DocID029918 Rev 1 19/48
AN4938 Clocks
47
3 Clocks
Four different clock sources can be used to drive the system clock (SYSCLK):
• HSI oscillator clock
• CSI oscillator clock.
• HSE oscillator clock
• Main PLL (PLL) clock
The devices have the two following secondary clock sources:
• 32 kHz low-speed internal RC (LSI RC) which drives the independent watchdog and,
optionally, the RTC used for Auto-wakeup from the Stop/Standby mode.
• 32.768 kHz low-speed external crystal (LSE crystal) which optionally drives the RTC
clock (RTCCLK).
Each clock source can be switched ON or OFF independently when it is not used, to
optimize power consumption.
Refer to device reference manual for a detailed description of the clock tree. In particular, a
complete view of clock usage by peripheral is provided in the Kernel clock distribution
overview.
3.1 HSE oscillator clock
The high speed external clock signal (HSE) can be generated from two possible clock
sources:
• HSE external user clock (see Figure 11).
• HSE external crystal/ceramic resonator (see Figure 12).
The resonator and the load capacitors have to be placed as close as possible to the
oscillator pins in order to minimize the output distortion and startup stabilization time. The
loading capacitance values must be adjusted according to the selected resonator.
Figure 11. HSE external clock Figure 12. HSE crystal/ceramic resonators
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