DocID026304 Rev 3 27/44
AN4488 Clocks
43
4.1.1 External source (HSE bypass)
In this mode, an external clock source must be provided. It can have a frequency from 1 to
50 MHz (refer to STM32F4xxxx 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 19 and Figure 18).
4.1.2 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 Figure 19. Using a 25 MHz
oscillator frequency is a good choice to get accurate Ethernet, USB OTG high-speed
peripheral, and I
2
S.
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 dedicated Application Note (AN2867 - Oscillator design guide for ST
microcontrollers) and electrical characteristics sections in the datasheet of your product for
more details.
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