DocID024966 Rev 1

13/27

AN4325

Clocks

26

2.1

High speed external clock signal (HSE) OSC clock

The high speed external clock signal can be generated from two possible clock sources:



HSE external crystal/ceramic resonator



HSE user external clock

The resonator and the load capacitors have to be placed as close as possible to the

oscillator pins in order to minimize output distortion and startup stabilization time. The

loading capacitance values must be adjusted according to the selected resonator.

External crystal/ceramic resonator (HSE crystal)

The 4 to 32 MHz external oscillator has the advantage of producing a very accurate

frequency on the main clock. Refer to the electrical characteristics section of the datasheet

for more details about the associated hardware configuration.

The HSERDY flag in the Clock control register (RCC_CR) indicates if the HSE oscillator is

stable or not. At startup, the clock is not released until this bit is set by hardware. An

interrupt can be generated if enabled in the Clock interrupt register (RCC_CIR).

The HSE Crystal can be switched on and off using the HSEON bit in the Clock control

register (RCC_CR).

Figure 5. HSE/ LSE clock sources

Clock source

Hardware configuration

External clock

Crystal/Ceramic

resonators