Clocks

AN4467

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DocID026156 Rev 2

2.2

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).To fine tune the REXT value ,refer to AN2867(Oscillator design guide for ST

microcontrollers)

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

refer to Section 5.4: Decoupling to minimize its value.

2.2.1

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 can be used as a GPIO. For Category 2

devices, when the package does not provide OSC_IN, PA0 can receive the external clock

source, see CK_IN definition in RM0377. From current consumption standpoint a square

signal is preferred (see Figure 8).

drop as explained in Section 1.1.3: Voltage regulator.

2.2.2

External crystal/ceramic resonator (HSE crystal)

The external oscillator frequency ranges from 1 to 25 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.

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

Figure 8. External clock

Figure 9. Crystal/ceramic resonators