ST STM32F10 Series, Application Note

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AN2586 - Application note Clocks
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2.2 LSE OSC clock
The low-speed external clock signal (LSE) can be generated from two possible clock
sources:
●
LSE external crystal/ceramic resonator (see Figure 10 )
●
LSE user external clock (see Figure 9 )
2.2.1 External source (LSE bypass)
In this mode, an external clock source must be provided. It must have a frequency of
32.768 kHz. The external clock signal (square, sine or triangle) with a duty cycle of about
50% has to drive the OSC32_IN pin while the OSC32_OUT pin must be left high impedance
(see
Figure 10 and Figure 9 ).
2.2.2 External crystal/ceramic resonator (LSE crystal)
The LSE crystal is a 32.768 kHz low speed external crystal or ceramic resonator. It has the
advantage of providing a low-power, but highly accurate clock source to the real-time clock
peripheral (RTC) for clock/calendar or other timing functions.
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 start-up stabilization time. The load
capacitance values must be adjusted according to the selected oscillator.
2.3 Clock-out capability
The microcontroller clock output (MCO) capability allows the clock to be output onto the
external MCO pin. The configuration registers of the corresponding GPIO port must be
programmed in alternate function mode. One out of four clock signals can be selected as
the MCO clock:
●
SYSCLK
●
HSI
●
HSE
●
PLL clock divided by 2
Figure 9. External clock Figure 10. Crystal/ceramic resonators
OSC32_OUT OSC32_IN
External source
(Hi-Z)
ai14371
Hardware configuration
OSC32_OUT OSC32_IN
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STM32F10xxx
C
L1
C
L2
Hardware configuration