DocID17496 Rev 7

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AN3216

Clocks

32

3.2

HSE OSC clock

The high-speed external clock signal (HSE) can be generated from two possible clock

sources:

•

HSE user external clock (see Figure 8)

•

HSE external crystal/ceramic resonator (see Figure 9)

(resonator series resistance).

pin capacitance and board or trace PCB-related capacitance. Typically, it is between 2 pF and 7 pF. Please

refer to Section 6: Recommendations on page 25 to minimize its value.

3.2.1

External source (HSE bypass)

In this mode, an external clock source must be provided. It can have a frequency of up to

32 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 9).

3.2.2

External crystal/ceramic resonator (HSE crystal)

The external oscillator frequency ranges from 1 to 24 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 9.

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.

pF range (typical), designed for high-frequency applications and selected to meet the

crystal manufacturer typically specifies a load capacitance that is the series combination of

Refer to the electrical characteristics sections in the datasheet of your product for more

details.

Figure 8. External clock

Figure 9. Crystal/ceramic resonators

OSC_OUT

OSC_IN

External source

(Hi-Z)

ai14369

Hardware configuration

OSC_OUT

OSC_IN

ai14370b

STM32L1xxx

(1)

Hardware configuration