ST STM32F10 Series, Примечание к применению

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Clocks AN2586 - Application note
12/23
2.1 HSE OSC clock
The high speed external clock signal (HSE) can be generated from two possible clock
sources:
●
HSE external crystal/ceramic resonator (see Figure 8 )
●
HSE user external clock (see Figure 7 )
1. C
L1
and C
L2
represent the load capacitances.
2. The value of R
EXT
depends on the crystal characteristics. Typical value is in the range of 5 to 6 R
S
(resonator series resistance).
2.1.1 External source (HSE bypass)
In this mode, an external clock source must be provided. It can have a frequency of up to
25 MHz. 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 8 and Figure 7 ).
2.1.2 External crystal/ceramic resonator (HSE crystal)
The 4-to-16 MHz external oscillator has the advantage of producing a very accurate rate on
the main clock. The associated hardware configuration is shown in
Figure 8 .
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
L1
and C
L2
it is recommended to use high-quality ceramic capacitors in the 5 pF-to-
25 pF range (typ.), designed for high-frequency applications and selected to meet the
requirements of the crystal or resonator. C
L1
and C
L2,
are usually the same value. The
crystal manufacturer typically specifies a load capacitance that is the series combination of
C
L1
and C
L2
. The PCB and MCU pin capacitances must be included when sizing C
L1
and
C
L2
(10 pF can be used as a rough estimate of the combined pin and board capacitance).
Refer to the electrical characteristics sections in the STM32F101xx and STM32F103xx
datasheets for more details.
Figure 7. External clock Figure 8. Crystal/ceramic resonators
OSC_OUT OSC_IN
External source
(Hi-Z)
ai14369
Hardware configuration
OSC_OUT OSC_IN
ai14370
STM32F10xxx
R
EXT
(1)
C
L1
C
L2
Hardware configuration