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Reset and clock control (RCC) RM0401

96/771 RM0401 Rev 3

5.2.1 HSE clock

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

sources:

• HSE external crystal/ceramic resonator

• HSE external user 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 oscillator.

External source (HSE bypass)

In this mode, an external clock source must be provided. You select this mode by setting the

HSEBYP and HSEON

bits in the RCC clock control register (RCC_CR). The external clock

signal (square, sinus or triangle) with ~50% duty cycle has to drive the OSC_IN pin while the

OSC_OUT pin should be left HI-Z. See

Figure 13.

External crystal/ceramic resonator (HSE crystal)

The HSE has the advantage of producing a very accurate rate on the main clock.

The associated hardware configuration is shown in Figure 13. Refer to the electrical

characteristics section of the datasheet for more details.

The HSERDY flag in the RCC clock control register (RCC_CR) indicates if the high-speed

external 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

RCC clock interrupt register

(RCC_CIR).

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

Figure 13. HSE/ LSE clock sources

Hardware configuration

External clock

Crystal/ceramic

resonators

OSC_OUT

External

source

(HI-Z)

OSC_IN OSC_OUT

Load

capacitors

ST STM32F410 User Manual

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