Clocks AN4488
26/44 DocID026304 Rev 3
4 Clocks
Three different clock sources can be used to drive the system clock (SYSCLK):
• HSI oscillator clock (high-speed internal clock signal)
• HSE oscillator clock (high-speed external clock signal)
• PLL clock
The devices have two secondary clock sources:
• 32 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 reference manual for the description of the clock tree.
4.1 HSE OSC clock
The high-speed external clock signal (HSE) can be generated from two possible clock
sources:
• HSE user external clock (see Figure 18)
• HSE external crystal/ceramic resonator (see Figure 19)
1. The value of R
EXT
depends on the crystal characteristics. Typical value is in the range of 5 to 6 R
S
(resonator series resistance).
2. Load capacitance C
L
has the following formula: C
L
= C
L1
x C
L2
/ (C
L1
+ C
L2
) + C
stray
where: C
stray
is the pin
capacitance and board or trace PCB-related capacitance. Typically, it is between 2 pF and 7 pF. Please
refer to Section 7: Recommendations on page 35 to minimize its value.
Figure 18. HSE external clock Figure 19. HSE crystal/ceramic
resonators
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