Clocks; High Speed External Clock Signal (Hse) Osc Clock - ST STM32F0 Series Application Note

AN4080
3

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 RTC
● HSI 14MHz RC oscillator (HSI14) dedicated for ADC
Each clock source can be switched on or off independently when it is not used, to optimize
the power consumption. Refer to the STM32F0xxx reference manual (RM0091) for a
description of the clock tree.
3.1

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.
Clock source
External clock
Crystal/Ceramic
resonators
HSE/ LSE clock sources
Doc ID 023035 Rev 2
Hardware configuration
OSC_IN OSC_OUT
GPIO
External
source
OSC_IN OSC_OUT
C
L1
Load
capacitors
Clocks
C
L2
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