Hse Clock; Figure 10. Hse/ Lse Clock Sources - ST STM32F100 Series Reference Manual

RM0041
Manual.http://infocenter.arm.com/help/topic/com.arm.doc.ddi0337e/DDI0337E_cortex_m3_r1p1
_trm.pdf
6.2.1

HSE clock

The high speed external clock signal (HSE) can be generated from two possible clock
sources:
•
HSE external crystal/ceramic resonator
•
HSE user external clock
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. The
loading capacitance values must be adjusted according to the selected oscillator.
Clock source
External clock
Crystal/Ceramic
resonators
External source (HSE bypass)
In this mode, an external clock source must be provided. It can have a frequency of up to
24 MHz. This mode is selected by setting the HSEBYP and HSEON bits in the
register (RCC_CR). The external clock signal (square, sinus or triangle) with ~50% duty
cycle has to drive the OSC_IN pin while the OSC_OUT pin should be left hi-Z. See
Figure 10.

Figure 10. HSE/ LSE clock sources

Hardware configuration
External
source
OSC_IN OSC_OUT
C
L1
RM0041 Rev 6
Reset and clock control (RCC)
OSC_OUT
(HiZ)
C
L2
Load
capacitors
Clock control
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