AN2586 Datasheet, PDF (12/28 Pages) STMicroelectronics – Getting started with STM32F10xxx hardware development

Clocks
AN2586
2.1.1
2.1.2
External source (HSE bypass)
In this mode, an external clock source must be provided. It can have a frequency of up to:
● 24 MHz for STM32F100xx value line devices
● 25 MHz for STM32F101xx, STM32F102xx and STM32F103xx devices
● 50 MHz for connectivity line devices
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 and Figure 6).
External crystal/ceramic resonator (HSE crystal)
The external oscillator frequency ranges from:
● 4 to 16 MHz on STM32F101xx, STM32F102xx and STM32F103xx devices
● 4 to 24 MHz for STM32F100xx value line devices
● 3 to 25 MHz on connectivity line devices
The external oscillator has the advantage of producing a very accurate rate on the main
clock. The associated hardware configuration is shown in Figure 7.
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 datasheet of your product for more
details.
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Doc ID 13675 Rev 7