AN4206 datasheet(16/34 Pages) STMICROELECTRONICS

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Clocks

AN4206

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DocID023944 Rev 4

Clocks

Three different clock sources can be used to drive the system clock (SYSCLK):



HSI 8 MHz RC oscillator clock (high-speed internal clock signal)



HSE oscillator clock (high-speed external clock signal)



PLL clock

The devices have other secondary clock sources:



40 kHz low-speed internal RC (LSI RC) that drives the independent watchdog and,

optionally, the RTC used for Auto-wakeup from the Stop/Standby modes.



32.768 kHz low-speed external crystal (LSE crystal) that optionally drives the real-time

clock (RTCCLK)

Each clock source can be switched on or off independently when it is not used, to optimize

the power consumption. Refer to the STM32F3xx reference manuals (RM0316, RM0313,

RM0364, RM0365 and RM0366) for a description of the clock tree.

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 oscillator.

Figure 9. HSE/ LSE clock sources

Clock source

Hardware configuration

External clock

Crystal/Ceramic

resonators

OSC_OUT

External

source

GPIO

OSC_IN

OSC_IN OSC_OUT

Load

capacitors

CL2

CL1

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