AN2586

Clocks

Doc ID 13675 Rev 7

11/28

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:

●

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 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 STM32F10xxx or STM32F100xx reference manual (RM0008 or RM0041,

respectively) for a description of the clock tree:

●

RM0008 for STM32F101xx, STM32F102xx, STM32F103xx and STM32F105xx/107xx

microcontrollers

●

RM0041 for STM32F100xx value line microcontrollers

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 20 to minimize its value.

Figure 6.

External clock

Figure 7.

Crystal/ceramic resonators

OSC_OUT

OSC_IN

External source

(Hi-Z)

ai14369

Hardware configuration

OSC_OUT

OSC_IN

ai14370

STM32F10xxx

(1)

Hardware configuration