimate of the combined pin and board capacitance).
L2
Refer to the electrical characteristics sections in the datasheet of your product for more
details.
and
Figure
6).
2
S.
it is recommended to use high-quality ceramic capacitors in the 5 pF-to-
L2
. The PCB and MCU pin capacitances must be included when sizing C
L2
Doc ID 18267 Rev 2
Figure
and C
are usually the same value. The
L1
L2,
Clocks
7. Using a 25 MHz
and
L1
13/29
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