Clocks

AN3320

12/29

Doc ID 18267 Rev 2

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 Figure 7)

●

HSE user external clock (see Figure 6)

1.

(resonator series resistance).

2.

capacitance and board or trace PCB-related capacitance. Typically, it is between 2 pF and 7 pF. Please

refer to Section 5: Recommendations on page 21 to minimize its value.

Figure 6.

HSE external clock

Figure 7.

HSE crystal/ceramic

resonators

OSC_OUT

OSC_IN

External source

(Hi-Z)

ai14369

Hardware configuration