AN3320

Clocks

Doc ID 18267 Rev 2

13/29

2.1.1

External source (HSE bypass)

In this mode, an external clock source must be provided. It can have a frequency from 1 to

16 MHz (refer to STM32F20xxx/21xxx datasheets for actual max value).

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).

2.1.2

External crystal/ceramic resonator (HSE crystal)

The external oscillator frequency ranges from 4 to 26 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 7. Using a 25 MHz

oscillator frequency is a good choice to get accurate Ethernet, USB OTG high-speed

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.

25 pF range (typ.), 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.