Clocks; Hse Osc Clock; Figure 6. Hse External Clock; Figure 7. Hse Crystal/Ceramic Resonators - ST STM32F20 Series Application Note

Clocks

2 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 STM32F20xxx/21xxx reference manual RM0033 for the description of the clock
tree.
2.1

HSE OSC clock

The high-speed external clock signal (HSE) can be generated from two possible clock
sources:
●
HSE external crystal/ceramic resonator (see
●
HSE user external clock (see
Figure 6.
1. The value of R
(resonator series resistance).
2. Load capacitance C
capacitance and board or trace PCB-related capacitance. Typically, it is between 2 pF and 7 pF. Please
refer to
12/29
HSE external clock
Hardware configuration
OSC_IN OSC_OUT
(Hi-Z)
External source
depends on the crystal characteristics. Typical value is in the range of 5 to 6 R
EXT
has the following formula: C
L
Section 5: Recommendations on page 21
Doc ID 18267 Rev 2
Figure
Figure 6)
Figure 7.
ai14369
= C x C / (C
L L1 L2
to minimize its value.
7)
HSE crystal/ceramic
resonators
+ C ) + C where: C
L1 L2 stray stray
AN3320
S
is the pin